Characterization of alfalfa germplasm expressing resistance to silverleaf whitefly, Bemisia argentifolii

Abstract: Thirty‐eight plants were taken from a University of California alfalfa selection nursery for developing resistance to silverleaf whitefly, Bemisia argentifolii Bellows & Perring. Seventeen of the plants had low whitefly infestation and were categorized as ‘potentially resistant’; 21 of the plants had high whitefly infestation and were categorized as ‘presumed susceptible’. Plants were propagated vegetatively so that replicated measurements of whitefly performance could be made on each genotype. Two colonies of silverleaf whiteflies were used: one reared on alfalfa (alfalfa‐experienced whiteflies), and the other on cotton (alfalfa‐naive whiteflies). The effect of variation among alfalfa genotypes on whitefly performance was similar for both whitefly sources, although on all genotypes, the alfalfa‐experienced whiteflies generally performed better than their alfalfa‐naive counterparts. In greenhouse tests, fecundity of newly eclosed adults (over a 5‐day period) on the 17 potentially resistant genotypes was relatively consistent in being lower than fecundity on the presumed susceptible genotypes. However, in nymphal survival tests, the response on the 17 potentially resistant genotypes was not consistent. Nymphal survival (egg to adult) on some of these was very low, as expected, while nymphal survival on others was as high as on the presumed susceptible genotypes. Fecundity and nymphal survival data were not correlated for alfalfa‐naive whiteflies, and were only weakly correlated (r² = 0.13, d.f. = 32, P = 0.04) for alfalfa‐experienced whiteflies. Thirteen genotypes then were examined in the greenhouse in stage‐specific survival tests, where four genotypes demonstrated high resistance (<10% nymphal survival) and three demonstrated moderate resistance (11–34% survival) compared with the three presumed susceptible genotypes that were tested (51–73% survival). Most of the mortality on the resistant genotypes occurred in the first instar, while mortality was more evenly distributed across the life stages on the susceptible genotypes. Interestingly, if nymphs survived to second instar on the resistant genotypes, then their subsequent survival to adult eclosion was similar to survival of second instar to adult on susceptible genotypes. Six of the genotypes used in the greenhouse stage‐specific survival test also were evaluated in the field for nymphal survival, and these results were consistent with the greenhouse tests.     

Comparative studies of predation among feral,commercially-purchased,and laboratory-reared predators

The predatory activities of commercially-purchased adult Hippodamia convergens Guérin-Méneville and two laboratory-reared strains of adult Geocoris punctipes (Say) were compared with their feral counterparts. In single prey choice feeding tests, commercially-purchased and feral H. convergens were provided copious amounts of silverleaf whitefly, Bemisia argentifolii Bellows & Perring adults or eggs of pink bollworm, Pectinophora gossypiella (Saunders). Commercially-purchased H. convergens devoured more pink bollworm eggs and at a faster rate than the feral H. convergens. In contrast, feral H. convergens consumed more adult whiteflies and at a faster rate than commercial H. convergens. In multiple feeding choice tests, two distinctly different laboratory-reared strains and feral G. punctipes were provided a cotton leaf disk containing copious amounts of silverleaf whitefly eggs, nymphs, and adults. Virtually no predation was observed on whitefly eggs, but both laboratory strains of G. punctipes fed on more whitefly nymphs and adults than the feral G. punctipes. Moreover, both of the laboratory strains had a significantly faster feeding rate on adult whiteflies and spent significantly less time feeding on plant tissue than the feral strain. These results suggest that the augmentative biological control candidates retained their ability to prey on these selected prey items, even after being displaced from their overwintering site (H. convergens) or being reared in captivity (G. punctipes) for over 40 generations. Handling editor: Patrick De Clercq. This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or a recommendation for its use by the USDA.     

Field Evaluation of Amitus bennetti (Hymenoptera: Platygasteridae), a Parasitoid of Bemisia argentifolii (Hemiptera: Aleyrodidae), in Cotton and Bean

Field cage experiments were conducted in Riverside, California to quantify the impact of releases of the parasitoid Amitus bennetti Viggiani & Evans on mortality of the whitefly Bemisia argentifolii Bellows & Perring. Single-row 50-m-long plots were planted with either cotton or bean. Cages were erected over the plants in each row, and adult whiteflies were released into the cages. Approximately 10 days later, adult parasitoids were released. Marked individual whiteflies were scored every 4 days for 6 weeks. Paired life tables were then constructed from census data from release and control cages over a single whitefly generation. Total whitefly mortality in release cages (71% in bean, 61% in cotton) was significantly greater than in control cages (25% in bean, 34% in cotton). The marginal rate for mortality attributable directly to the parasitism was 0.535 in the bean plots and 0.201 in the cotton plots. In addition, other mortality was greater in the release plots, possibly reflecting death of parasitized hosts before larval parasitoids could complete development. Parasitism was the greatest mortality factor in the study.     

Beauveria bassiana ARP14 a potential entomopathogenic fungus against Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae)

The sweet potato whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), and the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), are important pests of protected crops grown in warm climates. We compared efficacy of a new strain of the entomopathogenic fungus Beauveria bassiana (ARP14) isolated from Riptortus pedestris (Hemiptera: Alydidae) with a commercial strain (GHA) against different life stages of both B. tabaci and T. vaporariorum. Eggs, nymphs, and adults were exposed to 1 × 10⁸ conidia/mL of each strain using the leaf-dipping method. The mycosis rate of B. tabaci eggs (as a proportion) was relatively low (0.13 for B. bassiana ARP14 and 0.10 for B. bassiana GHA), while, for T. vaporariorum eggs, mycosis rate was 0.44 for B. bassiana GHA and 0.27 for B. bassiana ARP14. However, mycosis rate of 1st instars of both whiteflies was much higher than for eggs, for both strains (ARP14 and GHA). The developmental period of B. tabaci eggs exposed to ARP14 was significantly shorter than for either eggs treated with GHA or the control. For 2nd and 4th instar nymphs and adults of both whiteflies there were no differences in mycosis rates between the two B. bassiana strains. These results suggest that, B. bassiana ARP14 could be commercialized as a native biological control agent for control of B. tabaci and T. vaporariorum.     

Effect of cotton nitrogen fertilization on Bemisia argentifolii populations and honeydew production

The impact of nitrogen fertilization on cotton plants, Gossypium hirsutum L., silverleaf whitefly, Bemisia argentifolii Bellows & Perring, population dynamics and honeydew production were investigated in the field at Riverside, California, USA. Treatments were soil applications of 0, 112, 168 and 224 kg nitrogen per hectare, and a soil application of 112 kg of nitrogen plus a foliar application of 17 kg nitrogen per hectare. Increased numbers of both adult and immature whiteflies occurred during population peaks with increasing amounts of applied nitrogen. Higher numbers of whiteflies resulted in increased levels of honeydew. Increasing plant nitrogen also enhanced cotton foliar photosynthetic rates and stomatal conductance, and altered concentrations of glucose, fructose and sucrose in cotton petioles. However, at our treatment levels nitrogen had no effect on seedcotton yield. Petiole glucose levels were significantly correlated with numbers of whitefly adults on leaves during their peak populations. Significant correlations between whitefly numbers and other cotton physiological parameters occurred on only a few sampling dates.     

Variation in biological characteristics and esterase patterns among populations of Bemisia tabaci, and the association of one population with silverleaf symptom induction

Biological characteristics (oviposition and survival rates) and esterase banding patterns in native PAGE were investigated to evaluate variation among three populations of Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). Reproductive capabilities of whiteflies from cotton (Gossypium hirsutum L.) and pumpkin (Cucurbita maxima Duchesne) populations were similar on the three host plant species tested. These populations, which had the same wild-type field origin, reproduced better on either cotton and pumpkin than on poinsettia (Euphorbia pulcherrima Willdenow). In contrast, poinsettia whiteflies exhibited relatively similar reproductive capabilities for the three host species tested. Pumpkin and cotton whiteflies had similar esterase banding patterns (A type), while poinsettia whiteflies yielded a different banding pattern (B type). In transmission studies, whiteflies from cotton or pumpkin sources did not induce silverleaf (SSL) or white stem (WS) symptoms in Cucurbita spp. tested. In contrast, poinsettia whiteflies were associated routinely with SSL and WS symptoms in Cucurbita spp. following colonization by whitefly adults. From these data, it was possible to correlate a specific esterase banding pattern (A or B) with reproductive capabilities and the ability to induce SSL and WS symptoms.     

The effect of NeemAzal-T/S®, a commercial neem product, on different developmental stages of the common greenhouse whitefly Trialeurodes vaporariorum Westwood (Hom., Aleyrodidae)

The effects of NeemAzal-T/S^®, a commercial neem product, on different life development stages of the common greenhouse whitefly Trialeurodes vaporariorum Westwood (Hom., Aleyrodidae) were tested in laboratory and greenhouse experiments. Treatment of eggs of T. vaporariorum did not affect either larval emergence or the time until larval emergence. However, the proportion of pupal formation (based on the numbers of emerged larvae) was significantly reduced after a treatment of the eggs 3, 5 and 7 days after oviposition, respectively. The proportion of emerged adults (based on the number of formed pupae) was only significantly reduced when eggs had been treated 5 and 7 days after oviposition. In all neem treatments the time until adult emergence was significantly delayed. A neem treatment of early larval instars of T. vaporariorum resulted in a significantly reduced proportion of pupal formation, but subsequent adult emergence (based on the number of formed pupae) was not affected. Treating early larval instars of T. vaporariorum significantly prolonged the time until adult emergence. A neem treatment of T. vaporariorum pupae significantly reduced the proportion of emerged adults but had no effect on the time until adult emergence. Exposing female whiteflies to fresh, 24-h-old and 72-h-old residues of NeemAzal-T/S^® had no effect on the mortality of the insects, though the number of eggs laid per female was significantly reduced in T. vaporariorum exposed to fresh neem residues. These findings are discussed within the context of integrated control of whiteflies in the greenhouse environment.     

Factors that affect the selection of tomato leaflets by two whiteflies,Trialeurodes vaporariorum and Bemisia tabaci (Homoptera: Aleyrodidae)

In Japan, although greenhouse whitefly, Trialeurodes vaporariorum (Westwood), and sweet potato whitefly, Bemisia tabaci (Gennadius), co-occur on tomato plants under greenhouse conditions, the two whiteflies are distributed differently with regard to leaf position. To elucidate the factors that determine the leaf position of these whiteflies, we investigated traits for leaflets collected from three positions on tomato plants. Furthermore, we examined leaflet selection by and fertility of the two whiteflies under choice and non-choice conditions. In addition, the effect on whitefly behavior of volatile compounds released from leaflets was evaluated by use of a Y-tube olfactometer test. Nitrogen and carbon content were highest for upper leaflets. In choice tests, more T. vaporariorum and B. tabaci adults selected upper and middle leaflets, respectively. Similarly, they oviposited more eggs on upper and middle leaflets. In non-choice tests, T. vaporariorum oviposited more eggs on upper leaflets, but B. tabaci oviposited equally on each leaflet. In Y-tube olfactometer tests, more T. vaporariorum adults moved to upper leaflets whereas more B. tabaci adults moved to middle leaflets. These results suggest that different leaflet selection by adults of these two whiteflies is likely to be associated with the different volatile compounds emitted by tomato leaflets at each position.     

Bemisia tabaci (Hemiptera: Aleyrodidae) biotype B colonisation and leaf morphology relationships in upland cotton cultivars

Cultivars of upland cotton, Gossypium hirsutum L., are widely grown throughout the world for fibre production. They are also good reproductive hosts for Bemisia tabaci (Gennadius) biotype B. We studied the relationships between cotton leaf morphology and whitefly population densities in eight United States Deltapine and six Australian cotton cultivars and breeding lines at Holtville, California, in 1996. Results showed that okra-leaf cultivars and lines were colonised with fewer whitefly adults, eggs and nymphs compared to normal-leaf cultivars. The distance from underleaf surfaces of cotton leaves to the centres of nearest minor vascular bundles was negatively correlated with whitefly adult, egg and nymphal densities on leaves for all genotypes with the exception of the Australian breeding line 89013–114. Our results suggest that okra-leaf and distance from underleaf surfaces to the centre of nearest minor vascular bundles of cotton leaves are genetic traits that have potential for breeding whitefly-resistant upland cotton cultivars.     

Whitefly control in cut gerbera: is it possible to control Trialeurodes vaporariorum with Encarsia formosa?

We investigated the impact of inundative releases of the parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae), for control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on cut gerbera (Gerbera jamesonii L.) under controlled greenhouse conditions. Experimental units consisted of ten plants covered and separated from other units by gauze tents. We assessed three release rates of the aphelinid parasitoid: a 7-week experiment with a standard release rate (10 m⁻²/14 days), and a subsequent 3-month trial with high (100 m⁻²/week) and very high (1,000 m⁻²/week) release rates. Experimental units without release of parasitoids served as control treatment. Gerbera plants were infested initially with 50–100 juvenile and 50–70 adult whiteflies in the first experiment, and in the second experiment with less than 50 juveniles per plant and 50–70 adults. Whitefly and parasitoid population density were assessed in weekly intervals using infestation and activity categories. Results show that parasitized whiteflies were present in all treatments within 2 weeks after initial release. Unfortunately, it was not possible to control whiteflies with standard release rates of E. formosa. Although parasitism rates slightly increased, the effect on whitefly populations was negligible. Large amounts of honeydew and growth of sooty mold fungi caused the termination of the first experiment. In a second experiment, E. formosa was tested at 10–100 times higher release densities. In contrast to the first experiment, whitefly densities increased steadily during the first 8 weeks, but remained constant until the end of the experiment in both treatments. Parasitism by E. formosa reached its maximum after 8 weeks. We discuss possible reasons for the low efficiency of E. formosa as a whitefly antagonist in greenhouse production of gerbera.     

Effects of Bt cotton and crylac toxin on survival and development of pink bollworm (Lepidoptera: Gelechiidae).

We evaluated the effects of Bacillus thuringiensis (Bt) toxin CrylAc on survival and development of a susceptible strain and laboratory-selected resistant strains of pink bollworm, Pectinophora gossypiella (Saunders). For susceptible and resistant strains tested on artificial diet, increases in CrylAc concentration reduced developmental rate and pupal weight. In greenhouse tests, survival of resistant larvae on transgenic cotton that produces CrylAc (Bt cotton) was 46% relative to their survival on non-Bt cotton. In contrast, Bt cotton killed all susceptible larvae tested. F1 hybrid progeny of resistant and susceptible adults did not survive on Bt cotton, which indicates recessive inheritance of resistance. Compared with resistant or susceptible larvae reared on non-Bt cotton, resistant larvae reared on Bt cotton had lower survival and slower development, and achieved lower pupal weight and fecundity. Recessive resistance to Bt cotton is consistent with one of the basic assumptions of the refuge strategy for delaying resistance to Bt cotton. Whereas slower development of resistant insects on Bt cotton could increase the probability of mating between resistant adults and accelerate resistance, negative effects of Bt cotton on the survival and development of resistant larvae could delay evolution of resistance.     

Development, survival and fecundity of the urine fly, Scatella (Teichomyza) fusca and predation by the black dumpfly, Hydrotaea aenescens

Development, survival and fecundity for Scatella (Teichomyza) fusca Maquart (Diptera: Ephydridae) were studied at 20 ± 1 °C and 85 ± 10% r.h. Mean (± S.E.) developmental times of eggs, larvae and pupae were 4.0 ± 0.06, 14.9 ± 0.19 and 14.6 ± 0.11 days, respectively, the mean (± S.E.) survival of the original egg cohort to the start of larval, pupal and adult stage being 77.2 ± 3.2%, 54.5 ± 2.6% and 47.9 ± 3.5%, respectively. Females and males displayed approximately straight survivorship curves during adult life, implying constant mortality rates. Mean (± S.E.) adult longevity was 41.6 ± 2.98 days for females and 51.2 ± 3.91 days for males. Assuming a stable age distribution the population consisted of 56% eggs, 31% larvae, 6% pupae and 7% adults. Oviposition peaked when females were 25 days old, and the highest reproductive values (RV_x) (mean ± S.E.) ranged from 129.1 ± 7.57 to 138.5 ± 6.83 for individuals 17–27 days old. A female deposited a mean (± S.E.) of 614.7 ± 35.9 female eggs over a maximum life span of 93 days. The basic reproductive rate (R ₀) (mean ± S.E.) was 173.0 ± 14.2 female offspring per female and the intrinsic rate of natural increase of female individuals (r) (mean ± S.E.) was 0.088 ± 0.001 day^-1. The mean (± S.E.) generation time (T) was 57.8 ± 0.78 days. In cultures with equal numbers of first instar S. fusca larvae and predacious third instar larvae of Hydrotaea aenescens (synonymous Ophyra aenescens) Wiedemann, mean (± S.E.) survival to the adult stage of S. fusca (16.7 ± 8.8%) was significantly lower than in controls with S. fusca alone (58.3 ± 7.4%). The potential significance of predation by H. aenescens on S. fusca in pig farms is discussed.     

Baseline susceptibility and development of resistance to pyriproxyfen in Bemisia argentifolii (Homoptera: Aleyrodidae) in Arizona

Pyriproxyfen (Knack) was registered in Arizona cotton, as the crucial component of a resistance management plan, to control whitefly Bemisia argentifolii (Bellows & Perring) in 1996. A statewide monitoring program was implemented at the same time to detect and monitor whitefly resistance to this novel insecticide. Bioassays involving dipping of leaves infested with whitefly eggs showed that all Arizona whiteflies tested were highly susceptible to pyriproxyfen in 1996. The LC50 estimates were in the range of 0.0020-0.0067 microg (AI)/ml. Two diagnostic pyriproxyfen concentrations, 0.01 and 0.1 microg (AI)/ml, were established for efficient identification of resistant whiteflies. No resistance to pyriproxyfen was detected in whiteflies in statewide surveys conducted in 1997 and 1998. Mean mortality at 0.01 microg (AI)/ml dropped significantly, and survivors were detected for the first time at 0.1 microg (AI)/ml in 1999, the fourth year of use of pyriproxyfen in Arizona cotton. Among the five cotton locations monitored each year since 1996, four of them had whiteflies with significantly reduced susceptibility to pyriproxyfen in 1999. Similarly, reduced susceptibility to pyriproxyfen was detected in whiteflies collected from fall melons and greenhouses in 1999. Although there have been no reports of field failures of pyriproxyfen in Arizona cotton, the reduced susceptibility of whiteflies from statewide survey in 1999 was significant. The results may indicate the development of an early stage of resistance to pyriproxyfen, and the findings should serve as early warning and substantiation of the high resistance risk of pyriproxyfen.     

Life history and demography of the predatory mite,Amblyseius longispinosus Evans

The life history and demography of Amblyseius longispinosus Evans were studied under laboratory conditions 25–28°C and 65–85% RH using the red form of the twospotted spider mite, Tetranychus urticae Koch complex as prey. The entire development from egg to adult averaged 102.5 h for both sexes with a survival rate of 90.0%. Immature females consumed more prey eggs than males, averaging 7.9 and 6.4 eggs/day, respectively. The embryonic development time for male eggs was longer (average 45.2 h) than for female eggs (average 42.6 h). The average duration of succeeding stages did not differ between the sexes: larva, 15.7; protonymph, 21.1; and deutonymph, 23.0 h. Larvae were not observed to feed. Statistically significant differences between sexes were noted with respect to consumption of the deutonymphs (female, average 4.0±0.2; male, average 2.9±0.3 eggs/day) but not of the protonymphs. Mating occurred on the same day as adult emergence and was repeated several times during the reproductive life. Egg laying started on the second day after emergence. Parameters relating to oviposition were: fecundity, gross (50.7) and net (43.3) eggs/female; hatchability, 99.6% (gross) and 99.5% (net); the oviposition lasted at most 28 days and showed a peak in the first week. Net reproductive rate (R₀) was 36.7 female offspring/female/generation time (T) of 9.0 days. The sex ratio was biased toward the females (71.9%), while the intrinsic rate of increase (r_m) was 0.4 with a doubling time (DT) of 1.7 days. Males lived longer (36 days) than females (30 days) with a LT₅₀ for males of 26 days and for females of 22 days.     

Adult survival of Delphastus catalinae (Coleoptera: Coccinellidae), a predator of whiteflies (Hemiptera: Aleyrodidae), on diets of whiteflies, honeydew, and honey

Delphastus catalinae (Horn) is a coccinellid predator that is commercially sold for the management of whiteflies. A study was conducted to assay the effect of selected diets on the survival of adult D. catalinae. Treatments of water (as a control), 10% honey, honeydew, and whiteflies [Bemisia tabaci (Gennadius)] were provided to the beetles in laboratory assays. Newly emerged, unfed adult insects were used at the start of a survival experiment with trials lasting 50 d. Another survival experiment used mixed-aged adults from a greenhouse colony, and the trials lasted 21 d. Survival was poor on a diet of solely water; ~1% survived beyond a week at 26°C. Survival using the newly emerged insects was similar between those fed honeydew and honey diets, but those on the whitefly diet had the greatest survival (~60% on day 50). However, in the experiment with mixed-aged beetles, adults on honey, and whitefly diets performed the same over a 21-d experiment. Excluding those on the water diet, survival of beetles on the various diets ranged from ~50-80% after 21 d. In an open choice assay across 7 h, D. catalinae adults were found on the whitefly diet in a much greater incidence than on the other diets, and the number of beetles found on the whitefly diet increased over time. The data supports that when D. catalinae are employed in greenhouses or fields for whitefly management, during low prey populations, honeydew from the whitefly can help sustain the population of this predator. Moreover, a supplemental food such as a honey solution can help sustain the population of D. catalinae when the prey is decreased to low numbers. These results may help in the development of strategies to enhance the utility of predators for the management of whiteflies.     

Impact of Bemisia whiteflies (Homoptera: Aleyrodidae) on alfalfa growth, forage yield, and quality

A 3-yr project was initiated in 1993 to examine the effects of insecticides and sustained whitefly, Bemisia argentifolii Bellows & Perring [aka. B tabaci Gennadius (Strain B)], feeding on alfalfa plant growth and vigor in greenhouse cage studies, and to determine the impact of natural Bemisia whitefly populations on alfalfa forage yields and quality in a large-plot field experiment. Alfalfa plant growth and vigor after exposure to imidacloprid and a mixture of fenpropathrin and acephate insecticides did not differ from untreated plants in the greenhouse. Consequently, foliar and soil applied insecticides were used to manipulate whitefly densities on alfalfa plants to measure whitefly feeding effects on plant growth and forage yield. Heavy whitefly densities on untreated alfalfa plants in the greenhouse resulted in significant reductions in relative growth rates and net assimilation rates as compared with imidacloprid-treated plants that were maintained relatively whitefly-free. Reductions in alfalfa plant growth measured between infested and treated plants were proportional to whitefly densities. Field plot results derived from three crop seasons were relatively consistent with our greenhouse trials. Both experimental approaches clearly showed that alfalfa plants exposed to high densities of whitefly immatures and adults grew at a significantly slower rate and produced less foliage. As a result of reduced growth rates, alfalfa maturity in the naturally infested plots was estimated to be approximately 7-10 d behind managed plots. Delays in maturity resulted in significant reductions in forage yields of 13-18% during August-September harvests when whitefly populations reached peak abundance. Whitefly feeding stresses also effected hay quality through the reduction of crude protein content and contamination of foliage with honeydew and sooty mold. The status of the Bemisia whiteflies as an economic pest to alfalfa is clearly evident from these studies, but the damage potential of whiteflies in the southwestern United States appears to be restricted to one or two harvest periods during the summer coinciding with peak adult populations and their dispersal from alternate host crops.     

Effect of continuous light on the biology of the greenhouse whitefly,Trialeurodes vaporariorum,on roses

Continuous light is a promising method to reduce the problems with rose powdery mildew in greenhouse rose production. The effects of such a light regime on the performance of insect pests on roses have so far not been investigated. In the present study, survival, developmental time, and reproduction during one generation of the greenhouse whitefly, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae), were characterized on roses, Rosa×hybrida cv. Passion, grown in climate chambers with long‐day conditions (L20:D4) or continuous light (L24:D0) at 21 °C and fluctuating relative humidity (mean 74%, range 47–96%). Whiteflies reared under continuous light had lower immature survival and fecundity and shorter female longevity than whiteflies reared under long‐day conditions, but immature developmental time was only slightly affected. Life‐table analysis showed that the net reproductive rate (R_o) and intrinsic rate of natural increase (r_m) were reduced by 85 and 76%, respectively, and the time for the population to double its size (D) was 4.2 times longer under continuous light. This mean that the whitefly population growth under continuous light was strongly reduced compared with the traditional light regime used in rose production.     

Repellency of ginger oil to Bemisia argentifolii (Homoptera: Aleyrodidae) on tomato

The whitefly Bemisia argentifolii Bellows & Perring is an economically important pest of tomatoes, Lycopersicon esculentum Mill., inducing an irregular ripening disorder of fruit and transmitting plant pathogenic viruses. With the goal of investigating ginger oil as a protectant for tomato plants, we tested the effects of concentration of ginger oil and application methods on repellency to whitefly in a vertical still-air olfactometer. In choice and no-choice experiments conducted in a greenhouse, we evaluated whether ginger oil would protect tomato seedlings from whitefly settling and oviposition. Ginger oil repelled whitefly adults in the vertical olfactometer. The repellency of ginger oil was attributed to its odor, effective at the concentrations used over a distance of 1-2 mm. Tomato leaf disks dipped in ginger oil repelled whiteflies at concentrations of 0.5, 0.75, and 1%, but not at concentrations <0.5%, in a dose-response experiment conducted in the olfactometer. Repellency increased with increasing ginger oil concentration when leaf disks were dipped in ginger oil but not when ginger oil was sprayed onto the leaf disks. Higher quantities of monoterpenes and sesquiterpenes were deposited on leaf disks dipped in ginger oil than on sprayed leaf disks according to gas chromatographic quantification. In the greenhouse, both choice and no-choice tests were conducted with tomato seedlings dipped in 0.25% ginger oil solution or 2% Tween 20, as treatment and control, respectively. In the choice test, 35-42% fewer whitefly adults settled and 37% fewer eggs were laid during the 24-h exposure period on tomato plants dipped in ginger oil solution than on plants dipped in 2% Tween 20. In the no-choice test, 10.2-16.7% fewer whiteflies settled on treated plants compared with control plants but no significant differences were detected in the number of eggs laid. Higher concentrations of ginger oil could not be used without causing severe wilting of tomato leaves. Ginger oil has potential as a protectant of tomato seedlings against B. argentifolii, but issues of phytotoxicity and coverage need to be addressed.     

Host plant effects on resistance to bifenthrin in silverleaf whitefly (Homoptera: Aleyrodidae)

Effects of host plants on resistance to bifenthrin in the silverleaf whitefly, Bemisia argentifolii Bellows & Perring, were determined by LC50 bioassay. In addition, inheritance of resistance to bifenthrin was investigated beginning with a single source of a bifenthrin-susceptible population. Overall, the resistance ratio between the bifenthrin-susceptible population and the selected bifenthrin-resistant population from the same source population was 915-fold after 1 yr in the greenhouse. Responses to bifenthrin among the susceptible and the resistant populations were changed when whiteflies were reared on three different host plants, i.e., cotton, cabbage, and squash. In the resistant populations, the LC50 value of whitefly fed on squash was increased as much as 7.5-fold, while the LC50 value of whitefly fed on cabbage was similar to cotton that served as the control plant. The host plant on which whiteflies feed appears to be an important factor in selection for resistance to bifenthrin, but these effects are crop specific. Based on an analysis using LC50 values of the reciprocal F1 cross on cotton, resistance of whitefly from a single-source whitefly population was inherited as an incompletely dominant factor. A model used to estimate loci numbers showed that resistance of whitefly to bifenthrin is probably controlled primarily by a few or a single locus. In addition, the difference in the ratio of LC50 values between males from unmated mother and males from mated mother was approximately fivefold, suggesting that insecticide resistance in whitefly males is in some way affected by mating.     

Effect of Host Plant on the Potential ofPaecilomyces fumosoroseus(Deuteromycotina: Hyphomycetes) for Controlling the Silverleaf Whitefly,Bemisia argentifolii(Homoptera: Aleyrodidae) in Greenhouses

We determined host plant effect on susceptibility of the silverleaf whitefly,Bemisia argentifolii, to the entomopathogenic fungusPaecilomyces fumosoroseus. Whiteflies were reared on three vegetable species (cucumber, cabbage, and tomato) and three cultivars of tomato (Heatwave, Better Boy, and Rutgers). Second instars were sprayed with 5 × 10⁴conidia/cm²ofPfr97, aP. fumosoroseusstrain, used as a microbial control agent of whiteflies. Trials were conducted in an experimental greenhouse, where temperature and relative humidity were adjusted to favor infection (22–33°C, and 68–100% RH). Larval susceptibility to fungal infection was high and not significantly affected by the host plant. Mortality was > 70% 1 week after treatment and increased further during the second week. Percentages of cadavers with subsequent production of conidia observed in the greenhouse did not vary significantly either with the host vegetable species (85–93% 7 days after treatment and 99–100% 14 days after treatment), or with the cultivar of tomato (96–97% 7 days after treatment and 99–100% 14 days after treatment). After incubation under optimal laboratory conditions, the percentages based on the total number of sporulating cadavers (includingin situsporulating individuals and cadavers sporulating afterin vitroincubation) were not significantly influenced either by host vegetable or cultivar of tomato. According to the conditions prevailing in the series of experiments with the three vegetable species or in the series of experiments with the three cultivars of tomatoes, the production of newly formed conidia varied from approximately 10,000 to 18,000 conidia/cadaver. However, in both series, there was no significant influence of the host vegetable species or cultivar. The survival of the newly formed conidia harvested 7 days following treatment reached more than 50% but was not affected by host plant. These results indicate thatP. fumosoroseusshows potential as a microbial control agent for controllingB. argentifoliion greenhouse crops.