Impact of the Stem Borer, Dectes texanus , on Yield of the Cultivated Sunflower, Helianthus annuus

Impact of the stem borer, Dectes texanus, on yield ofthe cultivated sunflower, Helianthus annuus.

J.P. Michaud1, Angela K. Grant and J.L. Jyoti

1 Kansas State University, Agricultural Research Center – Hays, 1232 240th Ave, Hays, KS, 67601,785-625-3425

AbstractFoliar and soil-drench insecticide treatments were used in attempts to manipulate infestation ofcultivated sunflower plants, Helianthus annuus LeConte (Asterales: Asteraceae) by Dectes texanusLeConte, (Coleoptera: Cerambycidae) a serious pest of sunflowers in the High Plains of the USA. Seedyields were assessed on a per-plant basis for both oilseed and confection type sunflower hybrids in twoyears. Both insecticide treatments (foliar λ-cyhalothrin and soil-drench carbofuran) improved yield ofoilseed sunflowers in 2004, but not in 2005. Yield of confection hybrids was improved by a systemicfungicide (thiophanate methyl) in 2005, but insecticides did not improve yield in either year. Bothinsecticide treatments gave good control of various stalk-boring insects such as Cylindrocopturusadspersus (Coleoptera: Curculionidae), Mordellistena sp. (Coleoptera: Mordellidae), and Pelochristawomanana (Lepidoptera: Tortricidae), but neither gave better than 50% control of D. texanus. Plantswere sorted according to the presence or absence of D. texanus larvae and no reduction was found intotal seed weight, seed size, or oil content as a result of infestation. However, mature larvae of D.texanus girdle stalks at the base in preparation for overwintering, a behavior that reduced stalkbreakage force by 34–40%, leading to yield losses through lodging. At harvest in 2005, there weredifferences between cultivars and among treatments in the proportions of D. texanus larvae that hadgirdled their plants at harvest. It was concluded that further research aimed at reducing crop losses toD. texanus should focus on means of delaying stalk desiccation and/or deterioration, factors that appearto trigger girdling behavior.

Keywords: Ataxia hubbardi, Cylindrocopturus adspersus, girdling, insects, lodging, Mordellistena sp., oil content,Pelochrista womanana, seed weight, yieldCorrespondence: [email protected]: 19 May 2006 | Accepted: 21 October 2006 | Published: 12 April 2007Copyright: This is an open access paper. We use the Creative Commons Attribution 2.5 license that permits unrestricted use,provided that the paper is properly attributed.ISSN: 1536-2442 | Volume 7, Number 21Cite this paper as:Michaud JP, Grant AK, Jyoti JL. 2007. Impact of the stem borer, Dectes texanus, on yield of the cultivatedsunflower, Helianthus annuus. 14pp. Journal of Insect Science 7:21, available online: insectscience.org/7.21

Journal of Insect Science | www.insectscience.org ISSN: 1536-2442

Journal of Insect Science: Vol. 7 | Article 21 1

IntroductionThe indigenous longhorned beetle, Dectes texanusLeConte, has become a serious pest of cultivatedsunflowers (Philips et al. 1973; Rogers 1985) andsoybeans (Falter 1969; Hatchett et al. 1975;Campbell and van Duyn 1977) in the Americanmidwest. Originally described from the wildsunflower Helianthus annuus (LeConte 1862), thenatural host plants of D. texanus are weeds suchas Ambrosia and Xanthium spp. (Compositae)(Hatchett et al. 1975), making its pest status insoybeans somewhat anomalous. Yield losses inboth crops arise when stalks of mature plants aregirdled from the interior by larvae in preparationfor overwintering, causing plants to lodge eitherprior to, or during, harvest.

There is only one generation per year, andlate-instar larvae of D. texanus overwinter in thestubble of their host plants. Adult beetles emergeover an extended period from late spring tomid-summer and females begin to lay eggs atabout two weeks of age (Crook et al. 2004;Michaud and Grant 2005). Little is known of therange of adult dispersal, but insects likely moveonly as far as necessary to find suitable foodplants (Michaud and Grant 2005). Female feedingbehavior is strongly correlated with egg laying(Michaud and Grant 2005) and ovipositionpunctures are normally initiated in feeding scars(Hatchett et al. 1975). Eggs are laid in the leafpetioles (Hatchett et al. 1975) and hatching larvaebore down the petiole into the core of the mainstalk where they feed on the pith. In soybean, thisbehavior leads to premature leaf senescence andinfested plants can be identified early in theseason by the presence of wilted leaves. There isno tendency for females to avoid previouslyattacked plants, and multiple eggs are often laidby a single female in the same plant, or even thesame petiole (Hatchett et al. 1975; Michaud andGrant 2005). Larvae are highly aggressive towardconspecifics and eliminate one another by combatuntil typically only one larva remains in a plant byseason’s end. As the stalk dries down, the larvamoves to the base of the plant, prepares anoverwintering chamber, and then girdles the baseof the stalk above the chamber before pluggingthe stalk cavity with shredded fibers.

Michaud and Grant (2005) showed that D.texanus females preferred cultivated sunflower tosoybean for both feeding and oviposition and thatsoybean appears to be a relatively inferior hostplant for both larvae and adults, leading to

significantly reduced pupal weight and shorteradult life. Since cultivated sunflower fields in theHigh Plains frequently have 80% or more ofplants infested by D. texanus, it was of interest toknow whether or not the boring damage done bylarvae had an impact on the productivity and yieldof sunflower plants independent of the lossesattributable to lodging of girdled plants. Insunflowers grown for the confection market,premiums are paid for large seed size, whereas inthe oilseed market, premiums are paid for high oilcontent. The present study was initiated with theobjective of measuring sunflower seed yields on aper-plant basis to determine whether or not D.texanus larval boring affected plant productivityin terms of either total seed weight, seed size, oroil content. Various insecticide treatment regimeswere applied with the objective of generating aseries of plants with and without D. texanuslarvae and then protecting the yield of theseplants from other sources of damage until harvest.However, other species of Coleoptera andLepidoptera also cause boring damage tosunflower stalks in the area of this research,potentially confounding measurements of D.texanus yield impact. Primary among these arethe sunflower stem weevil, Cylindrocopturusadspersus (LeConte) the sunflower root moth,Pelochrista womanana Kearfoot, and thetumbling flower beetle, Mordellistena spp.Fortunately, these species are all effectivelycontrolled by insecticide treatments that are farless effective against D. texanus.

Materials and MethodsExperiments were conducted at Kansas StateUniversity, Agricultural Research Center- Hays,KS (38° 51′N 99° 20′W) on a crete silty clay loamsoil (22% sand, 48% silt, and 30% clay) with pH6.2 and 1.8% organic matter. The confectionhybrid used was Triumph 757c and the oilseedvariety was a Nusun hybrid, Triumph 665. In eachyear of the study, a field of 0.6 hectares wasplanted ‘no-till’ with each hybrid variety using aJohn Deere plated planter at a 76 cm row spacing.All fields were fallow the year prior to plantingsunflowers. Prior to planting, fields were fertilizedwith 57 kgs N per acre in the form of anhydrousammonium and then given a pre-emergentherbicide treatment of 2.0 L / ha S-metolachlor(Dual II Magnum, Syngenta Crop Protection Inc.,www.syngenta.com) and 0.225 L / hasulfentrazone (Spartan DF, FMC Corp.,www.fmc.com). Following seedling emergence,



plants were hand-thinned to a density ofapproximately 45,000 stems / ha (confection) or55,000 stems / ha (oilseed). In 2004, both fieldswere planted on 14 May and emergence was notedon 21 May. Seedling destruction by black-tailedjack rabbits, Lepus californicus, reduced plantpopulation to ca. 30,000 stems / ha in theconfection field. The stand in the oilseed field wasso poor that it had to be replanted on 4 June. In2005, a rabbit control program was initiated inApril and seedling losses were minimal. In orderto achieve similar planting dates for each cultivarfor the two years, the confection hybrid wasplanted on May 11 and the oilseed hybrid on June6. There was sufficient moisture for promptgermination in both cases. Weather data wasobtained for both growing seasons from the KSU,ARCH weather station located within 100m of theexperimental fields.

The experiment was laid out in a completelyrandomized design with five replications of eachtreatment. When plants reached the 4–6 leafstage, a series of 15 test plots were staked out ineach field. Each test plot consisted of a section ofa row with 30 plants or, in some cases, twoadjacent rows with 15 plants in each. Plots wererandomly assigned to one of three treatments:Control (no insecticide applied); λ-cyhalothrin(Warrior with Zeon technology, Syngenta) appliedtwice as a foliar spray at the equivalent of 0.288 L/ ha, once at the 6-leaf stage (V6) and again atthe12-leaf stage (V12), and carbofuran (Furadan4F, FMC) applied twice as a soil drench at theequivalent of 0.720 L / ha, once at V6 and again atV12. In 2005, an additional treatment was addedin the confection field which consisted of threesoil drenches of the fungicide thiophanate methyl(3336F, Cleary Chemical Corp.www.clearychemical.com) at a rate equivalent to4.8 L / ha, once each at V6, V12 and flower budinitiation (R1).

At bloom, all flower heads in test plots weredusted with a Bacillus thuringensis formulation(Dipel Dust, Southern Agricultural InsecticidesInc. www.southernag.com) every three days toprotect against seed damage by sunflower moth,Homoeosoma ellectulum Hulst while sparinghymenopterous pollinators. At this time, a seriesof 15 plants in each test plot were selected forcontinued protection from seed-damaging pests,henceforth referred to as ‘protected yield’ plants.Flagging tape was used to tag the head and stalkof each selected plant with a numerical code

indicating treatment, plot, and plant number. Atpetal fall, each selected flower head was coveredwith a brown paper bag secured with staples toprevent seed loss to bird predation. Once plantsreached physiological maturity, heads wereharvested in their bags and dried in an oven at 38°C for a period of 20–30 days. Harvest dates were11 September and 23 September, 2004 and 2September and 13 September, 2005, forconfection and oil cultivars, respectively. Thestalks of protected plants, including the roots,were then uprooted and brought to the laboratorywhere each stalk was measured for basalcircumference at soil level and then carefullydissected to identify and tally all the stalk-boringinsect larvae.

After completion of the drying process, flowerheads were removed from bags, measuredvertically and horizontally to estimate flowerdiameter, and then threshed individually using animprovised mechanical single-head threshingmachine. The seed was cleaned, analyzed formoisture content using a Grain AnalyticalComputer (GAC 2000, Dickey John Corp.www.dickey-john.com), and then weighed on ananalytical balance. A sample of 100randomly-selected seeds was then counted outand weighed to provide an estimation of seed size.All seed weights were corrected for moisturecontent prior to analysis. Subsamples of 40 ml.were taken from each oilseed flower head andplaced in a labeled envelope for subsequentdetermination of oil content by nuclear magneticresonance (Gambhir and Agarwala 1985).

In 2005, measurements were made of stalkstrength on the plants remaining in all test plotson the day after harvest of protected yield plants.This was accomplished with a digital force gauge(FGE-50X Dart, Shimpo Instruments,www.shimpoinst.com) attached to a 16 gauge wirelooped around the stalk at a height of 50 cm.Force was applied to the wire loop until the stalkbroke, whereupon the peak force was recordedand the circumference of the stalk measuredbasally at the point of breakage. Since stalkbreakage force demonstrates a positive linearrelationship with stalk circumference, forcemeasurements were divided by basalcircumference in order to generate a value of forceper unit basal circumference (henceforth referredto as the ‘adjusted stalk breakage force’) so thatstalk strength could be compared amongtreatments independently of variation in stalk



Figure 1. Maximum, minimum and median daily temperatures in experimental fields at the Agricultural ResearchCenter – Hays during the growing seasons of 2004 and 2005. Curved lines depict seasonal trends.

girth. Since some plants had been girdled by D.texanus and others not, the presence / absence ofgirdling in the stalk was also noted. In the oilseedfield, many girdled plants had lodged so theproportion of lodged plants was tallied in all plots.Data on rates of D. texanus infestation from theprotected yield plants were used in each plot toestimate the proportions of D. texanus larvae thathad girdled their plants prior to harvest.

Data for insects and components of yield wereanalyzed for effects of treatment separately foreach hybrid using Proc. GLM (SAS Institute 1998)followed by Tukey’s honestly significant different(HSD) test (α < 0.05) for separation of means.For each hybrid in each year, plants were thengrouped according to presence/absence of D.texanus larvae and compared for components ofyield using ‘treatment’ and ‘plot’ as fixed factors.Regression analyses were then used to testrelationships between flower diameter, stalkcircumference, and various components of yield.A χ2, goodness of fit test was used to comparerates of oilseed plant lodging in insecticidetreatments to those in control plots in 2005.

ResultsWeatherThe summer of 2004 was substantially coolerthan that of 2005 (Figure 1). Although the twoyears had similar total rainfall by end ofSeptember, the growing season of 2004 beganwith much drier conditions in spring, but receivedmore than twice the rainfall during June and Julythan in 2005 (Fig. 2), the two months duringwhich sunflowers achieve most of their growth.

The confection field, established almost onemonth earlier than the oilseed field, was severelystressed by a strong wind storm on 4 July, 2004that snapped off some plants and left many otherslying prostrate on the ground. Surviving plantswere provided special care (stalks and heads weresupported off the soil) until maturity and mostyielded well, but it was impossible to estimate theyield impact of this wind damage that occurred ata time when most plants were in full bloom. Incontrast, the later-planted oilseed flowers werestill in vegetative stages during the storm andwere not adversely impacted beyond some leafpunctures caused by hailstones.



Figure 2.Monthly rainfall recorded in experimental fields at the Agricultural Research Center – Hays fromJanuary to September in 2004 and 2005.

Insect infestationThe rate of infestation of control oilseed plants byD. texanus was significantly higher in 2004 thanin 2005 (F = 10.655; df = 1,146; P = 0.001) butthere was no difference between years forconfection plants (F = 0.197; df = 1,131; P =0.666). Patterns of D. texanus infestation across

pesticide treatments are depicted in Fig. 3. Therewere significant differences among treatments inrate of infestation in oilseed plants in both 2004(F = 8.685; df = 2,215; P < 0.001) and 2005 (F =18.530; df = 1,218; P < 0.001). However,treatment differences were not significant forconfection plants in 2004 (F = 1.496; df = 2, 200;

Figure 3. Effect of various pesticide treatments on mean percentages (± SEM) of oilseed and confectionsunflowers infested by Dectes texanus larvae in two years at the Agricultural Research Center – Hays. Columnsbearing the same letter were not significantly different (Tukey HSD, α < 0.05).



Table 1. Mean (± SEM) values for components of yield for Triumph 665 oilseed sunflower plants with and withoutboring by larvae of Dectes texanus

D. texanusYear / Variable absent present F 1,14 P

2004n 91 127

Total seed weight (gm) 150.0 ± 2.8 149.9 ± 2.8 0.28 ns100 seed weight (gm) 7.1 ± 0.1 7.0 ± 0.1 0.01 ns

Seed weight/unit area head (mg/cm2) 291.1 ± 2.5 295.1 ± 3.0 1.06 nsFlower diameter (cm) 25.5 ± 0.2 25.3 ± 0.2 0.02 ns

Basal stalk circumference (cm) 10.2 ± 0.1 10.4 ± 0.1 2.16 nsOil content (%) 41.6 ± 0.3 42.2 ± 0.2 0.82 ns

2005n 140 81

Total seed weight (gm) 44.8 ± 1.2 49.6 ± 2.0 1.94 ns100 seed weight (gm) 3.1 ± 0.1 3.2 ± 0.1 0.02 ns

Seed weight/unit area head (mg/cm2) 290.9 ± 3.7 291.1 ± 4.6 0.19 nsFlower diameter (cm) 13.8 ± 0.2 14.5 ± 0.2 1.17 ns

Basal stalk circumference (cm) 6.4 ± 0.1 6.6 ± 0.1 0.76 nsOil content (%) 47.5 ± 0.2 47.5 ± 0.2 2.86 ns

P = 0.227). The larger cerambycid, Ataxiahubbardi Fisher, which also bores the central pithcore of stalks, infested 23.3, 15.5 and 16.5 percentof plants in the control, λ-cyhalothrin, andcarbofuran treatments, respectively. This specieswas not observed in oilseed plants in either year,but is known to be more aggressive than D.texanus in larval combat (Michaud and Grant2005). Although the treatments did notsignificantly affect infestation by A. hubbardi in2004 (F = 0.625; df = 2,200; P = 0.536), it islikely that some larvae of D. texanus wereeliminated from certain plants by this speciesearlier in the season.

There were significant effects of treatment oninfestation by D. texanus in confection sunflowersin 2005 (F = 11.127; df = 3,292; P < 0.001). Thepercentage of plants with D. texanus larvae wassimilar in control and thiophanate methyltreatments, but the λ-cyhalothrin and carbofurantreatments significantly reduced D. texanusinfestation (Figure 3). Treatment also affectedinfestation rate by A. hubbardi in 2005

(F = 13.504; df = 3,292; P < 0.001); control andthiophanate methyl treatments averaged morelarvae per plant (0.22 ± 0.05 and 0.24 ± 0.05,respectively) than the λ-cyhalothrin (0.01 ± 0.01)or carbofuran (0.0) treatments.

Infestation of oilseed plants by other stem-boringinsects was relatively low in both years of theexperiment. In 2004, control plants contained anaverage of 5.5 ± 0.03 larvae of C. adspersus, 1.4 ±0.01 larvae of Mordellistena spp., and 0.0 larvaeof P. womanana. No larvae of these insects werefound in plants receiving either of the insecticidetreatments. In 2005, oilseed control plantscontained an average of 0.76 ± 0.18 C. adspersuslarvae, 0.12 ± 0.04 Mordellistena spp. larvae, and0.05 ± 0.03 P. womanana larvae, but no larvae ofthese species were found in plants receiving eitherof the insecticide treatments.

Infestation of confection plants by variousstalk-boring insects was sufficient to permitstatistical comparisons among treatments. In2004, control plants contained a mean of 11.5 ±

Table 2. Mean (± SEM) values for components of yield for Triumph 757 confection sunflower plants with andwithout boring by larvae of Dectes texanusor Ataxia hubbardi.

D. texanus or A. hubbardiYear / Variable absent present F df P

2004n 96 107

Total seed weight (gm) 88.4 ± 3.8 86.2 ± 3.9 0.01 1,14 ns100 seed weight (gm) 14.1 ± 0.2 14.7 ± 0.2 5.99 1,14 ns

Seed weight / unit area head (mg/cm2) 249.7 ± 8.7 240.8 ± 9.0 0.06 1,14 nsFlower diameter (cm) 21.2 ± 0.3 21.2 ± 0.3 0.01 1,14 ns

Basal stalk circumference (cm) 14.8 ± 0.2 15.0 ± 0.2 0.25 1,14 ns2005

n 97 186Total seed weight (gm) 77.3 ± 1.6 79.7 ± 2.4 0.01 1,19 ns100 seed weight (gm) 10.7 ± 0.1 10.5 ± 0.1 4.49 1,19 < 0.05

Seed weight/unit area head (mg/cm2) 319.5 ± 4.7 338.8 ± 10.8 1.72 1,19 nsFlower diameter (cm) 17.5 ± 0.2 17.3 ± 0.1 0.740 1,19 ns

Basal stalk circumference (cm) 8.6 ± 0.1 8.8 ± 0.1 0.14 1,19 ns



Table 3a. Effects of year on components of yield for oil seed sunflowers (Triumph 665) across all treatments.

Mean ± SEM

YearDry weight of seed /

flower (gm)Dry weight of 100

seeds (gm)Seed weight / cm2

flower (mg)Basal stalk circumference

(cm)Flower diameter

(cm)Percent oil

2004 150.04 ± 1.27b 7.06 ± 0.04b 293.4 ± 2.0a 10.30 ± 0.07b 25.42 ± 0.16b 41.92 ± 0.13a2005 46.16 ± 1.21a 3.10 ± 0.04a 291.1 ± 2.9a 6.45 ± 0.03a 14.07 ± 0.13a 47.44 ± 0.12bF 1,4 195.49 274.02 0.59 1109.91 246.96 94.67

Values bearing the same letter were not significantly different within columns (P < 0.05).

1.02 C. adspersus larvae per plant, significantlymore (F = 85.216; df = 2,200; P < 0.0001) thanplants treated either with λ-cyhalothrin (2.1 ±0.54) or carbofuran (1.04 ± 0.23). Control plantsalso contained more Mordellistena sp. larvae(0.65 ± 0.15) than either λ-cyhalothrin- orcarbofuran-treated plants (0.08 ± 0.04 and 0.15 ±0.05, respectively, F = 11.621; df = 2,200; P <0.001). No larvae of P. womanana were recoveredfrom plants in any treatment.

In 2005 confection sunflowers, infestation by C.adspersus larvae averaged 19.1 ± 2.43, 6.0 ± 0.76,0.03 ± 0.03, and 0.0 larvae per plant,respectively, in control, λ-cyhalothrin, carbofuranand thiophanate methyl plants (F = 50.981;3,292; P < 0.0001). There was a significantpositive relationship between numbers of C.adspersus larvae and total seed weight of controlplants (F = 8.48; df = 70; P = 0.005; r2 = 0.108)and the basal circumference of these plants (F =39.82; df = 70; P < 0.001; r2 = 0.363). Infestationby Mordellistena sp. averaged 0.86 ± 0.13, 0.0,0.03 ± 0.02 and 1.0 ± 0.13 larvae per plant in thefour treatments, respectively (F = 35.209; df =3,292; P < 0.0001). Control confection plantscontained a mean of 0.62 ± 0.08 P. womananalarvae per plant, significantly more (F = 37.481; df= 3,292; P < 0.0001) than any pesticidetreatment, the latter being not significantlydifferent from one another with 0.04 ± 0.02, 0.03± 0.02, and 0.11 ± 0.04 larvae per plant for theλ-cyhalothrin, carbofuran, and thiophanatemethyl treatments, respectively.

Impact of Dectes texanus on componentsof yieldAlthough chemical treatments reduced infestationby D. texanus, they did not generate a level ofdifferential infestation sufficient to permitanalysis of yield impact by plot or treatment.Consequently, plants were sorted into two groupsaccording to presence/absence of D. texanuslarvae, ignoring treatment, using plot as a fixedfactor. Analysis of data for the oilseed hybridrevealed that plants with and without D. texanuslarvae were not significantly different with respectto any component of yield in either 2004 or 2005(Table 1). For confection sunflowers, plantsinfested by A. hubbardi were included togetherwith those infested by D. texanus since larvae ofboth insects cause similar plant damage byhollowing out the pith core of the stalk during thegrowing season. Once again, there was nosignificant effect of cerambycid infestation on anycomponent of yield in either year (Table 2), exceptthat the mean weight of 100 seeds from plantscontaining larvae was significantly lower than thatfrom uninfested plants in 2005, despite anon-significant trend in the opposite direction in2004.

Components of yield for oilseedsunflowersAll components of oilseed yield were higher on aper-plant basis in 2004 than in 2005, except forseed weight per unit area of flower that did notdiffer, and oil content that was significantly lower(Table 3a and Table 3b). In 2004, both insecticide

Table 3b. Effects of treatments on components of yield for oil seed sunflowers (Triumph 665) within years.

Mean ± SEM

Year / TreatmentDry weight of seed

/ flower (gm)Dry weight of 100

seeds (gm)Seed weight / cm2

flower (mg)Basal stalk circumference

(cm)Flower diameter

(cm)Percent oil

2004Control 142.37 ± 4.13a 6.91 ± 0.14a 286.9 ± 3.8a 10.07 ± 0.12a 24.96 ± 0.33a 42.10 ± 0.35a

λ-cyhalothrin 155.00 ± 2.80b 7.10± 0.09a 293.5 ± 3.4ab 10.46 ± 0.10b 25.91 ± 0.21b 41.50 ± 0.31aCarbofuran 152.47 ± 3.18b 7.20± 0.10a 299.7 ± 3.2b 10.38 ± 0.12b 25.37 ± 0.24ab 42.08 ± 0.31a

F 2,8 0.92 0.26 1.75 1.27 0.76 0.32005

Control 48.97 ± 1.73b 3.16 ± 0.07b 292.3 ± 4.2b 6.52 ± 0.05a 14.46 ± 0.20b 47.39 ± 0.18aλ-cyhalothrin 41.40 ± 1.80a 2.92 ± 0.06a 277.7 ± 5.3a 6.50 ± 0.07a 13.51 ± 0.26a 46.98 ± 0.27aCarbofuran 49.17 ± 1.81b 3.24 ± 0.07b 302.4 ± 5.0b 6.39 ± 0.06a 14.21 ± 0.22b 48.04 ± 0.14b

F 2,8 0.61 0.61 2.3 0.41 0.5 1.17




Figure 4 Linear regressions of seed oil content on flower diameter in 2004 (a) and 2005 (b) for Triumph 665sunflowers grown at the Agricultural Research Center – Hays.

treatments resulted in increased stalk size,measured as basal circumference, and increasedyield, measured as dry weight of seed per flower,without affecting seed size, as estimated by weightof 100 seeds, or oil content. This yield increasewas associated with an increase in flower size inthe λ-cyhalothrin treatment, but not in thecarbofuran treatment, where it was associatedwith an increase in flower productivity (seedweight per unit area of head). There were nosignificant effects of treatment on seed oil contentin 2004. In 2005, the λ-cyhalothrin treatmentappeared to reduce components of yieldcompared to the carbofuran and controltreatments. This yield reduction was accompaniedby reductions in flower size, weight of 100 seeds,and productivity per unit area of flower head. In2005, the carbofuran treatment improved seed oilcontent over that observed in either control orλ-cyhalothrin-treated flowers that did not differfrom each other.

Linear regression revealed that seed oil contentwas negatively correlated with flower diameter in2004 (F = 85.02. df = 216; P < 0.0001; r2 =

0.282; Figure 4a), and that seed size, as estimatedby weight of 100 seeds, exhibited a positiverelationship with flower diameter (F = 66.56; df =216; P < 0.0001; r2 = 0.236). By contrast, in2005, oil content exhibited a slightly positiverelationship with flower diameter (F = 12.45; df =220; P = 0.001; r2 = 0.054; Figure 4b) and therelationship between flower diameter and seedsize was not significant (F = 2.45; df = 220; P =0.119). However, the relationship between flowerdiameter and stalk circumference was stronglypositive in both years (2004: F = 72.43; df = 216;P < 0.0001; r2 = 0.251; 2005: F = 48.78; df = 219;P < 0.0001; r2 = 0.182).

Components of yield for confectionsunflowersAll components of yield were higher on a perplant basis for confection flowers in 2004 than in2005 (Table 4a and Table 4b). The smallerflowers in 2005 resulted in a greater weight ofseed per unit area of head, although the seedsthemselves were substantially smaller. Neitherinsecticide treatment affected any component ofyield significantly in 2004, but sample sizes were



Table 4a. Effects of year on components of yield for confection sunflowers (Triumph 757c) across all treatments.

Means (± SEM)

YearDry wt. of seed / flower

(gm)Dry wt. of 100 seeds

(gm)Seed wt / cm2 flower

(mg)Basal stalk circumference (cm) Flower diameter (cm)

2004 87.23 ± 2.69b 14.40 ± 0.16b 245.0 ± 6.30b 12.07 ± 0.14b 21.18 ± 0.21b2005 78.11 ± 1.72a 10.51 ± 0.10a 331.9 ± 7.38a 8.74 ± 0.06a 17.94 ± 0.41aF 1,4 3.79 790.69 23.98 443.41 17.71


reduced by wind damage in that year. In 2005,the thiophanate methyl treatment produced moreweight of seed per flower than either the controlor carbofuran treatments, apparently as a result ofgreater flower productivity measured as seedweight per unit area of flower. The λ-cyhalothrintreatment was not significantly different from anyother treatment. However, the control andλ-cyhalothrin treatments resulted in larger seedsthan either the carbofuran or thiophanate methyltreatments.

The flower diameter of confection plants wasstrongly and positively correlated with seed size(weight of 100 seeds) in both 2004 (F = 62.92; df= 201; P < 0.0001; r2 = 0.238) and 2005 (F =65.50; df = 293; P < 0.0001; r2 = 0.188). Therelationship between flower diameter and stalkcircumference was also strongly positive in bothyears (2004: F = 81.27; df = 201; P < 0.0001; r2 =0.288; 2005: F = 133.54; df = 288; P < 0.0001; r2

= 0.317).

Girdling, lodging, and stalk breakageforcesGirdled oilseed stalks had a mean adjustedbreaking force of 330 ± 28.6 gm/cmcircumference compared to a mean of 502 ± 11.0gm/cm for non-girdled stalks, corresponding to a34.3% reduction in stalk strength. Similarly,girdled confection stalks had a mean adjustedstalk breakage force of 463.6 ± 174.0

gm/cmcircumference compared to a mean of774.5 ± 20.5 gm/cm for ungirdled stalks,corresponding to a reduction of 40.1%.Considering only ungirdled plants, the carbofurantreatment significantly increased the adjustedstalk breakage forces for both oilseed plants (F =8.551; df = 2,8; α < 0.05; Figure 5a) andconfection plants (F = 3.361; df = 3,11; α < 0.10;Figure 5b).

In the oilseed field, 36.9% of control plants hadlodged and were lying on the ground by date ofharvest as a result of D. texanus girdling. This wassignificantly greater than the 7.9% lodgingobserved in λ-cyhalothrin-treated plants (χ2 =44.33, P < 0.02) and the 0.6% lodging observed incarbofuran-treated plants (χ2 = 10,472.11, P <0.001). The mean rates of D. texanus infestationobtained from protected yield plants in thesetreatments were used to estimate that 66.6% oflarvae completed the girdling behavior in controlplots, compared to 18.6% in λ-cyhalothrin plots,and 5% in carbofuran plots. The frequency ofgirdling in non-lodged plants that was used forstalk breakage force measurements wasdetermined and were significantly reduced for theinsecticide treatments (F = 23.489; df = 2,8; P <0.0001), 33.3% for the control, versus 6.4% and0.0% for the λ-cyhalothrin and carbofuran plots,respectively, were girdled in the three treatments(Tukey HSD, P < 0.05).

Table 4b. Effects of treatments on components of yield for confection sunflowers (Triumph 757c) within years.

Means (± SEM)

Year / TreatmentDry wt. of seed /

flower (gm)Dry wt. of 100

seeds (gm)Seed wt / cm2

flower (mg)Basal stalk circumference (cm)

Flower diameter(cm)

2004Control 82.37 ± 5.10a 14.17 ± 0.28a 227.7 ± 11.6a 11.92 ± 0.26a 21.21 ± 0.36a

λ-cyhalothrin 89.66 ± 5.48a 14.97 ± 0.28a 245.0 ± 12.2a 11.85 ± 0.28a 21.54 ± 0.49aCarbofuran 89.00 ± 3.82a 14.18 ± 0.25a 257.2 ± 9.4a 12.33 ± 0.20a 20.91 ± 0.28a

F 2,8 3.46 1.73 1.47 0.7 0.62005

Control 73.34 ± 1.66a 10.98 ± 0.13b 319.0 ± 5.4a 8.67 ± 0.12a 17.12 ± 0.18aλ-cyhalothrin 79.14 ± 2.13ab 10.64 ± 0.20b 332.1 ± 4.2ab 8.68 ± 0.10a 17.51 ± 0.20aCarbofuran 74.52 ± 1.91a 10.17 ± 0.18a 314.7 ± 5.5a 8.46 ± 0.09a 17.32 ± 0.20a

Thiophanate methyl 85.19 ± 5.81b 10.15 ± 0.23a 361.6 ± 3.4b 9.17 ± 0.12b 17.56 ± 0.21aF 3,12 1.5 3.22 2.01 1.8 1.12




Figure 5. Mean (± SEM) adjusted stalk breakage forces (grams of force per unit cm basal stalk circumference)measured at harvest for two sunflower hybrids receiving various pesticide treatments in 2005 at the AgriculturalResearch Center – Hays. Columns bearing the same letter were not significantly different (Tukey HSD, α < 0.05).

Very few plants had lodged as a result of girdlingin the confection field by date of harvest (5 plantsin control plots, 0 plants in λ-cyhalothrin andcarbofuran plots, and 8 plants inthiophanate-methyl plots). Examination of plantsused for stalk breakage measurements revealedthat 14.0%, 13.2%, 0.0%, and 7.5% of plants weregirdled by D. texanus in control, λ-cyhalothrin,carbofuran and thiophanate methyl plots,respectively, but these differences were notstatistically significant (F = 2.367; df = 3,11; α >0.05).

DiscussionEfficacy of treatmentsAlthough both insecticide treatments appearedeffective in controlling infestation by sunflowerstem weevil, tumbling flower beetle, and root

moth, neither could be considered adequate forcontrol of D. texanus. λ-cyhalothrin has beenshown to have high toxicity to D. texanus in thelaboratory (Kazmareck et al. 2004) but adultbeetles are active in the field for an extendedperiod in summer and may have laid eggs inplants during periods when residual activity of thematerial was low. Failure of application timing tocorrespond adequately with female D. texanusactivity is the most likely explanation for thefailure of λ-cyhalothrin to reduce rates of larvalinfestation in oilseed plants in 2005 and inconfection plants in either year, although why thistreatment appeared to increase infestation inconfection plants in 2005 is difficult to surmise.Campbell and van Duyn (1977) tested variousinsecticides and application techniques against D.texanus in soybeans and concluded that theirtreatments were ineffective against larvae and



that numerous, carefully timed applicationswould be necessary to control adults.

The carbofuran treatment targeted larvae withinplants and was applied as a soil drench to obtaingood systemic activity. It may be that failure ofthis material to provide adequate control of D.texanus is partially derived from the larva’spropensity to confine its feeding to the centralpith core of the plant, avoiding conductive tissuesthat would presumably contain most of theinsecticide. Rogers (1975) noted that systemicinsecticides were ineffective in controlling D.texanus larvae in either soybean or sunflower.The two-fold reduction in stalk circumference inoilseed plants in 2005 relative to 2004 may havebrought more larvae into contact with conductiveelements of the stalk and resulted in bettercontrol in the former year. Some of the differencesobserved in insecticide efficacy between the twoyears may also have resulted from differences inenvironmental conditions. The higher summerrainfall of 2004 compared to 2005 would beexpected to result in more rapid weathering ofλ-cyhalothrin residues on foliage, and perhapsimprove plant uptake of the carbofuran soildrenches. However, such effects do not appearimplicated in levels of D. texanus control becauseλ-cyhalothrin was generally less effective in 2005,whereas the reverse was true for carbofuran.

Impact of D. texanus on sunflower yieldsWhen treatments were pooled and all plants withcerambycid larvae were compared to thosewithout, there was no effect of D. texanus larvalboring on any component of yield (Tables 1 and 2)except in confection plants in 2005 when plantswith larvae had reduced seed weights comparedto those uninfested. However, the magnitude ofthe difference was small (0.2 gm) and theopposite trend, although non-significant, wasevident in 2004, suggesting variation due tochance effects.

There are various possible explanations for whyrate of D. texanus infestation was lower in oilseedplants in 2005 than in 2004 while this was not thecase in confection plants. If the peak period offemale oviposition occurred later in 2004, femalesmay have oviposited more eggs in thelater-planted oilseed cultivar relative to theconfection cultivar in response to their earlierstage of development. However, it is also possiblethat the small stalks of oilseed plants in 2005reduced their acceptabilty for oviposition by D.texanus females.

Components of yieldBoth insecticide treatments increased the seedyield of oilseed plants in 2004, although neitherdid so in 2005. Interestingly, the mechanisms ofyield increase in 2004 appeared slightly differentfor the two materials tested; the λ-cyhalothrintreatment resulted in larger flowers than controlplants, whereas the carbofuran treatment resultedin flowers that were more productive per unit areaof head (Table 3). Both insecticide treatmentsincreased stalk girth. Given the absence ofstalk-feeding insects in control plants (apart fromD. texanus), it seems likely that insects feeding onother plant parts were implicated in the observedyield losses. Although the foliar λ-cyhalothrintreatment would presumably be limited toimpacts on foliage-feeding insects, the carbofuransoil drenches would presumably affect root, stem,and foliage insects to some degree. Thusdefoliation might primarily impact flower size,whereas root and stem feeding might have moreimpact on flower productivity in terms of seed fill.

Although no particular folivorous insect wasobserved in numbers approaching an economicinjury level in 2004, a complex of larvalLepidoptera were observed defoliating plants andmay have collectively been responsible forreducing the mean stalk circumference, flowerdiameter, and seed weight of control oilseedplants. The primary species observed includedEstigmene acrea (Drury) (Lepidoptera, Arctiidae)and Vanessa cardui L. (Lepidoptera:Nymphalidae). Muma et al. (1950) found variableeffects of foliar insecticide treatment on sunfloweryields in Nebraska that they attributed todifferential impacts on lepidopterous larvae andtheir natural enemies in different years. However,only the λ-cyhalothrin treatment would haveimpacted beneficial insects active on sunflowerfoliage as the carbofuran was applied as a soildrench.

It is notable that the seed weight of oilseedsunflowers was 222% higher in 2004 relative to2005, whereas it was only 11% higher forconfection sunflowers in 2004 which was asuperior growing season. However, it is difficult tounderstand this contrast because of the winddamage to confection plants during seedmaturation in 2004 and because variability instand establishment between the two hybrids inboth years probably had significant effects onplant size. Since the thiophanate methyltreatment produced the highest seed yield inconfection flowers in 2005, it is possible that



stalk- and root-infecting fungi were more limitingto yield than were insects in that particular fieldand year. However, the effectiveness of bothinsecticide treatments in increasing the yield ofoilseed plants in 2004 suggests that herbivorousinsects (other than D. texanus) were limiting toyield in that particular year. The lack of suchtreatment effects in 2005 is consistent withfactors other than herbivory, most likely soilmoisture, being more limiting to yield. Thereduced yields of λ-cyhalothrin-treated oilseedplants relative to controls in 2005 is difficult toexplain, but may reflect a chance effect of thesetreatment plots being located in less productiveparts of the field. Some bract necrosis induced byheat scorch was observed on oilseed flowers in2005 and this damage impeded the developmentof some flowers, particularly in two λ-cyhalothrinplots.

Reliable yield data for confection plants wasobtained only in 2005 and the λ-cyhalothrintreatment produced the best combination of seedweight and seed size, again suggesting thepotential benefits of protecting foliage. Thepositive correlation between numbers of C.adspersus larvae and total weight of seed fromcontrol confection plants probably reflects thetendency of bigger stalks to harbor largernumbers of larvae, and clearly indicates that amean of 19 larvae per stalk is well below aneconomic impact threshold for this pest. Theconsistent positive relationship between basalstalk circumference and flower diameter suggeststhat wider plant spacing is one tactic forincreasing stalk girth along with flower size forconfection varieties where seed size is alsocorrelated with flower diameter. However, anegative relationship between flower diameterand seed oil content was evident in 2004 whenoilseed flowers ranged between 20 and 30 cm indiameter, consistent with the inference that suchlarge flowers are not desirable for oil production.Mean seed oil content was 13.4% higher in 2005when most flowers ranged between 10 and 20 cmin diameter and the negative correlation withflower diameter disappeared, suggesting that thisrelationship might only be a concern whenflowers become larger than 20 cm.

Stalk breakage forces, lodging, andgirdling behaviorFungicide treatment increased the breakageforces of ungirdled stalks significantly more thaneither insecticide treatment in the 2005confection plants, suggesting that fungi may playa role in stalk deterioration at crop maturity that

is worthy of further investigation. Similarly, thegenerally higher stalk breakage forces obtainedfrom the carbofuran treatments compared to theλ-cyhalothrin treatments are likely a function ofthe systemic activity of the former material indelaying various processes associated with stalkdegradation, but data from multiple years wouldbe required to confirm this.

Girdling by D. texanus larvae reduced stalkbreakage forces by 34–40%. However, larvaeappear physically unable to fully girdle sunflowerstalks that are greater than 2.5 cm in diameter.Similarly, Wapshere (1974) noted that larvae ofthe cerambycid Mecas saturnina LeConte wereonly able to partially girdle cocklebur plants withstems > 2.0 cm in diameter. Confectionsunflowers are planted at lower densities thanoilseed plants with the goal of obtaining largerheads that, as our data clearly show, are not onlyassociated with larger seeds, the criterion forpremiums in confection markets, but also withthicker stalks. Farmers specializing in theproduction of confection sunflowers usuallymanage to avoid lodging problems simply becausestout confection stalks are robust enough toremain standing even when girdled by a D.texanus larva. Note also that the stalk breakageforces tended to be 50% higher for the confectionhybrid than for the oilseed hybrid afteradjustment for differences in plant size,suggesting that the former were intrinsicallystronger. Lodging losses caused by D. texanus are,therefore, a more serious concern for farmersgrowing oilseed cultivars, especially when theydepend upon rain, as moisture is often limiting.Oilseed sunflowers grown at high density underdry conditions not only have slender stalks thatcan be completely girdled by D. texanus, but theirstalks also dry out quickly after crop maturity,likely triggering earlier onset of the girdlingbehavior (see below). Reduction of plantingdensity to increase stalk girth is one possibletactic for oilseed farmers, provided flowerdiameter head does not increase above 20 cm.

In 2005, a large proportion of untreated plants inthe oilseed field had lodged by time of harvest,whereas this was not the case in untreatedconfection plants despite similar rates of D.texanus infestation. Although the larger girth ofconfection stalks may be implicated here, it wasalso apparent that fewer larvae had completedgirdling in the confection hybrid than in theoilseed hybrid by date of harvest. There were alsomarked differences among treatments in



completion of stalk girdling by larvae in theoilseed plants in 2005, as both insecticidetreatments appearing to delay the behavior. Oneexplanation consistent with these observations isthat girdling behavior may be triggered by stalkdesiccation because this determines the end of itssuitability as a food source. We have often rearedD. texanus larvae on fresh sunflower stalks in thelaboratory, but have never managed to elicitfeeding on dry stalk material. In contrast, larvaeof A. hubbardi do not girdle plants and readilyfeed and gain weight when provided withcompletely dry stalk pith (AKG, personalobservation). The absence of lodging in bothcultivars in 2004 is consistent with the moistconditions that prevailed up until harvest in thatyear. The oilseed stalks in 2005 sufferedsubstantial lodging, but were considerably moreslender than confection stalks, and also notablymore dry and brittle at time of harvest. Both largeplant size and pesticide treatment might delaystalk desiccation and this effect could explain theobserved delays in girdling behavior in confectionplants relative to oilseed plants and in pesticidetreatments relative to controls. Similarly,Richardson (1975) concluded that desiccation ofpith within stems represented the end of asuitable food supply and was a likely trigger for D.texanus girdling behavior in soybeans. Anecdotalsupport for this interpretation is provided byobservations of commercial fields wherenon-irrigated regions and dry spots typicallyexhibit lodging much earlier than moist orirrigated portions, despite similar rates of larvalinfestation.

Given that larval boring by D. texanus larvae hasno measurable impact on yield, further researchdirected at mitigating losses to this pest shouldfocus on factors triggering the girdling behaviorand possible means of delaying or otherwisemanipulating this behavior. Possibilities forfurther investigation include systemic materialsthat delay stalk deterioration, dessicants thataccelerate drying of flower heads relative to stalks,or the development of cultivars with ‘stay-green’stalks. The common objective of these approacheswould be to delay the onset of girdling behaviorby D. texanus larvae until well after seed has drieddown to harvestable levels.

AcknowledgementsWe are thankful to Triumph Seed Corporation forproviding sunflower seed for these experiments,and to FMC Corporation for supplying Furadan4F. We are also grateful to Phil Stahlman (KSU)

for his agronomic assistance in planting thesunflower fields, to Larry Charlet and Jerry Miller(USDA, ARS) for assisting with seed oil contentanalysis, and to Phil Sloderbeck for reviewing theMS. This work was supported, in part, by a grantfrom the National Sunflower Association. This iscontribution No. 06-278-J from the KansasAgricultural Experiment Station.

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Impact of the Stem Borer, Dectes texanus , on Yield of the Cultivated Sunflower, Helianthus annuus

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