Growth, reproduction and survival (= performance) of the aphid Myzus persicae Suizer was measured on virus-free and virus-infected potato plants. The principle objective was to evaluate if various viral infections affected aphid performance differently, and if so, whether any order in the performance response of the aphid was discernible according to the type of virus-vector relationship. Three viruses varying in their dependency on M. persicae as a vector were used. Plants infected with potato leafroll virus (PLRV), a circulative virus highly dependent upon M. persicae for dispersal and transmission, were superior hosts as determined by the significantly greater mean relative growth rate (MRGR) and intrinsic rate of increase (rm) ofM. persicae compared with those of aphids reared on other plants. Plants infected with potato virus Y, a noncirculative virus less dependent upon M. persicae for dispersal than PLRV, were intermediate in their quality based upon intermediate MRGR and r m values. Plants infected with potato virus X, a nonvectored virus independent of M. persicae, were least suitable hosts along with the group of virus-free plants according to the lower MRGR and r m values.

Titers of two systemic neonicotinoid insecticides in citrus trees were measured in conjunction with conventional evaluations of their impact on glassy-winged sharpshooter (Homalodisca coagulata (Say); GWSS) populations. Xylem fluid samples were collected at regular intervals and from multiple locations within field-grown citrus trees to determine imidacloprid and thiamethoxam concentrations using commercial ELISA kits. Uptake profiles varied considerably with peak mean titers of imidacloprid occurring 6-8 weeks after application compared with 2 weeks for thiamethoxam. The persistence of each compound also varied as near-peak levels of imidacloprid were sustained for another 6-10 weeks before gradually declining. In contrast, thiamethoxam titers declined more rapidly after the initial peak, possibly reflecting an application rate only one-quarter of that used for imidacloprid. Within-tree distributions were more similar for the two compounds, with no significant effect due to height of the sample (upper or lower half) or to the quadrant location within the tree, with the exception of one quadrant in the thiamethoxam-treated trees. Substantial reductions in GWSS nymphs and adults were observed in imidacloprid-treated trees during the 2001 trial and were sustained for 4-5 months after treatment. Treatment effects on nymphs were not as well pronounced in the 2002 trial, when overall GWSS infestations were much reduced from the previous year. However, consistently lower adult infestations were still observed in 2002 for both treatments compared with untreated trees. Information on the spatial and temporal profiles in citrus trees was obtained for both compounds to complement field impact data and improve understanding of their pest management potential.

No abstract

Two systemic neonicotinoids, imidacloprid and thiamethoxam, are widely used for residual control of several insect pests in cotton (Gossypium spp.), vegetables, and citrus (Citrus spp.). We evaluated their impact on six species of beneficial arthropods, including four parasitoid species--Aphytis melinus Debach, Gonatocerus ashmeadi Girault, Eretmocerus eremicus Rose & Zolnerowich, and Encarsia formosa Gahan--and two generalist predators--Geocoris punctipes (Say) and Orius insidiosus (Say)--in the laboratory by using a systemic uptake bioassay. Exposure to systemically treated leaves of both neonicotinoids had negative effects on adult survival in all four parasitoids, with higher potency against A. melinus as indicated by a low LC50. Mortality was also high for G. ashmeadi, E. eremicus, and E. formosa after exposure to both compounds but only after 48 h posttreatment. The two predators G. punctipes and O. insidiosus were variably susceptible to imidacloprid and thiamethoxam after 96-h exposure. However, toxicity to these predators may be related to their feeding on foliage and not just contact with surface residues. Our laboratory results contradict suggestions of little impact of these systemic neonicotinoids on parasitoids or predators but field studies will be needed to better quantify the levels of such impacts under natural conditions.

Laboratory bioassays were carried out with four neonicotinoid insecticides on multiple strains of Bemisia tabaci (Gennadius) to evaluate resistance and cross-resistance patterns. Three imidacloprid-resistant strains and field populations from three different locations in the southwestern USA were compared in systemic uptake bioassays with acetamiprid, dinotefuran, imidacloprid and thiamethoxam. An imidacloprid-resistant strain (IM-R) with 120-fold resistance originally collected from Imperial Valley, California, did not show cross-resistance to acetamiprid, dinotefuran or thiamethoxam. The Guatemala-resistant strain (GU-R) that was also highly resistant to imidacloprid (RR=109-fold) showed low levels of cross-resistance when bioassayed with acetamiprid and thiamethoxam. However, dinotefuran was more toxic than either imidacloprid or thiamethoxam to both IM-R and GU-R strains as indicated by low LC50s. By contrast, a Q-biotype Spanish-resistant strain (SQ-R) of B. tabaci highly resistant to imidacloprid demonstrated high cross-resistance to the two related neonicotinoids. Field populations from Imperial Valley (California), Maricopa and Yuma (Arizona), showed variable susceptibility to imidacloprid (LC50s ranging from 3.39 to 115 microg ml(-1)) but did not exhibit cross-resistance to the three neonicotinoids suggesting that all three compounds would be effective in managing whiteflies. Yuma populations were the most susceptible to imidacloprid. Dinotefuran was the most toxic of the four neonicotinoids against field populations. Although differences in binding at the target site and metabolic pathways may influence the variability in cross-resistance patterns among whitefly populations, comparison of whitefly responses from various geographic regions to the four neonicotinoids indicates the importance of ecological and operational factors on development of cross-resistance to the neonicotinoids.

No abstract

: A Ðeld-collected population of the silverleaf whiteÑy, Bemisia argentifolii, was selected with the nicotinyl compound, imidacloprid, over 32 generations to determine if resistance would develop when maintained under continuous selection pressure in a greenhouse. Resistance was slow to increase at Ðrst with low to moderate levels of resistance (RR from 6-to 17-fold) in the Ðrst 15 generations of selection. Further selection steadily led to higher levels of resistance, with the greatest resistance ratio at 82-fold, the gradual rise suggesting the involvement of a polygenic system. At the end of the selection, slopes of probit regressions were substantially steeper than earlier, indicating increased homogeneity of imidacloprid resistance in this strain.A hydroponic bioassay featuring systemic uptake of imidacloprid through roots was developed to monitor the changes in resistance to imidacloprid in the selected whiteÑy strain and in seven Ðeld-collected strains from Imperial Valley, California. Six out of seven Ðeld-collected strains exhibited low values LC 50 (0É002 to 0É512 mg ml~1) compared to the selected resistant strain, with one exception where the was 0É926 mg ml~1 (RR \ 15É0). Variation in LC 50 responses to imidacloprid in the Ðeld strains suggest that this technique is sufficiently sensitive to detect di †erences in susceptibilities of whiteÑy populations. The imidacloprid-resistant strain showed no cross-resistance to endosulfan, chlorpyrifos or methomyl (RR ranging from 0É4-to 1É5-fold). A low level of cross-resistance was observed to bifenthrin in the IM-R strain at 7-fold. The success of selection for resistance to imidacloprid has serious implications for whiteÑy control programs that rely heavily on imidacloprid.

The eVect of dietary supplements on the longevity of male and female Gonatocerus ashmeadi, G. triguttatus and G. fasciatus (Hymenoptera: Mymaridae), was determined in the laboratory. Treatments included: water only, 3:1 honey-water solution, Xoral and extraXoral nectars from Wve diVerent plants (excised stems from Fagopyrum esculentum, Lobularia maritima, Phacelia tanacetifolia, Anethum graveolens and Vicia faba), honeydew from Coccus hesperidum and Homalodisca vitripennis (formally H. coagulata), a commercially available food supplement (Eliminade) and citrus foliage. Additionally, the sugar composition of each food resource was determined using HPLC and whole Xower extracts. Honey-water and F. esculentum nectar signiWcantly increased longevity of male and female G. ashmeadi, G. triguttatus, and G. fasciatus up to 1860%, 1323% and 1459%, respectively, when compared with water. For both sexes and all three parasitoid species, survival on citrus foliage, H. vitripennis excrement, and P. tanacetifolia Xowers was equivalent to that on water only. The longevity of G. ashmeadi and G. triguttatus was up to 539% higher on Eliminade compared with water only, however there was no signiWcant eVect of Eliminade on survival of G. fasciatus. Coccus hesperidum honeydew increased survival times up to 665% for all mymarid species compared with citrus foliage alone. HPLC analysis indicated that food resources most beneWcial to Gonatocerus parasitoids possessed a high proportion of glucose (up to 44%) and fructose (up to 53%), suggesting that sucrose may not be as important for parasitoid survival. Citrus and P. tanacetifolia Xowers contained favorable proportions of glucose and fructose, but the inability of Gonatocerus spp. to beneWt from this may be related to Xower morphology which could prevent access to nectar.