Stephen P. Wraight

Biological control of Bemisia tabaci with fungi

Recent advances in production, formulation, and application of insect pathogenic fungi have resulted in improvements in long-standing whitefly mycoinsecticide products based on Verticillium lecanii, and development and registration of several new products based on Paecilomyces fumosoroseus and Beauveria bassiana. These products have the capacity to suppress and, in some instances, provide good control of whiteflies in both greenhouse and field crops. However, numerous factors continue to impede the commercial development of fungi as whitefly biological control agents. These include slow action, poor adulticidal activity, potentially negative interactions with commonly used fungicides, relatively high cost, limited shelf life, and dependence on favorable environmental conditions. Development of methods and strategies for overcoming these limitations has progressed, however, and various practices that enhance mycoinsecticide efficacy have been identified. Principal recommendations include: (1) initiating treatments against the early stages of the pest to prevent population buildup, (2) targeting pest populations developing under moderate environmental conditions (e.g., during spring or fall growing seasons), (3) selecting crops amenable to multiple, highly efficient spray applications, and (4) applying fungi asynchronously with incompatible fungicides. Commercial markets for these products have been slow to develop and remain unstable in the face of strong competition from less costly, highly efficacious chemical insecticides. Nevertheless, continuing problems with chemical insecticide resistance and environmental and food contamination support continued development of fungi as relevant tools in the whitefly biological control arsenal.

Liquid culture production of desiccation tolerant blastospores of the bioinsecticidal fungus Paecilomyces fumosoroseus

Liquid media with differing carbon concentrations and carbon-to-nitrogen ratios were tested for production of desiccation tolerant blastospores of Paecilomyces fumosoroseus. While all media tested supported sporulation in submerged culture, high blastospore concentrations (5·8 × 108) spores ml−) were produced in media containing 80 g glucose l− and 13·2 g Casamino acids l− (MS medium) and a significantly higher percentage (79%) of these blastospores survived air drying. Media containing glucose concentrations greater than 20 g l − and Casamino acid concentrations between 13·2 and 40 g l− supported maximal production of desiccation tolerant blastospores. All 23 isolates of P. fumosoroseus grown in MS media produced high concentrations of desiccation tolerant blastospores. When stored at 4 °C, more than 60% of the lyophilized blastospores produced in MS medium were still viable after 7 months storage while less than 25% of the air-dried blastospores survived after 90 d storage. Standard whitefly bioassays were performed to compare air-dried blastospores of P. fumosoroseus ARSEF 4491 with solid substrate-produced conidia of Beauveria bassiana ARSEF 252. Air-dried blastospores of P. fumosoroseus gave LD50s of 60 and 113 blastospores mm− for the silverleaf whitefly (Bemisia argentifolii) in two separate bioassays with potency ratios (LD50 B. bassiana/LD50 P. fumosoroseus)of 3·9 and 3·8, respectively. These results have demonstrated that high concentrations of blastospores of P. fumosoroseus can be rapidly produced in liquid culture, remain viable following drying, and infect and kill silverleaf whitefly.

Debilitation in conidia of the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae and implication with respect to viability determinations and mycopesticide quality assessments

Germination of Beauveria bassiana (Bb) and Metarhizium anisopliae (Ma) conidia determined from a fast-rehydration (FR) protocol were compared to those obtained when dry conidia were subjected to slow rehydration (SR) by holding under high humidity conditions prior to aqueous suspension. Differences in viability estimates obtained using the FR vs. SR protocols increased markedly after conidia were exposed to various stress factors in storage (high a(w), temperature, and O(2) concentrations), with the SR protocol producing higher estimates of viability in all cases. After Bb conidia were stored under moist conditions for 21 days at 25 degrees C, the SR estimate of viability was >21% greater than the FR estimate. In jars flushed with different O(2) concentrations and stored at 50 degrees C for 34 days, proportional differences between protocols varied, depending on water activity, from 18-44% in jars flushed with 0% O(2) (100% N(2)) to as high as 63-93% when treated with 21-22% O(2). For conidia stored over a broad range of moderate to high temperatures in the absence of O(2), SR-FR differences were <or= 9% at 25-40 degrees C but 30% at 50 degrees C. Germination of stressed Bb and Ma conidia increased substantially when incubation time on the germination substrate was increased from 24 to 72 h, whereas germination of non-stressed conidia showed little change. Conidia debilitated by stress were characterized by hypersensitivity to lethal imbibitional damage (damage that is mitigated by slow rehydration) and slow germination. Viability protocols that may provide more reliable assessments of overall mycopesticide quality are discussed.

Efficacy of Beauveria bassiana (Hyphomycetes) for control of Russian wheat aphid (Homoptera: Aphididae) on resistant wheat under field conditions

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) is considered the most important pest of wheat produced under dryland field conditions in South Africa. As part of an integrated pest management strategy, the entomopathogenic hyphomycete Beauveria bassiana (Balsamo) Vuillemin was evaluated in combination with antibiotic host plant resistance under dryland field conditions during 1998 and 1999. A commercial formulation, Mycotrol® ES, was applied at a rate of 2.4 L (5×1013 conidia) per hectare +0.1% organosilicone surfactant. During both years, two applications were administered, i.e., on growth stages 31 (first node detectable) and 39 (early flag leaf). An additional treatment, application at growth stage 39 only, was included during 1999. Over the duration of the 1998 trial, ca. 65% fewer aphids were observed on treated plots compared with controls. A similar level of population reduction was observed during the 1999 trial; however, treatment effects were only briefly evident due to a rapid field-wide decline in aphid populations caused by adverse (cool, wet) weather conditions. The early application (GS 31) resulted in some level of control only during 1998. It was hypothesized that this phenomenon was the result of greater exposure to the spray applications and/or greater secondary pick-up of fungal inoculum by the aphids due to the higher level of aphid activity observed on the cultivar employed during that year. In this regard, migration of D. noxia onto the flag leaves should be further investigated as a behavioural trait for possible exploitation when considering the use of a mycoinsecticide.

Effects of manipulating spray-application parameters on efficacy of the entomopathogenic fungus Beauveria bassiana against western flower thrips, Frankliniella occidentalis, infesting greenhouse impatiens crops

Abstract The effects of various spray application parameters on the efficacy of a clay-based wettable powder formulation of Beauveria bassiana strain GHA conidia against western flower thrips, Frankliniella occidentalis, were evaluated in a series of greenhouse tests. With the aim of optimizing spray application methods to maximize biopesticide efficacy, a series of independent experiments was conducted that varied four spray parameters: application interval, rate, volume, and spray-program timing. Impatiens crops infested with western flower thrips were treated with (1) multiple sprays at the rate of 5×1013 conidia in 935 L aqueous carrier ha−1 applied at 3-, 5-, and 7-day intervals, (2) weekly sprays at rates of 5×1013, 1×1014, and 2.0×1014 conidia in 935 L carrier ha−1, (3) weekly sprays at a rate of 2.0×1014 conidia in volumes of 935, 1870, and 3740 L water ha−1, and (4) multiple sprays at the rate of 2.0×1014 conidia in 3740 L carrier ha−1 applied at 5-day intervals in spray programs initiated before versus after the onset of flowering. Pollen-bearing impatiens flowers were sampled twice weekly to estimate thrips population density, and adult female and second-instar thrips were collected 24 h post application for determination of acquired dose (conidia/insect). Numbers of conidia inoculated onto thrips increased with increasing spray frequency and volume. Dose was unexpectedly not directly correlated with application rate when volume was held constant, suggesting that thrips avoided concentrated spray residues. Statistically significant thrips population reductions relative to controls were achieved only when three to four sprays were applied at the highest label rate in the highest volume at < 7-day intervals. Applications against thrips infesting young, preflowering impatiens crops were not consistently more effective than applications in older crops. The most effective treatment programs reduced pest populations by 30–40% compared to untreated controls; this slowed, but did not stop, the growth of pest populations. Results indicate that use of fungi for thrips management will require integration with other control agents.

Effects of temperature and instar on the efficacy of Bacillus thuringiensis var. israelensis and Bacillus sphaericus strain 1593 against Aedes stimulans larvae

Abstract Laboratory trials of Bacillus thuringiensis var. israelensis (serotype 14) and B. sphaericus strain 1593 against field-collected Aedes stimulans showed that susceptibility declined with increasing instar and decreasing temperature. Test results with B. sphaericus were more erratic than with B. thuringiensis, and the efficacy of the former declined more rapidly with decreasing temperature. B. thuringiensis was significantly more active than B. sphaericus under all treatment conditions. These results indicate that the effective use of this strain of B. sphaericus as a mosquito biological control agent may be limited to warm water situations against more susceptible species.

Application and evaluation of entomopathogens in potato

More than 290 million metric tons of potatoes (Solanum tuberosum) are produced throughout the world annually. According to the FAO 2004–2005 statistics, China, Russia, India, Ukraine, and the United States rank 1–5, respectively, in potato production. Potato production ranks fourth among agricultural crops, after wheat, maize, and rice. In some regions, potato is grown as a dryland crop; in others, it is overheadirrigated with a traveling gun-type irrigation or a central pivot irrigation system. Furrow irrigation is seldom used.

Conidial vigor vs. viability as predictors of virulence of entomopathogenic fungi.

We tested the hypothesis that debilitated conidia exhibiting slow-germination (requiring>16h to germinate) are less virulent than vigorous conidia exhibiting fast germination (requiring⩽16h to germinate). Preparations of Beauveria bassiana s.l. strain CG 1027 with variable ratios of vigorous to debilitated conidia were assayed against third-instar larvae of Spodoptera frugiperda. As the proportion of debilitated conidia in test preparations increased, LC50 expressed in terms of total viable conidia increased, while LC50 expressed solely in terms of vigorous conidia remained constant, indicating that vigorous conidia were responsible for nearly all mortality observed in the assays. Larvae treated with conidia from low-quality batches (with high proportions of debilitated conidia) survived consistently longer than those treated with comparable doses of conidia from high-quality batches. These results confirm our previous hypotheses that inclusion of debilitated conidia in viability assessments can lead to overestimation of the quality (potency) of mycoinsecticide preparations and support our recommendation for use of short incubation periods for assessing viability whenever viability is relied upon as an indicator of product quality.

Biology and Feeding Requirements of Larval Hunter Flies Coenosia attenuata (Diptera: Muscidae) Reared on Larvae of the Fungus Gnat Bradysia impatiens (Diptera: Sciaridae)

ABSTRACT The larval feeding requirements and biology of the generalist predatory muscid hunter fly Coenosia attenuata Stein 1903 (Diptera: Muscidae) were investigated at 25°C. Larval C. attenuata were fed second-, third, and fourth-instar (L2, L3, and L4) larvae of the fungus gnat Bradysia impatiens (Johannsen) (Diptera: Sciaridae) at variable rates to determine minimum and optimum numbers of these prey required for normal development. The proportion of C. attenuata larvae surviving to pupation differed significantly as a function of L2 and L3 prey numbers. When the number of prey/ d was increased from 10 to 15 L2 and from 5 to 7 L3 per day, the respective percentages of pupation increased from 0 to 77% and from 0 to 48%. In contrast, all numbers of L4 prey (1–7 prey per d) supported pupation, and the pupation rate did not vary with prey number. At the highest prey numbers tested, mortalities of C. attenuata larvae fed L2, L3, and L4 fungus gnat larvae were 7, 30, and 75%, respectively. The higher mortality of larvae fed L4 prey was clearly the result of lethal wounds inflicted by the fungus gnat larvae in defensive strikes against the predators. At prey numbers supporting maximum rates of adult emergence, larval development required 12–14 d, and duration of the pupal stage was ≈10 d. C. attenuata larvae killed large numbers of prey during their development (means of up to 232 L2,144 L3, or 87 L4 fungus gnats), and larvae provided with marginally inadequate numbers of prey survived for long periods (mean 14–22 d, maximum 34 d) before succumbing to apparent starvation. These are favorable attributes with respect to use of C. attenuata as a biological control agent, suggesting a strong potential to substantially impact high-density pest populations and to survive in low-density pest populations.

Developmental Times and Life Tables for Shore Flies, Scatella tenuicosta (Diptera: Ephydridae), at Three Temperatures

Abstract Development times and survivorship of immature shore flies and longevity and reproduction of adult shore flies, Scatella tenuicosta Collin, reared on algae-infested filter paper, were studied at three temperatures (constant 20, 26, and 28.5°C) through life table analysis. The development time for each individual life stage and the total time from egg to adult decreased with increasing temperature. Duration of the third (ultimate) larval instar ranged from 3.3 ± 0.09 d at 20°C to 1.4 ± 0.04 d at 28.5°C and was 1.7–1.9 times longer than the approximately equal first and second instars. Development of male and female shore flies from egg to adult needed an average of 14.5 ± 0.13, 8.2 ± 0.05, and 7.0 ± 0.04 d at 20, 26, and 28.5°C, respectively, and needed an estimated 154.4 ± 1.2 thermal units (degree days). At these respective temperatures, adult females lived 21.8 ± 2.2, 19.9 ± 2.4, and 15.0 ± 1.4 d and produced 379 ± 62, 710 ± 119, and 477 ± 83 eggs during oviposition periods of 14.3 ± 2.1, 15.0 ± 2.2, and 10.8 ± 1.4 d; daily lifetime egg production averaged 16.3 ± 2.3, 33.5 ± 3.8, and 29.7 ± 3.5. Developmental stage-specific mortality was relatively low for all life stages at all temperatures, with maximum percent mortalities of 5.7% occurring in both the egg stage and in the third instar. The highest net reproductive rate (Ro) was obtained for insects reared at 26°C and was 329.6. The intrinsic rate of natural increase (rm) was highest at 28.5°C and was 0.430. Generation time and doubling time of the population were shortest at 28.5°C and were 12.4 and 1.6 d, respectively. Results suggested that 26°C was near optimum for reproduction.