ENTOMOPHAGA 41 (1), 1996, 27-36

P O T E N T I A L OF F U N G I AS B I O C O N T R O L A G E N T S O F P O L Y P H A G O T A R S O N E M U S L A T U S

(A CARl : T A R S O N E M I D A E )

J. E. PF~A (1), L. S. OSBORNE (2) & R. E. DUNCAN (1)

(1) University of Florida, Tropical Research and Education Center, 18905 S.W. 280 th Street, Homestead, Florida 330331, USA

(2) University of Florida, Central Florida Research and Education Center, 2807 Bimon Road, Apopka, Florida 33703-9598, USA

Infection of broad mites, Polyphagotarsonemus latus, by conidia of Beauveria bassiana, Hirsutella thompsonii, and Paecilomyces fumosoroseus conidia was investigated in the laboratory under controlled temperature and moisture condi- tions. Infection of P. latus by the fungus (LCso) was related positively to the 1.16 x 106 B. bassiana conidia per ml, 2.39 x 103H. thompsonii conidia per ml, and 1.29 x 105 P.fumosoroseus conidia per ml. Mortality caused by B. bassiana occurred the fastest among densities fluctuating between 65 and 125 mites per leaf.

The pathogens B. bassiana and P.fumosoroseus, adjuvants (oil and molasses), and the acaricide ABG6364, a microbial insecticide containing the Beta-exotoxin of Bacillus thuringiensis were evaluated for efficacy against P. latus in a green- house test. The B. bassiana treated plants had the greatest and most persistent effect on percentage mortality of total broad mites present per leaf (88 %) followed by the acaricide ABG6364. Use of adjuvants (oil and molasses) did not increase infection of P. latus.

KEY-WORDS:Beauveria bassiana, biological control, Hirsutella thompsoni, infectivity, Paecilomyces fumosoroseus, Polyphagotarsonemus latus.

The broad mite, Polyphagotarsonemus latus (Banks) is a cosmopolitan pest of many important crops such as cotton, papaya, mango, bean, potato, peppers and foliag~ plants (Jeppson et al., 1975; Costilla, 1980; Almeida et al., 1967; Flechtman & Rosa, 1980; Hugon & Chapin, 1986; Schoonhoven et al., 1978). In contrast to other mite species of the families Tetranychidae and Eriophyidae, mite pests of the family Tarsonernidae thrive under warm, humid conditions characteristic of tropical coasts and greenhouses (Jones & Brown 1983). The mite's short generation time ( l l -19days) , high fecundity (ca. 30eggs per female) and small size combined with its protected habitat and patterns of dispersion, often result in sudden and devastating effects on host plants (Hugon, 1983; Pefia & Bullock, 1994). Several natural enemies of the broad mite are known. Biological control has focused on predacious mites which are efficacious. Sixteen species of predators have been reported associated with P. latus (Kolodochka & Prutenskaya 1987; Zaman & Karimullah, 1987; Ochoa et al., 1991; Wu, 1984; Moutia, 1958; Smith & Papaceck, 1985; Badii & McMurtry, 1984; Pefia et al., 1989).

Pathogens have potential as control agents of phytophagous mites. Only one pathogen, Hirsutella nodulosa has been reported in association with P. latus (Cabrera et al., 1987).

28 J . E . PEIqA, L. S. OSBORNE & R. E. D U N C A N

Fungus infecting Tetranychidae and Eriophyidae have been documented by different resear- chers (Van der Geest, 1985), but field applications of fungi against mites have been carried out by few investigators. One of the major constraints is that germination of spores and penetration of the fungus into the mite is poor at humidities below 100 % (Gardner et aL, 1982; Gerson et al., 1979).

Because development of tarsonemids, in this case P. latus, is positively related to relative humidities between 75 and 90 %, and development of entomopathogenic fungi require between 90-100 % RH, fungi might be a valuable biological control agent.

This study was designed to determine the infectivity for P. latus of Beauveria bassiana Balsamo (Vuillemin), Paecilomyces fumosoroseus (Wize) Brown & Smith and HirsuteUa thompsonii Smith and to compare the susceptibility of P. latus to the entomopathogens alone, entomopathogens and their combination with adjuvants, and an acaricide.

MATERIALS AND METHODS

SOURCES AND MAINTENANCE OF PATHOGEN FORMS

The inoculum of B. bassiana was a sample of the original isolate obtained from Bephratelloides cubensis Ashmead (Hymenoptera: Eurytomidae) and re-obtained after a first passage in P. latus. H. thompsonii was obtained from the USDA-ARS collection of entomopathogenic fungal cultures from an unidentified tarsonemid and P. fumosoroseus was obtained from an isolate obtained from B. tabaci. B. bassiana was grown on oatmeal agar + dodine media (Chase et al., 1986), and H. thompsoni and P. f-umosoroseus were grown in Milicu-semisynthetic media (Chase et al., 1986). Cultures were grown at 26 ~ + 1 ~ in an incubator (75-85 % RH) and 12:12 LD photoperiod. A 7-d-old culture of each isolate was scraped with a sterile blade to harvest a mixture of conidia and hyphal debris. The mixture was suspended in water containing tween 80 (0.05 %), vortexed for approximately 5 mi- nutes and filtered to remove debris. Conidial concentrations were ascertained with a haemocytometer under a microscope. A series of seven conidial suspensions ranging from 10 ~ to 106 conidia per ml was prepared for each isolate by dilution. An aliquof of each suspension was plated on dodine media, incubated at 28 ~ and the germination rate checked daily until the colonies were formed.

SOURCE AND REARING OF P. LATUS

The mite stock was obtained from a field population of P. latus infesting bean cv. Podsquad in Homestead, Florida. The mites were transferred to potted bean plants and maintained under greenhouse conditions. One week after infestation of the bean plants, infested leaves were collected for use in the bioassays.

BIOASSAY

Twenty replicates of seven conidial concentrations (10 ~ 101, 102, 103, 104, 105, and 106 conidia per ml) plus control (solution only with tween 8(~ were involved in the bioassay of each fungal isolate. Each infested leaf (approx. 4-5 cmZ) was placed in a 12 cm Petri dish with a moist filter paper and each concentration was applied with a micro-sprayer at a 0.5 ml rate per Petri dish. The Petri dishes were kept on larger trays covered with a plastic bag to ensure constant humidity close to saturation (85-95 % RH). Trays were maintained in an incubator 24 _+ 2 ~ with 12 h of daily illumination. The treated mites

BIOCONTROL OF POLYPHAGOTARSONEMUS LATUS 29

were observed at 2, 3, and 6 days after application to record mortality. Infection by the fungi was confirmed by microscopic examination of hypha and conidia visible externally from P. laws. Mortality data for each fungal isolate were probit-transformed and linearly regressed against log-transformed concentrations to determine LC~o.s for all effective concentrations with the PROC_PROBIT procedure (SAS Institute 1987).

EFFECT OF P. LATUS DENSITY ON FUNGAL EPIZOOTICS

Bean leaves (approx. 5 cm 2) with 6 different densities of P. latus, ranging from 15- 125 broad mites per leaf were placed in Petri dishes with moist paper and kept at 24 + 3 ~ 85 % R.H. Each mite density was replicated 20 times. Then, four mite cadavers, previously infected with B. bassiana and P. fumosoroseus were placed in the center of each leaf. Mite mortality was monitored every 24 h. Mites were categorized dead or dead with patent signs of mycosis. The entire experiment was repeated twice for each mite density.

EFFECT OF FUNGAL ISOLATES APPLIED UNDER GREENHOUSE CONDITIONS

Bean plants were grown in a greenhouse in Miter pots, under conditions resembling a field situation (20-29~ 60-85 % RH, and 10:14 [L:D]. Each plant was previously inoculated with 20-25 broad mite males and females. Pre-inoculation mite densities within each plant were followed from 5 days. Randomly chosen leaves were examined in each plant and the number of mites recorded. The plots were inoculated with 2.72 x l0 s conidia per ml of B. bassiana, 2.72 x l0 s conidia per ml of B. bassiana and 1.35 ml Brassica napus L. oil per liter, 2.72 x 10 s per ml B. bassiana and 29.41 rnl molasses per liter, 2.7 x 10 s co- nidia per ml B. bassiana and 0.06-ml of the B. thuringiensis Beta-exotoxin, ABG6364 (Abbott Laboratories, North Chicago, Illinois, U.S.A.) per liter, 0.06 ml ABG6364 per liter and 1.35 ml B. napus oil per liter. Control treatments consisted of distilled water. Each treatment was replicated 20 times. The effect of various treatments on mite survival and activity was evaluated every 24 h for 7 days. Mites were observed under a microscope, and cause of mortality recorded.

RESULTS AND DISCUSSION

TOXICITY BIOASSAY i

All isolates tested were able to infect P. lares in the laboratory. P. lares mortality resulting from infection by B. bassiana was observed beginning on day 2 posttreatment and reached a peak on day 6. The incubation period of mites exposed to the H. thompsonii and P. fumosoroseus isolates ranged from 2-6 days, with a well defined peak of mortality at day 6 post-exposure (fig. 1). Final mortality data for all isolates and treatments were transformed into probability units (probits) and regressed against log 10 (x + 1) transformed dosages of conidial suspension. The analysis for the B. bassiana isolate resulted in a regression slope of 0.30 (_+ 0.064 SE) and an intercept of-1.82 (+ 0.34 SE). The LC~o was 1.85 • 106 conidia per ml, which indicated an intermediate level of virulence of the B. bassiana isolate to P. latus (table 1). The analysis for the H. thompsonii isolate showed a regression slope of 0.74 (+ 0.24 SE) and an intercept of - 2.49 (+ 1.08 SE). The LCso was 2.39 • 103 conidia per ml, which indicated a higher level of virulence compared to B. bassiana. The analysis for the P. fumosoroseus isolate resulted in a regression slope of 2.69 (+ 1.19 SE) and an intercept of - 13.36 (+ 6.02 SE). The LCso was 2.69 x 102 conidia

30 J . E . PElqA, L. S. OSBORNE & R. E. DUNCAN

per ml, which indicated a lower level of virulence compared to H. thompsonii. In general, among the tested isolates, the tarsonemid derived isolate had the lowest LCso (10 3) followed by the homopteran derived isolate (10 5) and by the hymenopteran derived isolate (106) .

100 -

Bb- Dose 10 ~ --O-- Bb- Dose 106 . -A . - p f . Dose 10 ~ - -A-- Pf - Dose 106 ---B-. Ht - Dose 10 ~ - -o - . Ht - Dose 106

80 - ..,~ . . . . . .

. / ~ .

._> 60 -

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a~ ~ 4 0 - ." / O..

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..." ~ . . . - ~ "

oo-~~

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Days Post Treatment

Fig. 1. Cumulative mortality of P./atus inoculated with (A) B. bass/aria (lO~ 106conidia/ml). (]3) H. thompsonii (I0 ~ conidia/ml, 106 conidia/ml). (C) P. fumosoroseus (10 ~ conidia/ml, 106 conidia/ml).

BIOCONTROL OF POLYPHAGOTARSONEMUS LATUS 31

TABLE 1

Virulence of entomopathogenic fungi P. latus in a laboratory bioassay under controlled conditions, based on probit analysis

Isolate Mean 95 % FL LC

Fungus eonidia/ml Lower Upper

Beauveria bassiana LC2o 1.26 x l0 s 0,52 x 103 6.16 • 105 LCso 1,85 x 106 3,18 • 103 6.35 x 106 LCso 2.70 • 107 7.90 • 106 1.48 x l0 s LCgo 1.10 • l0 s 2.88 • 107 1,13 • 109 LCgs 3.50 • l0 s 7,58 x 107 6.69 • 109

Paecilomyces fumosoroseus LC2o 6.16 • 103 0.02 • 102 5.65 • 104 LCso 2.61 x l 0 s 2,15 • 104 9.81 • 106 LCm 1.10 • 107 9,21 • l0 s 4.44 x 1011 LCgo 7.86 • 107 3,58 • 106 2.20 • 1014 LCgs 3.96 • l0 s 1,02 • 107 3.98 • 1016

Hirsutella thomsonii LC2o 0.17 • los 4,55 • 10 -9 2.96 • 103 LCso 2.39 • 103 0,01 • 101 2.37 • 103 LCso 3.30 x 104 1.11 x los 5.46 x 106 LCoo 1.30 • los 1,25 • 104 3 34 • 10 LC.os 4.05 x 105 3.90 • 104 1.56 x 1012

While a mortality response to dose was evident in all isolates, the probit analysis indicated that a significant heterogeneity existed for all trials. It is possible that the physiological condition of the hosts might be influencing the results. While older mites are not generally as vigorous as younger individuals, the older ones in the present trial showed less motility which might influence contact with the inoculum.

SPREAD OF ENTOMOPATHOGENS AT DIFFERENT P. LATUS DENSITIES

Exposures of different densities of P. latus to cadavers infected with B. bassiana for 48 h showed that exposure to conidia is influenced by the population density of P. latus. There was a trend for mites with higher exposure to die earlier. B. bassiana performed better under high P. latus population densities than when the population was low. The data suggest that the application of B. bassiana will not enable an epizootic to develop earliel: in the season when the P. latus population has reached less than 50 mites per leaf. The minimum average incubation period of slightly over 3 days was reached when P. latus population levels reached 125 individuals per leaf. (table 2).

Exposures of different densities of P. latus to cadavers inoculated with P. fumosoroseus showed that spread of the pathogen was less related to population densities during the first day post-treatment. Infection of individuals when P. latus population densities fluctuated between 15 to 55 individuals per leaf remained almost constant during the three days of exposure indicating that population growth remained relatively suppressed after application of conidia of P. fumosoroseus. Infection of population densities between 65-125 individuals per leaf peaked at day 2 post-treatment. However, a slight increase in population growth was observed for 65-125 P. latus per leaf after application of conidia of P.fumosoroseus. This increase may be the result of two interrelated factors. First, fungal metabolism may have been enhanced at higher densities during the 2-3 days following exposure to the infected cadavers. Second, because of the high densities, healthy mites will tend to search

32 I . E . PElq'A, L. S. OSBORNE & R. E. DUNCAN

TABLE 2 Spread of infection of entomopathogenic fungi at different P. latus densities

Fungus

Beauveria bassiana

Paecilomyces fumosoroseus

P.. latu$ Individuals Infected Days Post-treatment

Density 1 2 3

15 39 + 30be 23 :t: 00e 28 + 1 30 32 + 16c 45 + 21bc 60 + 38 55 48 + 24be 59 + 5b 57 + 24e 65 60 + 00abe 80 + 00be 80 + 00be

100 84 + 44ab 101 + 44ab 103 + 14b 125 113 + 13a 152 + 30a 161 + 5a

15 37 + 13a 26 + 13e 35 + 4e 30 31 + 20a 57 + 41be 58 + 26bc 55 64 + 16a 60 + 14be 62 + 22bc 65 81 + 41a 100 + 5lab 108 + 59ab

100 76 + 25a 101 + 32ab 103 + 19ab 125 85 + 14a 137 + 8a 126 + 13a

Means within a column for each fungus isolate followed by the same letter are not significantly different (P > 0.05)

for available feeding or ovipositing sites within the leaf, increasing their movement and the contact with the infected ones. The high humidity in the Petri dish was possibly the major factor that favored infection by both fungal isolates.

E ~ - ~ c r OF FUNGAL ISOLATES UNDER GREENHOUSE CONDITIONS

For P. latus control on whole plants, it was necessary to include a conidial dose that resulted in > 50 % mite mortality. The doses used in the whole-plant experiment were chosen to correspond approximately to the LCso and LCgo values observed in the bioassay experiment (table 1). B. bassiana and P.fumosoroseus were chosen for this study mainly because of their outstanding spore production in vitro, hence, the comparative ,ease with which inocula could be produced for field application. The mean totals of mites recorded alive for all treatments at each assessment date are recorded in table 3. Because other non-fungal products (i.e., Beta-exotoxin of Bacillus thuringiensis) were used in this experiment, dead mites with and without external signs of infection were analyzed as proportion of dead mites for each assessment date.

Beginning at the first day post-treatment all treatments significantly decreased population growth (F= 2.01; d f = 7 , 149; P < 0.05) of P. latus. However, infection was only clearly evident at the second day post-treatment for the B. bassiana inoculum and observed in low numbers for the P.fumosoroseus inoculum. The differences in the proportion of dead specimens increased by the third day after application for both B. bassiana and P. fumoso- roseus (F=2.79; df=7,149; P<0.02) . Inoculation with an aerial spray of B. bassiana conidia was effective in reducing P. latus; total reduction within the plots of the treated plants was 31-88 % higher (P = 0.05) than in the control plots from the period 1 to 9 days posttreatment. Analysis of variance made on arcsin transformations of the percent mycoses

BIOCONTROL OF POLYPHAGOTARSONEMUS LATUS 33

"10

(D

E3

C .o

O Q. O k.=.

n

1 ~ --

0 . 8 --

0 . 6 - -

q

0.4 -i

0.2

- -O- - BbH -O-- - BbH + Oil

BbH + Mol BbH + AGB

.... O- . -AGB

.... V-"- Oil ---~I,"" Control .... �9 .... pf

. 0 "

~176176 �9

,~176

~ ...................................

; "~ j

.o .,

: o~ .,

i ,,.

~ o'~

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1 2 3 4 5 6 7 8 9 10

Days Post Treatment

Fig. 2. Cumulative proportion (p) of dead P. /atus treated with B. bassiana (Bbh), P. fumosoroseus (Pf), adjuvants: oil and molasses and ABG6364 under greehouse conditions; p = p'-c/1-c, c = untreated control, p' = total proportion dead in treated plants.

34 J .E . PEIqA, L. S. OSBORNE & R. E. DUNCAN

TABLE 3

Live E latus (mean number per leaf) after treatment with entomopathogenic fungi, adjuvants, ABG6364, and/or water

Treatment Days Post-treatment

1 2 3 6 7 8 9

B. bassiana 36 + 14b 18+7e 9_2b 12+3b 9+2d 13_3d 8+1d B. bassiana+oil 17+4b 18+4e 18+6b 12+3b 19+7bed 11+2d 21+6d B. bassiana + molasses 17 + 4b 27 + 6c 22 + 7b 28 + 7b 29 5: 5bed 32 + 4ed 46 + 7e B. bassiana+ABG6364 18+4b 15+5e 8 + l b 6+2b 13+3d 25+5ed l l + 3 d ABG6364 24+7b 23+5e 33+11b 31+5b 48+8b 37+6c 54+8bc Oil 47+16ab 52+8b 6 1 + l l a 77+13a 81+13a 67+11b 71+l lab Water(Control) 72+11a 83+16a 86+19a 58+6a 110+24a 89__+12a 88+14a P. fumosoroseus 45 + 14ab 44 + 13b 43 + 7b 59 + 6a 45 + 10be 46 + 7b 46 + 9b

Means followed by the same letter in a column are not significantly different according to a least-significant differences (LSD) test on square root (x + 0.05) transformed data (P > 0.05). Untransformed means are presented.

data showed these differences to be significant at P = 0.05 l e v e l The use o f adjuvants, i.e., molasses or oil d id not p lay any significant role in the induction o f infection for B. bassiana. At the final sampling t ime (day 9 after treatment), a not iceable difference (P < 0.01) was observed between the B. bassiana t reatment and B. bassiana treatments with an adjuvant and the P fumosoroseus t reatment with respect to the proport ion of dead specimens. Adjuvants appeared to decrease the effect of B. bassiana. Treatments with oil provided a faster decline of the populat ion compared to treatment with molasses, maintai- ning a mortal i ty increase of 58 % above the control (fig. 2).

The use o f the B. thuringiensis Beta-exotoxin (ABG 6364) formulat ion rapidly sup- pressed populat ion growth during the first 6 days post treatment and mainta ined the populat ion at low levels 7 through 9 days after treatment. Morta l i ty caused by A B G 6364 remained 50 to 80 % higher than the control. Contrary to what was expected P. fumosoro- seus fai led to contr ibute not iceably to an increase in the death rate o f P. latus.

Although the use o f a mycoacar ic ide against P. latus is not immedia te ly apparent, the fungus that might be selected for its control, should be one that causes epizootics within 2-3 days fol lowing application. The wide divers i ty found in this study, suggests that addit ional work is required to evaluate fungal pathogen efficacy under natural field condit ions.

ACKNOWLEDGMENT

We thank R. Humber for facilitating the H. thompsonii isolate. We thank S. Sundhari and D. Fox for their technical assistance, C. Wellbourn, A. Castifieiras, R. Giblin-Davis and two anonymous reviewers for critical review of the manuscript. This research was supported by the Florida Lime Committee. This manuscript is Florida Experiment Stations Journal Series R-04972.

BIOCONTROL OF POLYPHAGOTARSONEMUS LATUS 35

R~SUM~

Possibilit6s d'utilisation de champignons dans la lutte biologique contre P. latus (Acarina: Tarsonemidae)

Une &ude sur l'infection de Polyphagotarsonemus latus par des conidies de Beauveria bassiana, HirsuteUa thompsoni et P. fumosoroseus a 6t6 conduite en laboratoire dans des conditions de temp6rature et humidit6 contr616es. Une corr61afion positive a 6t6 observ6e entre l'infection de P. latus (LCso) et les traltements 1,16 x 106 conidies/ml de B. bassiana, 2,39 x 103 conidies/rnl de H. thompsonii et 1,29 x 105 conidies/mi de P.fumosoroseus. Le taux de mortalit~ le plus rapide a 6t6 caus~ par B. bassiana an sein des populations de P. latus oscillant entre 65 et 125 individus par feuille. Les pathog~nes B. bassiana et P. fumosoro.seus, des adjuvants 0auile et m61asse) et l'acaxieide ABG6364 ont 6galement 6t~ test6s en serre en vue de d~terminer leur efficacit6 contre P. latus. L'effet le plus marqu6 et le plus persistant sur le taux de mortalit6 de P. laws (88 %) par feuille a &~ enregistr6 chez les plantes trait~es an B. bassiana, suivies de celles trait~es ~t l'acaricide ABG6364. Les adjuvants (huile et m61asse) n'ont pas falt augmenter la population de pathog~nes.

Received : 26 January 1996 ; Accepted : 22 July 1996

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