Effect of planting date and nitrogen fertilization on photosynthesis and soluble carbohydrate contents of cotton in relation to silverleaf whitefly （Bemisia tabaci biotype “B”） populations

The impacts of planting date and nitrogen fertilization on cotton （Gossypium hirsutum L.） photosynthesis and soluble carbohydrate contents in relation to silverleaf whitefly, Bemisia tabaci （Gennadius） biotype “B”, populations were examined in field experiments. Cotton planted in late April and early June was treated with 0, 112, 168 and 224 kg/N hectare in soil using urea fertilizer. The mean photosynthetic rate of April-planted cotton was 4%-20% higher than that of June-planted cotton early in the season, but 10%- 18% lower than that of June-planted cotton late in the season. The photosynthetic rates for both planting dates were positively correlated with levels of added nitrogen. While levels of glucose for both planting dates were positively correlated with nitrogen levels, fructose and sucrose levels were not. The mean levels of fructose were up to 40% lower, while that of sucrose were up to 59% higher, in April-planted cotton than in June-planted cotton. Levels of photosynthetic rate or stomatal conductance were not correlated with adult whitefly densities for either planting date. Levels of glucose and fructose were positively correlated with whitefly densities only for June-planted cotton late in the season.     

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.     

Establishment of a silverleaf whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) population in western Sydney

A field survey in western Sydney found populations of Bemisia tabaci biotype B (silverleaf whitefly) that have the potential to damage greenhouse vegetable crops prevalent in the region. Adult whiteflies were collected from five farms at 2-week intervals and the proportion of silverleaf whitefly in the overall whitefly population was determined using polyacrylamide gel electrophoresis. High levels of silverleaf whitefly were detected at most sampling dates on two of the farms surveyed and a third farm exhibited lower but relatively continuous silverleaf whitefly abundance. The presence of permanent silverleaf whitefly populations must be considered by the greenhouse industry when formulating whitefly management plans for western Sydney.     

A thermoprotective role for sorbitol in the silverleaf whitefly,Bemisia argentifolii

Accumulation of polyols in insects is well known as a cold-hardening response related to overwintering or to protection against cold shock. The silverleaf whitefly (Bemisia argentifolii, Bellows and Perring) is a major insect pest in tropical and subtropical regions where heat stress and desiccation pose formidable threats to survival. We found that sorbitol levels increased ten-fold when whiteflies were exposed to elevated temperatures. Sorbitol levels rose from 0.16nmolwhitefly(-1) at 25 degrees C to 1.59nmolwhitefly(-1) at 42 degrees C. Sorbitol levels fluctuated diurnally under glasshouse and field conditions increasing ten-fold from morning to early afternoon. Feeding experiments on artificial diets showed that both temperature and dietary sucrose concentration were key factors influencing sorbitol accumulation. Cell free extracts prepared from adult whiteflies catalyzed NADPH-dependent fructose reduction, but were unable to reduce glucose with either NADPH or NADH. Radiotracer experiments with labeled glucose and fructose showed that fructose was the immediate precursor of sorbitol. Thus, sorbitol synthesis in the whitefly is apparently unconventional, involving conversion of fructose by a novel NADPH-dependent ketose reductase. We propose that sorbitol accumulation is a mechanism for thermoprotection and osmoregulation in the silverleaf whitefly, allowing the insect to thrive in environments conducive to thermal and osmotic stress.     

Population dynamics of Bemisia argentifolii (Homoptera: Aleyrodidae) on spring collard and relationship to yield in the lower Rio Grande Valley of Texas

Seasonal population dynamics of the silverleaf whitefly, Bemisia argentifolii Bellows & Perring [formerly known as the sweetpotato whitefly, B. tabaci (Gennadius) Biotype "B"], was investigated on collard (Brassica oleracea L. variety acephala) during spring 1998 and 1999 in the Lower Rio Grande Valley of Texas. Yield loss caused by whitefly was determined by using insecticides to suppress whitefly populations to a low level. Although B. argentifolii populations of adults and immatures fluctuated greatly from April to June during the two seasons, the relative values were similar. Adult whiteflies first appeared on the plants in early April, increased rapidly within the month, peaked in May, and declined at the end of the season in early or mid-June. Whitefly eggs appeared on plants soon after adults were found, but high numbers of eggs were observed on foliage until late May 1998 and mid- and late May 1999. Nymphs and pupae increased slowly before June 1998 and increased early in May 1999. Whitefly population levels appeared to be positively associated with the availability and the growth of host plants until plant maturation, afterward being negatively related with plant quality in the late season. Temperature, rainfall, and natural enemies were not key factors in regulating population dynamics during the two seasons. Collard plants with heavy infestations of whiteflies were unmarketable because of the damage caused by honeydew and sooty mold on the foliage. Application of a combination of fenpropathrin (Danitol) and acephate (Orthene) not only significantly reduced the whitefly infestation levels but also reduced plant foliar damage, resulting in marketable foliage with six to seven times greater yield and higher quality compared with the untreated plants.     

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.     

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.     

Influence of Bemisia argentifolii (Homoptera: Aleyrodidae) infestation and squash silverleaf disorder on zucchini seedling growth

We investigated the effect of different levels of infestation by whiteflies, Bemisia argentifolii Bellows & Perring, on the growth and pigment concentrations of seedlings of zucchini, Cucurbita pepo L., that differed in their tolerance to squash silverleaf disorder. Genetically similar sister lines that were either tolerant (ZUC76-SLR) or susceptible (ZUC61) to silverleaf disorder exhibited reduced plant height, internode length, plant dry weight, and petiole length in response to whitefly feeding. Similar plant growth responses to whitefly feeding were observed despite that the foliage of ZUC61 silvered severely, whereas the foliage of ZUC76-SLR showed no silvering in a greenhouse experiment conducted in the spring and showed only minimal silvering in a similar greenhouse experiment conducted in the fall. In plants of both sister lines infested with 50 pairs of whiteflies and their progeny, petioles, but not the leaf blades, of uninfested leaves had reduced chlorophyll content. In another experiment, two different genetic sources of tolerance to silverleaf disorder (ZUC33-SLR/PMR and ZUC76-SLR) and a commercial silverleaf-susceptible zucchini hybrid ('Zucchini Elite') responded similarly to whitefly feeding, except the tolerant genotypes did not exhibit leaf silvering. All genotypes, silverleaf tolerant or not, had reduced dry weight, plant height, and internode length that became more pronounced as whitefly infestation increased. All genotypes had reduced levels of chlorophylls and carotenoids in uninfested young leaf blades and petioles from infested plants. Petioles, however, were more affected by feeding than leaf blades, showing a 66% reduction in chlorophylls a+b and carotenoids at the lowest infestation level (30 pairs of whitefly and their progeny), whereas pigments in leaf blades declined more slowly in response to whitefly feeding density, averaging 14-15% less at the highest infestation level (90 pairs of whitefly and their progeny). We conclude that tolerance to silverleaf disorder does not prevent stunting in zucchini seedlings nor does it protect against the systemic loss of photosynthetic and protoprotectant pigments induced by feeding of B. argentifolii whiteflies.     

Plant nitrogen status rapidly alters amino acid metabolism and excretion in Bemisia tabaci

The effect of plant nitrogen (N) status on the content and distribution of free amino acids in the bodies and honeydew of silverleaf whiteflies Bemisia tabaci (Gennadius) Biotype B (= B. argentifolii Bellows and Perring) was determined. Whiteflies fed for 4 days on cotton leaves that received high or low N fertility. For low-N plants, photosynthesis and leaf total N levels were decreased, and a much-reduced amount of free amino acids was recovered in phloem sap. Low N fertility did not affect whitefly total N content, but did markedly decrease the free amino acid content. Glutamine, alanine and proline accounted for over half of the insect free amino acid pool for both N treatments. On a relative basis, adjustments in glutamine levels in response to plant N status were much larger compared to the other amino acids. Large amounts of amino N, especially asparagine, were excreted from whiteflies fed on high-N plants whereas amino N excretion essentially ceased for whiteflies fed on low N plants. The distribution of amino acids in the insects and honeydew was not closely related to the phloem sap amino acids. However, total amino acid excretion was quite indicative of the plant N status and the quality of the insect diet. The results indicated that whitefly free amino acid pools and excretion of amino N were rapidly altered by plant N status.     

Biotype-specific expression of dsRNA in the sweetpotato whitefly

This study was conducted to evaluate the effect of two different biotypes of the sweetpotato whitefly,Bemisia tabaci (Gennadius), on the induction of squash silverleaf (SSL), and to determine if double-stranded RNA (dsRNA) occurs in geographically remote populations of the two biotypes. Recently collected B-biotype whiteflies from Florida, Arizona, Mississippi, and Texas (SPW-B) all contained a 7.0 kb dsRNA molecule. Kb dsRNA molecule. Laboratory colonies of A-biotype whiteflies that were originally collected in 1981 from cotton in Arizona and California did not contain the 7.0 Kb dsRNA. When the two biotypes were compared only the SPW-B induced rapid onset, grade 5, SSL. DsRNA similar to that found in adult SPW-B was concentrated in whitefly nymphs, but host plant leaf tissue did not contain any consistent dsRNA molecules. SPW-A only induced low-grade SSL and progeny of SPW-A that were fed on pumpkin plants displaying SSL did not acquire the ability to express dsRNA or induce SSL. Our data suggest that dsRNA is not directly involved in the induction of SSL and that SSL is a host-specific response, to a feeding injury induced by B-biotype whiteflies. The origin and source of the 7.0 Kb dsRNA molecule remains enigmatic but its expression is constant in the whitefly biotype that is responsible for the induction of SSL and several other plant disorders in the U.S.     

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.     

寄主种类、距离和种群密度对烟粉虱扩散的影响

在笼罩条件下,分别以黄瓜、棉花、蕹菜为虫源寄主和诱集寄主,研究寄主植物、空间距离、虫源密度等因子对B型烟粉虱扩散的影响。结果发现,从虫源寄主上扩散的成虫量,与烟粉虱对诱集寄主和虫源寄主的嗜性差异程度有关,诱集寄主相对于虫源寄主的嗜性越强,扩散出来的成虫数量就越多,反之扩散的成虫数量就越少。成虫扩散量与诱集寄主的距离呈负相关,并且诱集寄主的嗜性越强,随着距离的增大,诱集量下降的速率越快。成虫扩散量与虫源种群密度呈正关,但非嗜好寄主上的虫口承载量相对较低,在虫口密度相对较高时,非嗜好寄主上烟粉虱向外扩散的相对量更大。烟粉虱的扩散量与寄主营养状况呈负相关,寄主营养胁迫时,非嗜好寄主上可以获得相对更高的诱集量。     

Whitefly Bemisia tabaci (Homoptera: Aleyrodidae) infestation on cassava genotypes grown at different ecozones in Nigeria

Large-scale screening of cassava, Manihot esculenta Crantz, genotypes for resistance to infestation by whitefly Bemisia tabaci Gennadius, the vector of cassava mosaic geminiviruses, is limited. A range of new cassava elite clones were therefore assessed for the whitefly infestation in the 1999/2000 and 2000/2001 cropping seasons in experimental fields of International Institute of Tropical Agriculture, Ibadan, Nigeria. On each scoring day, between 0600 and 0800 hours when the whiteflies were relatively immobile, adult whitefly populations on the five topmost expanded leaves of cassava cultivars were counted. All through the 6-mo scoring period, there was a highly significant difference in whitefly infestation among the new cassava elite clones. Vector population buildup was observed in Ibadan (forest-savanna transition zone) and Onne (humid forest), 2 mo after planting (MAP). Mean infestation across cassava genotypes was significantly highest (16.6 whiteflies per plant) in Ibadan and lowest in Zaria (0.2). Generally, whitefly infestation was very low in all locations at 5 and 6 MAP. During this period, cassava genotypes 96/1439 and 91/02324 significantly supported higher infestations than other genotypes. Plants of 96/1089A and TMS 30572 supported the lowest whitefly infestation across cassava genotypes in all locations. The preferential whitefly visitation, the differences between locations in relation to whitefly population, cassava mosaic disease, and the fresh root yield of cassava genotypes are discussed.     

The influence of planting date,transgenic Bt maize and hybrid relative maturity on European corn borer Ostrinia nubilalis (Lepidoptera: Crambidae) ovipositional patterns

A three year study was carried out at Hoytville and at Wooster, Ohio, USA from 2006 to 2008 to investigate the influence of planting date, transgenic maize and hybrid maturity on Ostrinia nubilalis (Hubner) population dynamics and oviposition patterns. Maize plants were planted in late April or early May, mid‐May and early June during each year. The moth flight pattern showed bivoltine generations during the three years. The first moth flight peaked in June, with the populations declining during July. The second moth flight peaked in August and declined towards the end of September or early October. Egg mass density did not differ significantly between transgenic and non‐transgenic maize of different maturities. Significant differences were observed, however, among planting dates, sampling dates, and sampling date × planting date interactions. Generally higher numbers of egg masses from second generation moths were deposited on late planted maize than middle and early plantings.     

Impact of Bemisia argentifolii (Homoptera: Auchenorrhyncha: Aleyrodidae) infestation and squash silverleaf disorder on zucchini yield and quality

Fruit yield and quality of zucchini, Cucurbita pepo L., plants infested with Bemisia argentifolii Bellows & Perring were evaluated in a screenhouse under spring and fall growing conditions by using closely related sister lines that were either susceptible (ZUC61) or tolerant (ZUC76-SLR) to squash silverleaf disorder. Our objective was to test separately the effects of level of whitefly infestation and expression of silverleaf symptoms on zucchini yield and quality. In a second experiment, yield and quality of fruit produced by silverleaf-tolerant zucchini genotypes incorporating two different sources of tolerance (ZUC76-SLR and ZUC33-SLR/PMR) were compared with that of 'Zucchini Elite', a silverleaf-susceptible commercial hybrid. Zucchini fruit yield was reduced in plants exposed to repeated infestations of whiteflies in spring and fall of both experiments. In addition, fruit grew to harvestable size more slowly under the highest whitefly infestations. Fruit quality was reduced at high infestations because of uneven and reduced pigmentation. The fruit yield and quality of ZUC61 and ZUC76-SLR were similarly affected by whitefly infestation despite differences in their susceptibility to squash silverleaf disorder. Fruit from infested plants showed decreased levels of chlorophyll and carotenoids causing the "blanching" of the fruit that is associated with loss of quality and reduced marketability. Leaves of infested plants of all genotypes had reduced levels of photosynthetic and photoprotectant pigments, possibly leading to reduced photosynthesis and consequently reduced yield. We conclude that feeding by high whitefly populations rather than expression of squash silverleaf disorder is responsible for yield and quality reduction in zucchini.     

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.     

Silicon influence on resistance induction against Bemisia tabaci biotype B (Genn.) (Hemiptera: Aleyrodidae) and on vegetative development in two soybean cultivars

The potential of populations of Bemisia tabaci (Genn.) to become resistant to insecticides has stimulated research into alternative tactics of integrated pest management such as the induction of host-plant resistance. Recent data have shown that silicon can increase the degree of resistance of host plants to insect pests. Therefore the aim of our work was to study the effects of silicon application on the vegetative development of soybean plants and on the induction of resistance to the silverleaf whitefly, B. tabaci biotype B. We performed choice and no-choice tests of oviposition preference on two soybean cultivars, IAC-19 (moderately resistant to B. tabaci biotype B) and MONSOY-8001 (susceptible), with and without application of silicon. Silicon did not affect silverleaf whitefly oviposition preferences, but caused significant mortality in nymphs. Thus, silicon increased the degree of resistance to silverleaf whitefly. Silicon decreased the production of phenolic compounds, but did not affect lignin production. However, when applied to cultivar IAC-19, it increased the production of non-protein organic nitrogen. Silicon had no effect on the vegetative development of soybean plants, but it increased the degree of resistance to the silverleaf whitefly. We conclude that silicon applications combined with cultivar IAC-19 can significantly decrease silverleaf whitefly populations, having a positive impact both on the soybean plant and on the environment.     

Cardinium infection alters cotton defense and detoxification metabolism of its whitefly host

Field monitoring revealed that the infection ratio of the bacterial symbiont Cardinium in the whitefly (Bemisia tabaci MED) was relatively low in northern China. However, the role of this symbiont and the symbiont–whitefly–host plant interaction mechanism are poorly understood. We investigated the influence of Cardinium on the competitiveness of the host whitefly and the physiological interaction between the host plants and host whiteflies. Cardinium-infected whiteflies were displaced by uninfected whiteflies after 5 generations, which showed that Cardinium infection reduced whitefly competitiveness. The defense response genes of cotton significantly decreased under infestation by infected whiteflies compared to uninfected whiteflies. The expression of detoxification metabolism genes, especially the uridine 5ʹ-diphospho-glucuronyltransferase and P450 genes, in infected whiteflies significantly decreased. These results demonstrated that Cardinium could inhibit the defense response of the host plant and decrease the detoxification metabolism ability of the host whitefly. The reduced competitiveness of infected whiteflies may be associated with the inhibition of the whitefly detoxification metabolism by Cardinium, resulting in the reduced performance of infected whiteflies. However, Cardinium infection can suppress plant defenses, which may benefit both infected and uninfected whiteflies when they coexist. This research illustrates the symbiont–whitefly–host plant interaction mechanism and the population dynamics of the whitefly.     

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.     

Interaction among planting dates,transgenic maize,seed treatment,corn rootworm damage and grain yield

A study was conducted from 2006 to 2008 at South Charleston and Wooster, Ohio, USA, to evaluate the potential use of planting dates in combination with Bacillus thuringiensis transgenic maize and insecticidal seed treatments to manage the root feeding of western corn rootworm (Diabrotica virgifera virgifera, Coleoptera: Chrysomelidae). Three planting dates (early, middle and late), targeting late April/early, mid‐May and early June, respectively, were used. We planted six hybrid treatments consisting of two seed‐treated hybrids with seed treatment, two transgenic hybrids and two untreated hybrids, each set represented by one short and one full season maturity hybrid. When root injury was high, significant lodging and stunted growth were observed on untreated maize and declined when planting was delayed. Root injury by rootworm larval feeding was significantly reduced by later planting maize, that is, early June. The use of transgenic maize and seed treatment also significantly reduced root injury by rootworm larvae. The influence of planting date on grain yield was inconsistent from year to year. Grain yields from short season hybrids were comparable to full season hybrids especially on later plantings. These results showed that the use of a seed treatment and transgenic maize might be beneficial only when rootworm population is high and planting is early.