Hymenoptera Parasitoid Response to Sheep Grazing, Tillage, and Herbicides in Wheat-Fallow Rotations

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Hymenoptera Parasitoid Response to Sheep Grazing, Tillage,and Herbicides in Wheat-Fallow RotationsAuthor(s): H. B. Goosey , J. P. Hatfield , M. G. Rolston , P. G. Hatfield and G. D.JohnsonSource: Journal of the Kansas Entomological Society, 86(1):22-35. 2013.Published By: Kansas Entomological SocietyDOI: http://dx.doi.org/10.2317/JKES110223.1URL: http://www.bioone.org/doi/full/10.2317/JKES110223.1

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Hymenoptera Parasitoid Response to Sheep Grazing, Tillage, andHerbicides in Wheat-Fallow Rotations

H. B. GOOSEY,1 J. P. HATFIELD, M. G. ROLSTON, P. G. HATFIELD, AND G. D. JOHNSON

Department of Animal and Range Sciences, Montana State University,

Bozeman, Montana 59717

e-mail: [email protected]

ABSTRACT: Sweep net sampling of spring and winter wheat (Triticum spp.) was conducted in

2007 and 2008 at the Fort Ellis Research and Extension Center, Montana State University,

Bozeman, MT to determine hymenoptera parasitoid family composition, abundance, and

diversity in two wheat-fallow cropping systems managed by either tillage, herbicides, or

domestic sheep (Ovis aries) grazing. Eleven hymenopteran families classified as parasitoids

were captured in 2007 and 16 families in 2008. The mean abundance of parasitoids was

greatest (P # 0.05) in crops where the fallow component of the rotation was managed with

sheep grazing, as opposed to tillage and herbicide systems. Family diversity, as indexed by

Simpson’s D, did not differ between fallow management treatments (P 5 0.88) or cropping

system (P 5 0.74) but did differ between study year (P # 0.01).

KEY WORDS: Biological control, fallow, Hymenoptera, insect diversity, parasitoid, sheep,

wheat

Hymenoptera parasitoids are recognized in agroecosystems as important natural

enemies and potential trophic regulators (Norris and Kogan, 2000; Altieri and

Nichols, 2004). Research suggests that parasitoids are most effective in cropping

systems with increased plant diversity because beneficial hymenoptera often require

a plant food source (nectar or pollen) during their adult life stage (Jervis et al., 1993).When adult wasps utilize weed pollen and nectar for nutrition, a direct trophic

connection is established which increases the parasitoid’s lifespan, fecundity, and

usually pest management success (Norris and Kogan, 2000). As an example, Pereyra

and Villanueva (1987) recorded 28 adult species of entomophagous wasps feeding on

the nectaries of several weeds in Mexico during their adult stage. Conventional

farming systems, however, typically apply selective herbicides to a crop in an attempt

to reduce plant diversity to a mono-culture. Radosevich et al. (2007) indicates that in

agroecosystems, plant-plant interactions can be negative resulting in yield reduction.However, there is also a growing body of evidence indicating that plant-plant

interactions can also be positive or neutral in terms of resource use (Radosevich

et al., 2007) and/or polytrophic interactions (Norris and Kogan, 2000). Therefore

each trophic relationship should be investigated to assess plant-plant as well as plant-

host-parasitoid interactions (Altieri and Nicholls, 2004).

Integrated crop/livestock systems partner crop and livestock enterprises through

mutually beneficial practices. In a crop/livestock system, animals typically graze cropresidues and weeds which reduce feed costs (Franzluebbers, 2007). Furthermore,

grazing is emerging as a legitimate integrated pest management (IPM) tactic when it

is implemented with the purpose of managing insect or weed pests of crops or

rangelands (Olson, 1994; O’Neill et al., 2003; Goosey et al., 2005; Hatfield et al.,

Accepted 25 February 2012; Revised 10 January 2013

E 2013 Kansas Entomological Society

1 Corresponding author.

JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY86(1), 2013, pp. 22–35

2007a, b; O’Neill et al., 2008; Goosey, 2012). Sheep grazing can significantly alter

plant communities and habitats (Courtois et al., 2004) and increase plant diversity

through suppression of dominant species (Fowler, 2002) as well as influence higher

trophic level (parasitoid) diversity and abundance (Kruess and Tscharntke, 2002).

In the northern Great Plains, sheep grazing can be integrated into a crop-fallow

system (Johnson et al., 1997; Entz et al., 2002). In a crop-fallow system, land will

produce a crop one year and lie fallow (dormant) the next year. Fallowing crop land

is used to conserve soil water, mineralize plant biomass into available nutrients,

manage problematic weeds, increase cropped year yields, and reduce the risk of crop

failure in limited rainfall areas (Aase and Pikul, 1995; Jones and Popham, 1997).

Using tillage and herbicides to manage weeds during fallow have been effective but

are expensive, resulting in some of the highest variable costs for small grains

production in Montana (Johnson et al., 1997). Other disadvantages of using these

practices are increased erosion and an increased risk of herbicide contamination of

soil and water (Fenster, 1997).

The influence sheep grazing has on parasitoid abundance in wheat-fallow systems

has not been investigated. We conducted a preliminary survey of parasitoid

abundance in two wheat-fallow cropping systems where weed growth on fallowed

land was managed either with conventional tillage, herbicides, or sheep grazing. The

objective was to determine if a fallow management practice influenced the

abundance of hymenopteran parasitoids in wheat.

Materials and Methods

Site Description

The research site is located at the Fort Ellis Research and Extension Center,

Montana State University, Bozeman, MT (45.67uN, 111.03uW; altitude 1468 m).

The overall study ran from 2004 to 2008. Data presented here were collected during

2007 and 2008 as a preliminary investigation and smaller component of that larger

study.

The previous 10 yr cropping history for the site was a perennial grass pasture

containing a mixture of smooth bromegrass (Bromus inermis L.), intermediate

wheatgrass (Thinopyrum intermedium L.), and Canada bluegrass (Poa compressa L.).

When this pasture system was terminated, the site was left un-seeded and fallowed

with tillage for 1 yr prior to plot seeding in 2004. Site mean monthly air temperatures

ranges between 25.7uC (January) and 18.9uC (July) and the 113-yr average annual

precipitation equals 465 mm.

Treatment Description

The experimental design was a randomized complete block (three blocks) with

plot as the experimental unit. The sites contains 45 total plots with each plot

measuring 15.2 3 91.4 m (0.13 ha). The three treatments were sheep grazing (Graze),

herbicide application (Chem), and mechanical tillage (Mech). Each treatment was

applied on three cropping systems (continuous spring wheat [CSW], spring wheat-

fallow [SW-F], and winter wheat-fallow [WW-F]) as a fallow management method

for weed control and soil water conservation. Treatments were applied during the

fallow period with each phase of the cropping sequence replicated three times and

present each year. In each year of study, a predetermined crop-fallow sequence

VOLUME 86, ISSUE 1 23

designated 36 plots to either WW-F or SW-F production. Within any study year, 18

of these 36 plots were equally cropped to either spring or winter wheat. The

remaining 18 plots lie fallow and, depending on treatment designation, weed growth

was managed by Chem, Graze, or Mech practices. The remaining nine plots were

cropped to CSW. Data presented here were collected each study year and during the

crop phase from the 18 plots in the SW-F and WW-F sequence. Financially, there

was only sufficient time and funds to sample and process specimens from cropped

plots in two of the three cropping systems. We choose to eliminate the CSW system

since the fallow treatments (i.e., Chem, Graze, Mech), were most heavily applied to

the SW-F and WW-F rotations.

The Graze treatment was implemented using western white-faced ewes (Ovis aries)

at stocking rates between 29 and 153 sheep days ha21 in fenced plots. Three or four

grazing events occurred each fallowing period with each grazing event ending when

about 47 kg ha21 or less of wheat residue and weed biomass remained in the plot.

Ending biomass was measured by collecting all above ground plant biomass from

three 0.1 m2 rings per plot. Samples were oven dried for 12 hrs. and weighted to

estimate kg of plot biomass. The value of 47 kg ha21 was calculated from data

collected during 2005 and 2006 from Graze plots. During this time, we visually

estimated when grazing should be terminated. Once that decision was made, we then

entered each plot and measured the remaining biomass by collecting three 0.1 m2

rings per plot. Each sample was oven dried for 12 hrs. and weighed to determine a

base line protocol for grazing termination. Post-treatment Graze biomass was less

than Chem but greater than Mech biomass values.

The Chem treatment was implemented three to four times each fallow period by

applying herbicides such as a mixture of glyphosate [N- (phosphonomethyl)-glycin]

at 1.17 L ha21 and dimethylamine salt of dicamba (3, 6-dichloro-o-anisic acid) at

1.75 L ha21. The Mech treatment was implemented with a Flexicoil harrow (John

Deere 100, Kennedy, MN), pulled at a depth of 15 cm two to three times each fallow

period and once yearly for seed bed preparation.

Crop Management

Nitrogen fertilizer was applied at rates between 200 (WW-F) and 250 kg N ha21

(SW-F), to meet SW-F and WW-F yield goals. Soil NO3-N was measured to a depth

of 60 cm, each fall, and after harvest. Nitrogen fertilizer as urea (45% N) was

broadcast in April–May to both spring and winter wheat. Urea was incorporated

using the Flexicoil harrow in SW-F rotations and was applied at the surface in WW-

F. No P and K fertilizers were applied based on lab results. Spring wheat (cultivar

McNeal, Foundation Seed, Montana State Univ., Bozeman, MT) was planted at

90 kg ha21 in late April-early May and winter wheat (cultivar Promontory,

Foundation Seed, Montana State Univ., Bozeman, MT) was planted at 73 kg ha21

in late September-early October using row spacing of 30 cm. Growing season

broadleaf weeds were controlled with selective post-emergence herbicides.

Degree Day Calculation

Degree days (DD) were calculated using the single-sine method and an on-line

calculator (Coop, 2002) which utilized temperature data from the ‘BZEM8

MONTANA STA AM MT’ weather station (45.66uN; 111.05uW; altitude 1524 m;

24 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

approx. 8 km from the study site). A generalized lower developmental threshold of

0uC was used to calculate DD values for both study years.

Insect Sampling

Treatments were applied to the fallow phase of the copping system; however,

adult hymenopteran were sampled weekly, for three weeks, in all WW-F and SW-F

cropped plots with a standard 38 cm diameter sweep net (Table 1). Our objective

was assessing if hymenopteran parasitoids differences existed in the wheat crop

phase of rotation based on the fallow designation the previous year. Sampling was

initiated in each crop when 50% of the main tillers were growth stage Zadoks 37

(flag leaf just visible) (Zadoks et al., 1974). Sampling within each plot consisted of

20 (180u) sweeps along a linear transect that both approximated the plot center

and paralleled the long axis. All adult hymenopterans were removed from each

sample and each specimen was identified to family using a Leica brand Wild M3C

(Solms, Germany), with a magnification range between 96x and 640x. The work

area was illuminated using a V-LUX 1000 fiber optic light (Volpi MFG, USA Co.

Inc., Auburn, NY) with two goosenecks. Specimen identification and classification

was referenced using the appropriate systematic keys found in Borror et al. (1989),

Gibson et al. (1997), and Triplehorn and Johnson (2005). Parasitoid specimens are

maintained in vials containing 90% ethanol at the Montana Wool Lab,

Table 1. Calendar, Julian, and Degree Dates of samples taken from winter and spring wheat crops

during 2007 and 2008 and the Fort Ellis Research and Extension Center, Montana State University,

Bozeman, MT.

Year

WW-F* SW-F*

Calendar date Julian date Degree date Calendar date Julian date Degree date

2007

Sample 1 15 June 166 1131 10 July 191 1656

Sample 2 22 June 173 1260 17 July 198 1830

Sample 3 29 June 180 1402 24 July 205 2019

2008

Sample 1 20 June 171 879 18 July 199 1417

Sample 2 27 June 178 1007 24 July 205 1534

Sample 3 3 July 184 1133 30 July 211 1662

* WW-F 5 Winter Wheat-Fallow cropping rotation; SW-F 5 Spring Wheat Fallow cropping rotation.

Table 2. Tests of residual variance homogeneity and normality and treatment effects.

Crop

rotation Yr

Homogeneity of variance Normality Treatment effects

F d.f. Probability W* Probability F d.f. Probability

WW-F{ 2007 1.1 2,24 0.36 0.97 0.71 12.9 2,22 ,0.01

2008 2.53 2,24 0.1 0.96 0.38 17.0 2,22 ,0.01

SW-F{ 2007 0.87 2,24 0.83 0.96 0.33 29.98 2,22 ,0.01

2008 1.42 2,24 0.26 0.96 0.29 2.16 2,22 50.05

* Shapiro-Wilk normality W-statistic.

{ WW-F 5 Winter Wheat-Fallow cropping rotation; SW-F 5 Spring Wheat-Fallow cropping rotation.


Department of Animal and Range Sciences, Montana State University, Bozeman,

MT 59717-2900.

Statistical Analysis

All statistical analyses were conducted using SASH (Littell et al., 2002). Parasitoid

family abundance was calculated for statistical analyses by totaling adult captures

across family. Homogeneity of variance was determined by plotting residual vs.

predicted values and by analyzing the absolute value of the residuals as the dependent

variable and block and treatment as class variables (PROC MIXED, Littell et al., 2002).

Independence of error terms were assessed by plotting the residuals vs. time. Residual

normality was determined using normal probability plots and the Shapiro-Wilk test.

Table 3. Hymenopteran Family expressed as a percent of the total Hymenopteran adults captured,

across treatments, during 2007 and 2008 at the Fort Ellis Research and Extension Center, Montana State

University, Bozeman, MT.

Aphelinidae Braconidae Ceraphronidae Cynipidae Encyrtidae Eulopidae Eupelmidae

WW-F*

2007{ 3.5 4.5 0.0 4.5 7.6 13.8 0.0

2008{ 0.0 7.4 2.6 1.3 6.2 15.5 0.5

SW-F*

2007 14.2 15.6 0.0 0.0 5.4 10.8 0.0

2008 9.6 7.5 0.6 0.5 12.1 19.3 0.5

* WW-F 5 Winter Wheat/Fallow cropping rotation; SW-F 5 Spring Wheat/Fallow cropping rotation.

{ Sampling Year.

Table 4. Hymenopteran Family expressed as a percent of the total Hymenopteran adults captured

during 2007 and 2008 from Mechanical, Grazed, and Chemical fallowed plots at the Fort Ellis Research

and Extension Center, Montana State University, Bozeman, MT.

Treatment Aphelinidae Braconidae Ceraphronidae Cynipidae Encyrtidae Eulopidae Eupelmidae

WW-F*

2007{ Mech{ 3.6 4.4 0.0 4.4 7.3 13.1 0.0

Graze{ 2.8 3.3 0.0 5.0 7.2 11.7 0.0

Chem{ 4.2 5.8 0.0 4.2 8.3 16.7 0.0

2008{ Mech 0.0 4.9 3.3 1.6 8.2 18.0 1.6

Graze 0.0 3.3 3.3 1.1 6.6 13.2 0.0

Chem 0.0 13.9 1.3 1.3 3.8 15.2 0.0

SW-F*

2007 Mech 10.5 14.9 0.0 0.0 5.3 14.9 0.0

Graze 12.9 14.3 0.0 0.0 6.4 7.9 0.0

Chem 19.3 17.5 0.0 0.0 4.4 9.6 0.0

2008 Mech 11.0 8.2 0.0 1.4 9.6 23.3 1.4

Graze 6.0 6.0 0.0 0.0 14.9 16.4 0.0

Chem 11.7 8.3 1.7 0.0 11.7 18.3 0.0


{ Sampling Year.

{ Mech 5 Mechanical tilled fallow; Graze 5 Sheep grazed fallow; Chem 5 Herbicide Fallow.


The diversity of insect families for each fallow treatment was calculated as the

Simpson-Yule Index of Diversity (Southwood and Henderson, 2002) and is termed

‘Simpson’s D’ which is calculated as: [D 5 (1{PFobs

i

p2i ), where Fobs is the total number

of insect families comprising a sample and where pi is the proportion of individuals

in the ith family of a sample]. This value gives a measure of the probability that two

specimens randomly selected from a sample will belong to different families and,

unlike species richness and the Shannon-Weiner Diversity Index H’, is not influenced

by the abundance of organisms and is thus useful when comparing samples from

habitats that vary in family or specie abundance (Lande et al., 2000).

One Simpson’s D value was calculated for each fallow treatment (Mech, Graze,

Chem) by cropping system (WW-F, SW-F) combination by entering plot specific

family identification data, collected over the three sampling dates, into the above

Eurytomidae Ichneumonidae Mymaridae Platygastridae Pteromelidae Scelionidae Sphecidae Trichogrammatidae Torymidae

0.0 8.7 12.2 8.1 12.2 11.5 0.0 13.3 0.0

4.4 6.8 11.8 7.4 13.2 3.5 0.4 14.8 1.8

0.0 8.2 10.2 6.2 4.9 8.6 0.0 16.2 0.0

6.6 14.7 11.8 2.5 4.0 3.2 3.1 6.6 0.0


{ Sampling Year

Table 3. Extended.

Eurytomidae Ichneumonidae Mymaridae Platygastridae Pteromelidae Scelionidae Sphecidae Trichogrammatidae Torymidae

0.0 6.6 12.4 9.5 16.1 12.4 0.0 10.2 0.0

0.0 9.4 11.7 8.9 12.2 12.2 0.0 15.6 0.0

0.0 10.0 12.5 5.8 8.3 10.0 0.0 14.2 0.0

3.3 1.6 13.1 6.6 16.4 4.9 0.0 13.1 3.3

2.2 8.8 12.1 6.6 13.2 4.4 0.0 19.8 2.2

7.6 10.1 10.1 8.9 10.1 1.3 1.3 11.4 0.0

0.0 12.3 10.5 5.3 6.1 4.4 0.0 15.8 0.0

0.0 7.1 11.4 7.9 5.0 10.0 0.0 17.1 0.0

0.0 5.3 8.8 5.3 3.5 11.5 0.0 15.8 0.0

4.1 19.2 16.8 2.7 1.4 1.4 2.7 5.5 0.0

7.5 14.9 11.9 3.0 9.0 1.5 1.5 7.5 0.0

8.3 10.0 6.7 1.7 1.7 6.7 5.0 6.7 0.0


{ Sampling Year

{ Mech 5 Mechanical tilled fallow; Graze 5 Sheep grazed fallow; Chem 5 Herbicide Fallow.

Table 4. Extended.


equation. These values were then used to test for differences in family diversity

between treatments, study years, and cropping systems by entering each, as a class

variable, into the model statement with block as a random effect. (PROC MIXED,

Littell et al., 2002).

Results

The assumption, within cropping system, of residual homogeneity of variance and

normality were met across treatment in each sampling year (Table 2). Eleven

hymenopteran families were collected and identified during 2007 and 16 families

during 2008 (Table 3). In 2007, the most abundant (greater than 10% of the total)

families across treatment in WW-F were Eulopidae, Mymaridae, Pteromalidae,

Scelionidae, and Trichogrammatidae. In 2008, the same families were most

prevalent, excluding Scelionidae. The most abundant families in SW-F were

Aphelinidae, Braconidae, Eulopidae, Mymaridae, and Trichogrammatidae in 2007

and Encyrtidae, Eulopidae, Ichneumonidae, and Mymaridae, in 2008 (Table 3).

These families represent 63 and 55.3% (WW-F) and 67 and 57.9% (SW-F) of the

total captures in 2007 and 2008, respectively, and contain many biologically

important control agents in cropping systems. Family rank abundance, expressed as

a percentage, varied between year and treatment (Table 4).

Analysis of treatment effects within year indicates that, during both sampling

years, more adult hymenopteran parasitoids were captured from the Graze than the

Chem fallowing system and that, with the exception of 2008 SW-F, captures were

also greater in the Graze over the Mech system (Fig. 1; Table 2). Furthermore, adult

parasitoid captures from the WW-F and SW-F systems did not differ between the

Chem and Mech systems in either study year (Fig. 1; Table 2). Analyses of Simpson’s

D values indicates that the diversity of parasitoid families captured in this study

did not differ between cropping system combined over study year and fallowing

treatment (F 5 0.11; d.f. 5 1,34; P 5 0.7396) (Fig. 2A) or between fallow management

treatments when combined over cropping system and study year. (F 5 0.13;

d.f. 5 2,31; P 5 0.8783) (Fig. 2C); however, when combined over cropping system and

fallowing treatment, diversity did differ (F 5 54.34; d.f. 5 1,34; P , 0.0001) between

study years (Fig. 2B).

Discussion

It is reported that herbivore grazing can directly and indirectly influence plant

(Crawley, 1997; Radosevich et al., 2007) and insect (O’Neill et al., 2003; Goosey

et al., 2005; Hatfield et al., 2007a; O’Neill et al., 2008; Goosey, 2012) communities.

Kruess and Tscharntke (2002) report, from anthropogenic grasslands, a trophic level

reduction of 50% in insect diversity and abundance attributed to grazing intensity,

suggesting that intensity appears to negatively affect the plant-insect associations.

Kruess and Tscharntke (2002) further report a resource heterogeneity response

between grazed and ungrazed pastures suggesting that increased insect diversity and

abundance were associated with taller vegetation which is typically located in

ungrazed areas. Debano (2006) reports from a study on public and private ranches in

Arizona that Hymenoptera, in general, are less rich and diverse in livestock grazed

pastures. However, our data, which is generated from cropland, reports that neither

fallow management nor cropping system influenced parasitoid family diversity, as


indexed by Simpson’s D. Diversity is a measure of both richness (total number of

families captured) and evenness (numerical closeness of families in an environment).

This suggests that the parasitoids captured in this study were not associated withspecific fallowing practices or crops but rather were transients, in search of both food

and hosts, throughout the research site. Our data does indicate that family diversity

was greater in 2008. Eleven families were captured in 2007 compared to 16 families in

2008 and diversity increases with increased family richness (Price, 2007). The five

families which were not captured in 2007 and the mean percent of the total catch

they represent in 2008 data are: Ceraphronidae (1.6%), Eupelmidae (0.5%),

Fig. 1. Total hymenoptera parasitoid abundance in spring and winter wheat as a function of fallowing

method for each sampling date (left column) and for each year’s mean (right column); for yearly means,

symbols followed by a different letter grouping differ at a 5 0.05 (Least significant difference test).

Degree-days calculated using the single-sine method with a lower developmental threshold of 0uC.


Fig. 2. Mean family diversity (1 – Simpson-Yule Index of Diversity) of hymenoptera parasitoids

separated by A) winter wheat-fallow (WW-F) or spring wheat-fallow (SW-F) cropping systems but

combined over study year and fallowing treatment, B) year of study but combined over cropping system

and fallowing treatment, or C) fallowing treatments of mechanical tillage (Mech), domestic sheep grazing

(Graze), and herbicides (Chem) but combined over cropping system and study year. Family diversity

values represent the probability the two specimens randomly taken from any sample will be from different

families. Larger diversity values suggest greater parasitoid diversity. Bars with different letter groupings

differ at a 5 0.05 (Least significant difference test).


Eurytomidae (5.5%), Sphecidae (1.8%), and Torymidae (0.9%). Excluding

Eurytomidae, the remaining four families occur in such low abundances, in 2008, as

to classify them as ‘rare’ and therefore, probability suggests that these families were

missed in sweep net samples during 2007. Based on this, we suggest that Simpson’s D

diversity was greater in 2008 due to increased family richness from captures of rare

Hymenopteran families rather than there being an underlying ecological condition

which favored their presence in 2008 but not during 2007. In contrast, members of the

family Eurytomidae represent a significant 5.5% of the total abundance in 2008 but are

absent from 2007 data. Eurytomidae specimens vary in their habits with most being

parasitic and a few species being hyperparasitic; however a few species are phytophagous

and are often serious pests of wheat (Borror et al., 1989). We identified specimens to the

level of family and so it is impossible to assess what percentage of the total Eurytomidae

abundance were parasitoids or hyperparasitoids and what percentage were phytoph-

agous plant feeders. If the Eurytoma specimens were phytophagous one would expect

this family to be represented in both study years; however, specimens were only collected

in 2008. Therefore, it is possible to suggest that 2008 specimens were in fact parasitoids

or hyperparasitoids and their presence, in 2008, and absence, in 2007, was a function of

host availability rather than sweep net sampling bias.

Sweep netting is probably the most common method used to sample invertebrates

from vegetation because equipment is lightweight and simple to use while sampling

is speedy and has a high return for little cost. However there is an inherent bias

associated with samples collected from sweep nets. Factors which can change the

efficiency and consistency of samples are associated with changes in: 1) habitat, 2)

species composition, 3) vertical distribution of specimens on the vegetation, 4)

weather conditions, and 5) the influence of diel cycle on vertical movements of

specimens (Southwood and Henderson, 2000). All our samples were taken from

spring and winter wheat on the same sampling days and nearly the same sampling

times therefore any bias associated with habitat, weather, and diel cycle

considerations should have been minimized. Our results do not indicate any

significant variability in family composition based on our analyses of richness and

the Simpson’s D diversity index (Table 3; Table 4; Fig. 2). However given that the

wheat plants were .30 cm tall indicates that the sweep net was encountering and

subsequently sampling less than 100% of the entire crop which indicates that bias

associated with the vertical distribution of specimens is contained within our

samples. To explain, if all specimens were vertically distributed randomly within the

crop, then one should conclude that a biased smaller proportion of the total

possible specimens were sampled. In contrast, if not all specimens were distributed

randomly then the proportion of possible specimens that could be collected by the

sweep net would be biased depending on each species preferred height within the

crop. For example, specimens, such as Specidae, which preferentially choose

positions that are closer to the ground, would be sampled at rates lower than actual

and, in contrast, species preferentially choosing positions higher and closer to the

crop canopy, such as Aphelinidae adults searching for aphid hosts, would be

sampled at rates higher than actual. Additionally Doxon et al. (2011) reports from

a study which evaluated the response of shrubland birds and invertebrates to fire

and grazing, that sweep net sampling, when compared to vacuum sampling, is

biased against specimens ,5 cm in length. All of the families we present here

contain species with body sizes ,5 cm; however, we utilized the sweep net to


complete all of our sampling which would distribute any bias equally across the

entire sampling program.

O’Neill et al. (2008) reports Miridae and Reduviidae abundances were lower in

uniformly grazed treatments compared to controls on a crested wheat grass pasture

in south-western Montana. Additional reports from Montana also suggest sheep

grazing has a negative influence on wheat stem sawfly (Hymenoptera: Cephidae)

(Goosey et al., 2005; Hatfield et al., 2007a) and alfalfa weevil (Coleoptera:

Curculionidae) (Goosey et al., 2004; Goosey, 2012) populations by reducing the

prevalence of these crop pests. There are examples of grazing increasing insect

(Orthoptera: Acrididae) abundance (Holmes et al., 1979), but typically grazing is

negatively correlated with insect populations. The question then becomes, since we

did not record a difference in diversity between fallow treatments, why did we record

a treatment difference in parasitoid abundance with more adults being captured

from cropped plots where the fallowing phase was managed by sheep grazing?

One explanation may be found by examining additional preliminary research

(Hanson et al., 2010) which was also conducted at the Fort Ellis Research and

Extension Center. This study reports an increase in weed species richness and

diversity in both WW-F and SW-F sheep grazed systems when compared with the

Mech and Chem treatments. Furthermore, we observed that the Graze treatment,

when compared to Mech and Chem, shifted weed species composition toward

communities dominated by perennial flowering weeds such as dandelion (Taraxacum

officinale Weber) and annual dicotyledonous species such as prostate pigweed

(Amaranthus graecizans L.) and shepherd’s purse (Capsella bursa-pastoris L.).

Sjodin (2007), who investigated changes in insect pollinator behavior as a response

to grazing intensity and timing in east-central Sweden, reported that pollinators

appeared to choose habitats based on flower density, suggesting that the number of

flowers in a given area may reflect the amount of reward for visitors. Norris and

Kogan (2000) report that hymenopteran parasitoids which utilize flower resources

(nectar and pollen) exhibit increased adult longevity, fecundity, and potentially

fitness. Since the Graze system in this study had greater flowering plant species

richness and diversity (Hanson et al., 2010) it is therefore logical to suggest that

hymenopteran parasitoids were more abundant in these grazed areas because adult

parasitoids actively selected them based on an increased potential for nectar and

pollen rewards.

To the contrary, it is also probable and logical to suggest that the greater

parasitoid abundance was not due to food rewards but rather to a greater abundance

of hosts being located in Graze treatment crops. We do not have abundance data on

treatment pests and hosts to report so it is impossible for us to know what, if any,

differences existed between treatments; however if hosts were more abundant in

Graze treatment crops, this resource becomes more available and is more easily

discovered which would increase the abundance of any associated parasitoids. We

did record a greater abundance of parasitoids in the Graze treatment (Fig. 1) and if

this was due to an increase in hosts which was an indirect result of livestock grazing;

then we suggest that grazing does not appear to change the diversity of the host

complex. For example, if sheep grazing wheat fallow only increased the abundance

of species of Aphididae, one should expect to see differences in treatment diversity

because parasitoids of Aphididae, such as Aphelinidae, Braconidae, and Encyrtidae,

would become more abundant, as a response to an improved host resource base, in


the Graze treatment and would have been captured at a higher rate which would

have changed the data evenness thus changing diversity. This, however, is not the

case as family diversity did not differ between treatments (Fig. 2C). This suggests

that no particular fallow management practice is, in the short term, selectively

beneficial or detrimental to any family of parasitoid. It also suggests that in this

study if parasitoids are actively selecting grazed plots due to a food reward, the

practice of grazing does not shift weed community structure to the point that

parasitoid diversity is also shifted. To explain, if grazing shifted weed diversity so

that parasitoid diversity also shifted but away from the most prolific species, grazing

could be considered a negative practice in the arena of biological control. However,

and to the contrary, if grazing, by shifting weed diversity, enhanced the abundance

and diversity of prolific parasitoids, grazing could be considered synergistic to

biological control.

Although we cannot distinguish if the greater abundance of parasitoids in the

crop phase of the Graze treatment was due to an increased potential of a food

reward or due to more hosts, we can conclude that grazing does not shift parasitoid

diversity either positively or negatively nor does it appear to shift the diversity of

the pest/host complex. Our data concludes that wheat plots in a crop/graze fallow

rotation harbor a greater abundance of hymenopteran parasitoids than do wheat

plots in either crop/tillage or crop/herbicide rotations. Price (2007) suggests that

moderate levels of disturbing events produce the greatest insect diversity because

disturbances open up new resources for colonists and prevent competitively

dominant species from monopolizing resources. In the data presented here,

diversity does not differ yet abundance does and so we leave further interpretation

on how grazing events may interact with the population dynamics of Hymenoptera

parasitoids to the reader.

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Hymenoptera Parasitoid Response to Sheep Grazing, Tillage, and Herbicides in Wheat-Fallow Rotations

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