Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix AZ 85040-8803.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: June-September 1999

Effect on Beneficial Arthropods of Biorationals (Insect Growth Regulators and Entomopathogenic Fungi) Used for Control of Silverleaf Whitefly, Bemisia argentifolii, in Upland Cotton in Arizona

Deltapine NuCOTN 33^B was planted and furrow irrigated in plots 109 ft. in length and 12 rows across (40-in. rows). Plots were separated by 2-fallow rows and 20 ft. alleys. Spray applications were made by a ground boom with 5 nozzles/row, 1 overhead, and 2 swivel nozzles angled upward on a drop on each side of the row, at 250 psi and 30 gal/ac. Eight sprays were applied weekly beginning July 22 and ending September 9. The 1999 cotton season was a good year in respect to abiotic factors favorable to growing cotton. Silverleaf whitefly populations were present in cotton from mid to late season.

Biorational entomopathogenic fungi used included: Beauveria bassiana, as Naturalis ® L, Troy Biosciences Inc. at 10 oz. product/ac, 2.3x 10⁷ conidia/ml, as Mycotrol ® ES, Mycotech Corp., 0.5 pt/ac, 2 x 10 ¹³ spores/qt, and Paecilomyces fumosoroseus PFR- 97 ™ Thermo Trilogy Corp., 0.025 lbs. / gal., 1x 10 ⁹ CFU (spores)/ gm equivalent 20% product. All three of these products were used at full rate for multiple applications. Biorational insect growth regulators used were at full rate as single or multiple applications and included: azadirachtin as Bollwhip ™ (Thermo Trilogy Corp.), 4.5% formulation 6 oz product /ac.(note, other action modes also); buprofezin as Applaud ™ 70 WP (AgrEvo), 0.35 lb. AI/ac; and pyriproxyfen as Knack ™ 0.86 EC (Valent USA) 0.054 lb. AI/ac. These treatments were part of a 10-treatment random block design that included a "Best Agricultural Practice" (BAP) treatment, and an embedded-control treatment, plus a single 1-ac block control. Treatment efficacy was measured as mean percent reduction from the block control. Weekly sweeps (25/plot) were taken in all plots for predators, parasites, the thrip, Frankliniella occidentali, and Lygus (primarily hesperus, Lygus reported elsewhere in this review). Treatment efficacy was measured as mean percent reduction (%) from the block control; statistically significant = s, and statistically insignificant = ns, by ANOVA with P< 0.05 or stated otherwise.

Efficacies of biorational-entomopathogenic fungi were as follows for Naturalis ® L, Mycotrol ® ES, and PFR- 97 ™ , respectively, on populations of: Frankliniella occidentalis, Drapetis, and Orius, 0% for all; Chrysoperla larvae, 0, 23, and 0%, Collops, 39, 39, and 22%; Pseudatomoscelis, 8, 16, and 14%; Spanagonicus, 80, 40, and 0%; (all ns,). Naturalis ® L, Mycotrol ® ES had efficacies on populations of Geocoris of 16 and 17% (ns); in contrast, PFR- 97 ™ had an efficacy of 26% (s, P< 0.001). Mycotrol ® ES had an efficacy on populations of Hippodamia of 60% (ns); in contrast, Naturalis ® L and PFR- 97 ™ had efficacies of 80 and 100% (s). On misc. spiders, Naturalis ® L, Mycotrol ® ES, and PFR- 97 ™ had efficacies of 28, 33, and 22 % (s). Efficacies of biorational insect growth regulators were as follows, respectively: 1) Bollwhip ™ -- 0 % on Drapetis, Frankliniella occidentalis, Orius, and Spanagonicus populations (ns), 39 and 19% on Collops and Pseudatomoscelis populations (ns), 23, 22, and 99% on Chrysoperla larvae, misc. spiders, and Hippodamia populations (s), and 22% on Geocoris populations (s, P<0.001); 2) Applaud ™ -- 0% on Chrysoperla larvae, Drapetis, Frankliniella occidentalis, Geocoris, Hippodamia, Orius, and Spanagonicus populations (ns), 63, 22 and 2% on Collops, misc. spiders and Pseudatomoscelis populations(ns); and 3) Knack ™ -- 0% on Chrysoperla larvae, Collops, Frankliniella occidentalis, misc. spiders and Pseudatomoscelis populations(ns), 23, 12, and 26% on Drapetis, Geocoris, Orius, populations (ns), 100 and 80% on Hippodamia and Spanagonicus populations(s, P<0.01 and 0.02).

Investigator’s Name(s): D. H. Akey and T. J. Henneberry.

Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040-8803.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: June-September 1999

Use of the Entomopathogenic Fungi, Beauveria bassiana, and Paecilomyces fumosoroseus as Biorational Agents Against the Silverleaf Whitefly, Bemisia argentifolii, in Field Trials in Upland Cotton

Deltapine NuCOTN 33^B was planted and furrow irrigated in plots 109 ft. in length and 12 rows across (40-in. rows). Plots were separated by 2 fallow rows and 20 ft. alleys. Spray applications were made by a ground boom with 5 nozzles/row, 1 overhead, and 2 swivel nozzles angled upward on a drop on each side of the row, at 250 psi and 30 gal/ac. Eight sprays were applied weekly beginning July 22 and ending September 9. The 1999 cotton season was a good year in respect to abiotic factors favorable to growing cotton. Silverleaf whitefly populations were present in cotton from mid to late season.

Biorational entomopathogenic fungi used included: Beauveria bassiana, as Naturalis ® L, Troy Biosciences Inc. at 10 oz. product/ac, 2.3x 10⁷ conidia/ml, as Mycotrol ® ES, Mycotech Corp., 0.5 pt/ac, 2 x 10 ¹³ spores/qt, and Paecilomyces fumosoroseus PFR- 97 ™ Thermo Trilogy Corp., 0.025 lb/gal, 1x 10 ⁹ CFU (spores)/gm equivalent 20% product. All three of these products were used at full rate for multiple applications. These treatments were part of a 10-treatment random block design that included a "Best Agricultural Practice" (BAP) regime and an embedded-control treatment, plus a single 1-ac block control.

Whitefly eggs, small nymphs, and large nymphs were sampled from one leaf taken from each of 10 plants per plot, from the 5^th main-stem node down from the 1^st expanded terminal leaf. Each sample was counted from a 2.22 cm diameter disk taken from the leaf between the main (central) and the adjacent lateral vein. All whitefly adults were counted on the 5^th main-stem leaf abaxial surface sampled from 30 leaves/plot, using the leaf turn method; the first 10 were from the same plants used for immature samples.

Efficacies, of Beauveria bassiana as Mycotrol ® ES and Naturalis ® L respectively, were as follows: 26 and 41% against eggs; 23 and 46% against small nymphs; and 56 and 74% against large nymphs. Efficacies of P. fumosoroseus as PFR -97 ™ were as follows: 50% against eggs ; 49% against small nymphs; and 78% against large nymphs. Compared to the block control, the efficacies of all 3 entomopathogenic fungi were significant at P<0.01 by ANOVA and (P<0.05) by LSD.

Investigator’s Name(s): Earl Andress¹, Juli Gould², & Mark A. Quinn³.

Affiliation & Location: USDA--APHIS, Phoenix Plant Protection Center, Brawley CA¹, Phoenix AZ ², Washington State University, Pullman WA³.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: September 1999 - January 2000

Assessing the Impact of Established Whitefly Parasitoids in the Imperial Valley Using Multivariate Techniques

Releases of introduced natural enemies of silverleaf whitely have been conducted in the Imperial Valley for several years. Trapping surveys indicate that whitefly populations in the Imperial Valley have been declining for the last four years. Although numerous parasitoids of the introduced species have been recovered, it is not known how much of this overall reduction in whitefly population is due to their effect. In order to assign the portion of this decline that is caused by introduced natural enemies, we have begun a program using a multivariate approach.

Whiteflies and their parasitoids were sampled from broccoli fields during the fall and winter of 1999-2000. Whitefly density and percent parasitism were compared among fields sampled. Crops and land use surrounding the sampled fields were factored into the analysis, and the effects of surrounding crops, land use, indigenous parasitoids, and exotic parasitoids separated. Preliminary results will be presented.

Investigator’s Name(s): James S. Buckner¹, Tadeusz J. Poprawski², Walker A. Jones², & Dennis R. Nelson¹.

Affiliation & Location: ¹USDA--ARS, Biosciences Research Laboratory, Fargo, ND; ²USDA--ARS, Subtropical Agricultural Research Center, Weslaco, TX.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1999

The Effects of Eretmocerus mundus Parasitism on the Cuticular Lipids of Bemisia argentifolii Nymphs

The cuticular lipid composition of Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) nymphs parasitized by Eretmocerus mundus Mercet was determined by capillary gas chromatography (CGC) and CGC-mass spectrometry (CGC-MS) and the results compared with the cuticular lipids of unparasitized nymphs and those nymphs parasitized by Encarsia pergandiella Howard. Previous studies with B. argentifolii nymphs had shown that wax esters were the major components of the cuticular lipids with lesser amounts of hydrocarbons, long-chain aldehydes and long-chain alcohols.

As compared to unparasitized controls, no appreciable changes in lipid composition were observed for the cuticular lipids of E. pergandiella-parasitized nymphs. However, the cuticular lipids from nymphs parasitized by E. mundus contained measurable quantities of two additional components in their hydrocarbon fraction. CGC-MS analyses and comparisons with an authentic standard indicated that the two hydrocarbons were the monomethyl-branched alkanes, 2-methyltriacontane (31 carbons) and 2-methyldotriacontane (33 carbons). The occurrences, mechanisms for biosynthesis and possible functions of 2-methylalkanes as cuticular lipid components of insects have been reviewed. Current studies are focusing on determining the site of synthesis of these methyl-branched alkanes and their possible function as chemical cues for host recognition, acceptance and discrimination by E. mundus and other whitefly parasitoids.

Investigator’s Name(s): D. R. Ellis¹, R. J. McAvoy¹, L. Abu Ayyash¹, M. Flanagan¹, & M. A. Ciomperlik².

Affiliation & Location: ¹Department of Plant Science, University of Connecticut, Storrs, CT and ²USDA--APHIS--PPQ, Mission Plant Protection Center, Mission, TX.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: August - December 1997

Evaluation of Serangium Parcesetosum (Coleoptera: Coccinellidae) for Biological Control of Silverleaf Whitefly, Bemisia Argentifolii (Homoptera: Aleyrodidae), on Poinsettia.

Silverleaf whitefly, Bemisia argentifolii Bellows & Perring, is the major insect pest on greenhouse poinsettia but biological agents capable of suppressing silverleaf whitefly (SLWF) to acceptable levels have been elusive. Serangium parcesetosum Sicard is a coccinellid predator that appears to have great potential for SLWF control. In this study, the population dynamics of B. argentifolii on caged poinsettias (Euphorbia pulcherrima Wild. var. ‘Freedom Red’) in response to Serangium were characterized. SLWF were introduced to caged poinsettias at rates of 1 or 10 adults per plant, and 6 weeks later, Serangium were introduced at rates of 0, 2 or 4 adults per plant. SLWF and Serangium populations were monitored weekly during the study.

SLWF mortality increased dramatically within 2 weeks of a single Serangium release, and SLWF densities remained at or near the levels observed at time of predator introduction for the ensuing 10-week study period. Serangium larvae were observed 2 weeks after adults were released in cages with high initial SLWF levels but not in cages with low initial SLWF. With high initial SLWF, Serangium larvae were recovered as late as 10 weeks after predator introduction. It appears that SLWF control was primarily due to the prolonged survival and continuous feeding of individual beetles. Our data suggest that Serangium may be a good candidate for inclusion in an interspecific biological control approach to SLWF management on greenhouse poinsettia.

Investigator’s Name(s): Juli Gould & Paul Merten.

Affiliation & Location: USDA--APHIS, Phoenix Plant Protection Center, 4125 E. Broadway Rd., Phoenix, AZ 85040.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January 1 - November 30, 1999

Comparison of Four Methods of Releasing Whitefly Parasitoids: Emergence, Survival, Dispersal, and Mating Success

Releasing whitefly parasitoids in field corps is somewhat more complex than releasing them in greenhouse situations. In the field, parasitoids are subjected to higher temperatures, greater temperature fluctuations, wind, rain, low humidity and other factors that are more controlled and constant in greenhouse settings. Once released, successful parasitoids must emerge from the pupal case (if released as pupae), locate each other for mating, and disperse sufficiently throughout the field so that they can find their host. Several methods have been proposed and used to release parasitoids in field crops, with varying success. The goal of our research was to quantify the differences in emergence, survival, dispersal and mating success among four release methods. We also quantified the cost of each release method to determine efficiency.

Parasitoids were released two times in melons (early June and late June) and two times in cotton (early August and late August). Method 1 was to release adult parasitoids, Method 2 was to release parasitoid pupae in gel-caps stuck to the bottom of leaves, Method 3 was to release parasitoid pupae in a paper cup on a wooden skewer and Method 4 simulated releasing parasitoid pupae mixed with vermiculite from a commercial drop-box.

We released parasitoids at rates of 40,000 per acre in ¼ acre plots. We therefore released 10,000 parasitoids per plot. The parasitoids were released in patterns that simulated that which was typical for that method. Adults were released at the center of each of the four quadrants (2,500 per quadrant), gel-caps were placed at the center of each quadrant, the paper cups were placed at the center of the plot (10,000 parasitoids per cup), and the vermiculite was dropped from a 2.5 ml spoon every 1.5 m on every other row throughout the entire plot. Samples were taken from each of 25 points in a 5 by 5 grid throughout each plot. Three days after parasitoid release we vacuumed the foliage around each point for two minutes. We counted the number of parasitoids captured at each point. Each female was dissected and we mounted and cleared the spermatheca to determine whether or not it contained sperm (evidence of mating). Parasitoid emergence was estimated two times per crop in a separate experiment with 30 replicates for each crop. We tested vermiculite that was dropped in the sun and the shade of the plant canopy separately.

Emergence: For three out of the four trials, percentage emergence was higher for the paper cup and gel-cap methods than for the vermiculite. In all four trials, percentage emergence in the vermiculite was greater in the shade than in the sun. Percentage survival of adults was highly variable and more likely reflected mortality due to aspiration than to mortality in the field since the adults could disperse immediately after release.

Survival: Significantly more parasitoids were recovered in the paper-cup plots than in the adult or drop-box plots. There were no significant differences among the gel-cap, adult, or drop-box methods.

Dispersion: In melons, the dispersion of the parasitoids throughout the plot was best for the gel-cap and adult release methods, but in cotton the parasitoids were dispersed throughout the plot well for all four methods.

Mating Success: All but two of the parasitoids we recovered were mated, and there were no significant differences in mating success by release method.

Cost: The cost of treating a 40 acre field was estimated to be $210 for adult release, $123 for the gel-cap method, $53 for the paper cup method, and $636 for the drop-box method.

We would recommend the paper-cup method be used, especially in cotton where we saw a relatively even dispersion after release. We recovered significantly more parasitoids when paper cups were used for release, all recovered females were mated, and the cost was much lower for this method.

Investigator’s Name(s): S. M. Greenberg¹, B. C. Legaspi, Jr.¹, & Walker A. Jones².

Affiliation & Location: ¹Joint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA, and Texas Agricultural Experiment Station, Weslaco, TX; ²Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1999

Comparison of Functional Response and Mutual Interference Between Two Aphelinid Parasitoids of Bemisia argentifolii (Homoptera: Aleyrodidae)

We compared functional responses and mutual interference in an indigenous parasitoid, Encarsia pergandiella Howard, with that of an exotic parasitoid, Eretmocerus mundus Mercet from Spain, attacking the silverleaf whitefly, Bemisia argentifolii Bellows and Perring. Over the experimental host densities tested, E. mundus was characterized as a Type 1 response, in contrast to the asymptotic Type II in E. pergandiella. The instantaneous attack rates (a’) between E. pergandiella and E. mundus were not significantly different (0.27 vs 0.18 per day, respectively). However, the handling time of E. pergandiella (0.05 d or 72 min) was significantly higher than that of E. mundus (0.0083 d or 12 min). The higher attack rate of E. mundus is largely attributable to its shorter handling times. The mutual interference coefficient m of E. mundus was numerically, but not statistically higher than that for E. pergandiella (0.238 versus 0.184, respectively). Although there were no significant differences in m, the comparison raises the interesting question of whether parasitoids with higher attack rates may also have higher levels of mutual interference under conditions of high parasitoid density (e.g. mass rearing).

Investigator's Name(s): S. M. Greenberg^l, B. C. Legaspi, Jr.^l, & Walker A. Jones².

Affiliation & Location: ¹Joint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA, and Texas Agricultural Experiment Station, Weslaco, TX; ²Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1999

Temperature Effects on Host Mortality and Parasitoid Survival

The effects of temperature on insect life history was studied for two whitefly hosts, the silverleaf whitefly, Bemisia argentifolii Bellows & Perring, and the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), as well as the parasitoid, Eretmocerus eremicus Rose & Zolnerowich (Hymenoptera: Aphelinidae) attacking both hosts. Mean egg number as a function of time was fitted to models for age-specific oviposition for each whitefly. In B. argentifolii, numbers of eggs increased with time at 15, 21, and 24 ° C. At 28 and 32 ° C, the curve declined after 6d, although the model fit was poor. The model did not fit the oviposition data at 32 ° C. The maximal oviposition rate occurred at 24 ° C (12 eggs/48-h period), and the model was almost linear. In T. vaporariorum, the model closely fitted mean eggs laid, with the highest rate of 12 eggs/48-h at 21 and 24 ° C. Numbers of whitefly eggs as a function of time and temperature were described by a 3-dimensional surface model which was also used to estimate temperature thresholds for oviposition (12.5 ° C in B. argentifolii and 10.9 ° C in T. vaporariorum). Increasing temperatures produced decreased preoviposition periods in B. argentifolii, whereas temperature extremes resulted in longer preovipositional periods in T. vaporariorum. Development times from egg to adult, percentage mortality, and estimated degree days for development were measured at 15, 21, 24, 28, and 32 ° C for both whiteflies, and for E. eremicus reared on both hosts. Development rate was higher for B. argentifolii than T. vaporariorum at 24 and 28 ° C. Development of E. eremicus was higher when B. argentifolii was used as the host than T. vaporariorum at 24, 28, and 32 ° C. By extrapolation of development rates, lower developmental thresholds ( ° C) were estimated as follows: T. vaporariorum, 2.92; B. argentifolii, 10.32, E. eremicus on T. vaporariorum, 5.44; and E. eremicus on B. argentifolii, 8.7. Mean degree-day requirements for egg to adult development were calculated at T. vaporariorum (483.4); B. argentifolii (319.7); E. eremicus on T. vaporariorum (417.3); and E. eremicus on B. argentifolii (314.4). Percentage mortality also was significantly affected by temperature in both species of whitefly. In T. vaporariorum, higher temperatures caused higher levels of mortality, with almost 98% killed at 32 ° C. The reverse occurred in B. argentifolii, where highest levels of mortality were found at the lowest temperatures. Mortality patterns in E. eremicus reflected those of the host: increasing with temperature on T. vaporariorum, decreasing on B. argentifolii. The life history of E. eremicus was profoundly affected by that of its host.

Investigator’s Name(s): S. M. Greenberg¹, Walker A. Jones¹, and T. X. Liu²

Affiliation and Location: ¹Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA 2413 E. Highway 83, Weslaco, TX 78596; ² Texas Agricultural Experiment Station, Texas A&M University, 2415 E. Highway 83, Weslaco, TX 78596-8399

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control

Dates Covered by the Report: 1999

Tritrophic Interactions Between Two Species of Whiteflies and Two Species of Eretmocerus (Hymenoptera: Aphelinidae) on Tomato

Laboratory experiments were conducted to determine the tritrophic interactions of two whitefly species, Bemisia argentifolii Bellows & Perring and Trialeurodes vaporariorum (Westwood) and two species of parasitoids, Eretmocerus eremicus Rose & Zolnerowich (a native species), and E. mundus Mercet (an exotic species) on tomato (‘Trust’ and ‘Floridade’). Tomato varieties did not have any significant impacts on whiteflies and their parasitoids. Natural mortality, developmental time, oviposition and progeny between B. argentifolii T. vaporariorum were not significant different. The two species of Eretmocerus responded differently to the host whitefly species. E. mundus developed faster, parasitized more nymphs, had more progeny, had greater parasitism and emergence rate on B. argentifolii than on T. vaporariorum; whereas E. eremicus performed similarly on either host species with one exception that its females parasitized more B. argentifolii nymphs than T. vaporariorum nymphs. The females of both parasitoid species emerged from T. vaporariorum were significantly larger than that from B. argentifolii. The data of this study increased our knowledge of tritrophic interactions of whiteflies-parasitoids-host plants, and the information can be useful for development rearing strategies of whitefly parasitoids, and biological control of the two species of whiteflies using Eretmocerus.

Investigator’s Name(s): Jing S. Hu and Dale B. Gelman

Affiliation & Location: USDA, ARS, Insect Biocontrol Laboratory, Beltsville, MD

Research & Implementation Area: Section D, Natural Enemy Ecology and Biological Control

Dates Covered by the Report: 1999

Development of Encarsia formosa in the Silverleaf Whitefly, Bemisia argentifolii: Effect of Host Age

The effect of host age (at the time of parasitization) on the growth and development of Encarsia formosa was studied. E. formosa was able to parasitize and complete its life cycle in all four instars of the silverleaf whitefly. Parasitoid development was significantly slower when 1st instar hosts were parasitized than when 2nd, 3rd or 4th instars were parasitized. The whitefly instar parasitized had no significant effect on the day in which E. formosa larval hatch was first observed, i.e., on the 3rd day post-oviposition. However, mean embryonic development was significantly longer (5 days) when 1st instar hosts were parasitized than when 3rd or 4th instar hosts were parasitized (3.4 and 3.5 days, respectively). The duration of the 1st instar parasitoid was also significantly greater when 1st instar hosts were parasitized (approximately 3 days) than when older host instars were parasitized (approximately 2 days). However, durations of the parasitoid 2nd and 3rd instars and pupa were slightly, but not significantly, longer in hosts parasitized as 1st instars than in hosts parasitized as 2nd, 3rd or 4th instars. Interestingly, no matter which instar was parasitized, the parasitoid did not molt to the 3rd instar until the host had reached stage 4-5 (depth 0.25 mm) of its last instar. It appears, then, that the parasitoid’s molt to its last instar depends upon some aspect of host physiology associated with the 4th-5th stage of the whitefly last instar. However, attainment of the 4th-5th stage did not necessarily trigger the parasitoid’s final larval molt. Although host instar parasitized had a significant effect on various aspects of parasitoid development, it did not significantly influence the mean size of the parasitoid larva, pupa, or adult. Nor did it significantly affect percent emergence rate or adult longevity. Larval length and adult head width were similar for all parasitoids, regardless of which host instar was parasitized, and percent parasitoid emergence and longevity of adults, while slightly greater for parasitoids developing in hosts parasitized as 3rd or 4th instars than those parasitized as 1st or 2nd instars, was not significantly different. Finally, adult parasitoid emergence was more synchronous when 2nd, 3rd and 4th instar whiteflies were parasitized than when 1st instars were parasitized.

Investigator’s Name(s): Rosalind R. James.

Affiliation & Location: USDA, ARS, Kika de la Garza Subtropical Agric. Res. Center, Weslaco, TX.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: Jan-Dec, 1999

Effect of Certain Nutrients on Germination of Whitefly Pathogen Spores

The silverleaf whitefly is susceptible to fatal infection by entomopat../../../../cotton.htmlungi such as Beauveria bassiana and Paecilomyces fumosoroseus. B. bassiana rarely occurs naturally in Bemisia populations, but it has been used successfully as a mycoinsecticide. Naturally occurring P. fumosoroseus has been seen to cause epizootics in localized Bemisia populations. Although the general biology of these fungi is well studied, we are always looking for ways to improve control, and a better understanding of the infection process might help accomplish this goal. The main infective propagule for these fungi are spores – usually conidia. Spores germinate on the insect cuticle and then penetrate into the hemoceol where they cause infection. We tested whether sugars (as might occur in association with Bemisia) increase spore germination rates. Spores do not germinate in water, and we found that the addition of simple sugars (sucrose, melezitose, and trehalose) had little effect on germination. These sugars stimulated <20% germination of B. bassiana, and <1% germination of P. fumosoroseus, even after 18 hr. On the other hand, an exogenous source of protein (peptone or yeast extract) stimulated 95-100% germination for both fungi. Although it is clear that an exogenous source of carbon is not important to spore germination, sugars might still play a role in fungal growth and infectivity once the spores have germinated.

To see if lack of germination was limiting B. bassiana infections in whitefly nymphs, we applied germinated spores to early 3rd instars. Germinating B. bassiana conidiospores before applying them to whitefly nymphs increased mortality levels as by as much as 245%. At a concentration where fresh spores caused only 12% mortality (37 spores/mm²), germinated spores caused 41%. Where fresh spores killed 45% (144 spores/mm²), germinated spores caused 65% mortality. Spores were germinated using yeast extract, but adding yeast extract without pre-germinating the spores had no effect on whitefly mortality levels. Not only were levels of mortality increased, but the rate of mortality was increased, and this effect was statistically significant. When fresh spores were used, the mean time to death of infected insects was 5.45 (SE=0.16) d for an application rate of 37 spores/mm², and 4.74 (SE=0.08) d when applied at 144 spore/mm². When the spores were germinated before application, the mean time to death dropped to 4.58 (SE=0.16) and 4.45 (SE=0.10) d for each rate, respectively. Thus, the greatest effect of pre-germinating spores was seen at the lower dose. Perhaps the reason such high doses of spores are needed in field applications of B. bassiana is because only a small percentage are germinating on whitefly cuticle.

Investigator’s Names: T.-X. Liu¹, P.A. Stansly², A. N. Sparks, Jr.¹, T. C. Knowles³, and C. C. Chu⁴.

Affiliation and Location: ¹Texas Agricultural Research & Extension Center, Texas A&M University System, 2415 E. Highway 83, Weslaco, TX 78596-8399; ²Southwest Florida Research & Education Center, University of Florida, P. O. Box 5127, Immokalee, FL 34143; ³University of Arizona, La Paz County Cooperative Extension, P. O. Box BL, Parker, AZ 85344; ⁴Western Cotton Research Laboratory, USDA-ARS, 4135 E. Broadway, Phoenix, AZ 85040.

Research & Implementation Area: Section D. Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1999

Managing Bemisia argentifolii Using Mycotrol and Naturalis-L (Beauveria bassiana) on Vegetables, Cotton and Ornamentals in Southern United States

Two different strains of Beauveria bassiana (Balsamo) Vuillemin, formulated as Mycotrol® and Naturalis-L®, were tested against Bemisia argentifolii Bellows & Perring in Arizona, California, Florida, and Texas. Laboratory bioassays, greenhouse and field experiments were conducted on hibiscus, sweetpotato, cantaloupe, cucumber, tomato, eggplants and cotton. Mycotrol caused 76-94% mortality of B. argentifolii nymphs under laboratory and greenhouse conditions, where relative humidity was maintained above 90%. In Arizona, on subsurface drip irrigated cantaloupe that was under moderate to light whitefly pressure, Mycotrol provided significant control of whitefly with 68_79% population reduction, whereas Naturalis-L did not. After 3 applications at the labeled rate (1.12 kg/ha), Mycotrol had the cumulative effect of maintaining adult whitefly populations below the economic threshold of 3 per leaf for 28 d after the initial treatment. In the Imperial Valley, California, on cotton and in southwest Florida on field tomato and eggplant, multiple applications of Mycotrol at weekly intervals did not reduce whitefly population compared with untreated control. Naturalis-L was effective against B. argentifolii under laboratory conditions, and provided fairly good control on cotton in southern Texas, but did not give sufficient control on cucumber in south Texas or on cantaloupe in Arizona. It appears that applications of B. bassiana products, i.e. Mycotrol and Naturalis-L, could play a role in whitefly management, although their usefulness may be limited to high humidity conditions and complete spray coverage.

Investigator’s Name(s): G. S. McCutcheon¹ & A. M. Simmons².

Affiliations & Location: ¹Clemson University Coastal Research and Education Center, Charleston, SC; ² USDA-ARS, U. S. Vegetable Laboratory, Charleston, SC.

Research & Implementation Area: Section D: Natural Enemy Ecology, and Biological Control.

Dates Covered by the Report: 1999

Influence of Temperature on Parasitism by an Indigenous Eretmocerus Species

The influence of temperature on rate of parasitism by an indigenous parasitoid, Eretmocerus sp., on the whitefly Bemisia argentifolii Bellows & Perring was examined in the laboratory. The taxonomic status of the parasitoid is under study by M. Rose. The study was conducted using a range of constant temperatures (20, 25, 30, 35, 40, and 45 ° C) while maintaining relative humidity at about 80%. The insects were reared on collard plants. Treatments were replicated at each temperature from 4 to 8 times in each of eight trials. The parasitoids were confined on whitefly-infested leaves for 12 hours. Host density was 30 to 40 whitefly nymphs per test arena. There was an increased rate of parasitism by Eretmocerus sp. at temperatures above 20 ° and below 40 ° C. The relationship between temperature and percentage parasitism was linear when the 40 ° C treatment was not included in data analysis. Rate of parasitism was highest between 25 and 35 ° C. At 45 ° C, the adult parasitoids did not survive during the 12 h oviposition exposure period and no parasitism was observed at this temperature. Like its host, B. argentifolii, the parasitoid survives the mild winters of coastal South Carolina. Populations of both increase with the warmer temperatures of the spring and summer. These data indicate that high temperatures of the summer are particularly conducive to parasitism by this parasitoid.

Investigator’s Name(s): Steven E. Naranjo.

Affiliation & Location: USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January 1999 - December 1999

Intraguild Predation on Whitefly Parasitoids

Studies were continued to quantify comparative predation by generalist predators on parasitized and unparasitized whitefly (Bemisia tabaci) hosts in cotton. Adult females of Geocoris punctipes, Orius insidiosus and Hippodamia convergens were provided equal numbers of parasitized (Eretmocerus emiratus) and unparasitized early 4^th instar whiteflies in petri dish arenas and allowed to forage for 24 h. Studies were conducted with early instar parasitoids (displacement of host mycetomes) and with pupal-stage parasitoids. All three predator species displayed a significant preference for parasitized hosts. Preference was consistently strongest for pupal-stage parasitoids in all three predators. Comparing predators, H. convergens exhibited the strongest bias for parasitized hosts; responses by G. punctipes and O. insidiosus were similar to one another. Early 4^th instar whitefly are very flat and almost translucent on the leaf surface. In contrast, once the immature parasitoid is large enough to displace the host's mycetomes the host begins to swell and become opaque. Thus, parasitized hosts may be more apparent to predators foraging on the leaf surface. This hypothesis was tested by presenting parasitized host along with late-stage 4^th instar whitefly (this stage also swells and is opaque). Both G. punctipes and O. insidiosus showed no significant preference in these arenas supporting the hypothesis that preference for parasitoids may be based on visual cues. In contrast, H. convergens still showed a significant (albeit lesser) preference for parasitized hosts suggesting that other factors are involved in prey choice by this beetle. These data will aid ongoing efforts to model biological control of B. tabaci in cotton, and help to more accurately estimate marginal mortality rates due to parasitism in ongoing life table analyses.