Amir D. Omer

Jasmonic acid induced resistance in grapevines to a root and leaf feeder.

Abstract We investigated the effects of induced resistance to the folivore Pacific spider mite, Tetranychus pacificus McGregor (Acari: Tetranychidae), as well as the root-feeding grape phylloxera Daktulosphaira vitifoliae (Fitch) (Homoptera: Phylloxeridae) in grapevines using exogenous applications of the natural plant inducer, jasmonic acid. Foliar jasmonic acid application at concentrations that caused no phytotoxicity significantly reduced the performance of both herbivores. There were less than half as many eggs produced by spider mites feeding on the induced leaves compared with control grapevine leaves. Induction reduced the numbers of phylloxera eggs and nymphal instars by approximately threefold and twofold, respectively, on induced compared with control grapevine roots. The negative demographic effects of jasmonic acid application appeared to be caused by changes in fecundity for the Pacific spider mite, and possibly changes in development rate and fecundity for grape phylloxera.

Chemically-induced resistance against multiple pests in cotton.

Plants can be treated with natural elicitors to induce resistance to herbivores. The effects of induced resistance to cotton aphids Aphis gossypii Glover (Homoptera: Aphididae), two-spotted spider mites Tetranychus urticae Koch (Acari: Tetranychidae), and western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in cotton plants were investigated using applications of the natural plant inducer, jasmonic acid (JA). Preference was reduced by more than 60% for aphids and spider mites, and more than 90% for thrips on JA-induced leaves compared with control leaves. Survival of aphid nymphs was reduced by two-fold and percentage of nymphs attaining the adult stage was reduced by about eight-fold on induced leaves compared with controls. Induction reduced survival and reproduction of adult aphids by about 40% and 75%, respectively. Induction did not affect survival in spider mites or thrips. However, induction reduced egg production in spider mites by more than 75% and feeding in thrips by up to 80% compared with controls. Induction reduced leaf area in cotton seedlings by about 28%. The implications of our results for cotton pest management are discussed.

Preference and performance responses of California grape phylloxera to different Vitis rootstocks

Laboratory experiments on host preference and performance were simultaneously conducted with Cabernet Sauvignon, AXR#1, and 5C Vitis rootstocks for the California biotypes A and B grape phylloxera. Preference bioassays recorded over a 3‐day period indicated that phylloxera exhibit differential host choice. AXR#1 is antixenotic to biotype A, and 5C is antixenotic to both phylloxera biotypes. Preferences observed with biotype B for Cabernet Sauvignon and AXR#1 were not statistically different, whereas biotype A had shown a significant preference for Cabernet Sauvignon. On 5C the preferences scored were significantly lower than on either of the other two rootstocks, and there was no significant difference beetween biotypes. Performance bioassays recorded over a 29‐day period indicated that survival, development, and reproduction of both biotypes were affected by Vitis rootstock. Both biotypes were unable to survive or develop on 5C suggesting the presence of antibiotic resistance in 5C against these phylloxera biotypes. The antibiotic effects observed with AXR#1 against biotype A were expressed as significant reductions in survival, development, and reproduction. Performance of biotype B on AXR#1 was similar to its performance on Cabernet Sauvignon. For both AXR#1 and 5C rootstocks and both phylloxera biotypes A and B the antibiotic mechanism was considerably stronger than the antixenotic mechanism. This research indicated that host preference and performance are positively correlated in grape phylloxera with the rootstocks and phylloxera biotypes tested.

Sweetpotato whitefly resistance to insecticides in Hawaii: intra‐island variation is related to insecticide use

Susceptibility to acephate, methomyl, and permethrin was determined with laboratory bioassays of field‐collected adults from 15 populations of the B biotype of sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera:Aleyrodidae), from Hawaii. Comparisons at the LC50 showed up to 24‐fold resistance to acephate, 18‐fold resistance to methomyl, and 4‐fold resistance to permethrin. Analysis of variance showed significant intra‐island variation in susceptibility to each insecticide, but no significant variation among islands. Insecticide use varied from 4 to 103 insecticide sprays per site per season. Acephate and methomyl were used more often than permethrin. The frequency of application and LC50 for each insecticide were positively correlated across sites. These results suggest that local variation in insecticide use was a primary cause of variation in susceptibility. If local insecticide use is a key determinant of resistance, as our results suggest, growers can retard resistance development locally by reducing their own insecticide use.

Demographic analyses of organophosphate‐resistant and susceptible strains of greenhouse whitefly, Trialeurodes vaporariorum, on three cotton cultivars

Laboratory experiments were conducted using organophosphate‐resistant and susceptible strains of greenhouse whitefly, Trialeurodes vaporariorum Westwood, to assess age‐specific vital rates in individually‐held adults, and development and survival in preadults on three cotton cultivars at 27 ± 1 °C, 50 ± 10% RH, and a photoperiod of 16:8 (L:D). Female whiteflies lived longer than males, with a maximum life expectancy of 29 days. Heaviest egg laying occurred at ages between 7 and 18 days when individual whiteflies laid > 10 eggs/day. Greenhouse whitefly populations doubled weekly, with stable age distribution of 63% eggs, 28% larvae, 5% pupae, and 4% adults. Analysis of various life history parameters that combine aspects of survival, developmental rates, and fecundity indicated no consistent differences in reproductive fitness between the two greenhouse whitefly strains. Of the three cotton cultivars tested, Pima S‐6 was most susceptible, Acala SJ‐2 was intermediate and Gumbo 500 was most resistant to greenhouse whitefly. Resistance of Gumbo 500 was expressed as slower developmental rates, reduced survival to adulthood, lower reproductive rates, and lower intrinsic rate of increase.

Realized heritability of resistance to dicrotophos in greenhouse whitefly

Realized heritability (h2) of resistance to dicrotophos in greenhouse whitefly, Trialeurodes vaporariorum Westwood, was estimated from a laboratory selection experiment. Five generations of selection increased the LC50 approximately 13‐fold. Estimated h2 of resistance to dicrotophos was 0.40 when calculated with the method of Tabashnik (1992) and 0.35 with the method of Tanaka & Noppun (1989). These results suggest that 35 to 40% of the total phenotypic variation in resistance was caused by additive genetic variation. For thirteen previously reported estimates of h2 of insecticide resistance in other insect pests, the mean was 0.29. The relatively high h2 of dicrotophos resistance for T. vaporariorum is consistent with rapid resistance development in field populations.

Influence of Plant Growth Stage on Grape Phylloxera (Homoptera: Phylloxeridae) Populations

Abstract To understand the effects of plant growth stage on the performance of grape phylloxera, Daktulosphaira vitifoliae (Fitch), demographic analyses were done using attached roots in a Vitis vinifera L. ‘Chardonnay’ vineyard during the vegetative, mid-ripening, and postharvest periods of grapevines. Survival of first instars was <30%. First instars coincide with the time of gall formation, and their survival was higher during postharvest than during the vegetative and mid-ripening periods. Survival after the second instar did not decrease with development but was stable, generally above 90% for the postharvest period and below 75% for the other experimental periods. Sucrose levels in root phloem parenchyma tissues were highest during postharvest compared with vegetative and mid-ripening periods and reflect a more rapid development during this period but only by a few days. The limiting step of grape phylloxera population growth appears to occur simultaneously with gall formation and is associated with survival. Survival and developmental rate of a virulent grape phylloxera strain were limiting with greenhouse vines of the resistant rootstocks 5C and SO4. Survival and developmental rates were more limiting for attached than detached roots.

Seasonal Capacity of Attached and Detached Vineyard Roots to Support Grape Phylloxera (Homoptera: Phylloxeridae)

Abstract Field experiments were conducted to evaluate population densities and survival, developmental rate, and fecundity of grape phylloxera, Daktulosphaira vitifoliae (Fitch), as influenced by root attachment or detachment from mature, field-grown, Vitis vinifera L. grapevines through the growing season. Experiments were performed using artificial infestations of California biotype A grape phylloxera. Thirty-day bioassays on attached- and detached-roots were repeated monthly from May to September in 1997 (cultivar ‘Carignane’) and April to September in 1998 (cultivar ′Thompson Seedless′). The bioassays showed that attached roots had lower population densities than detached roots in all months of both years. Densities varied by month, tending to be higher in spring than in summer. Of the population parameters studied, survival was most influenced by attachment condition, being higher on detached than on attached roots by up to 25-fold. These results imply the importance of vine-related mortality factors to grape phylloxera population density. Influence of root attachment condition on developmental rate and fecundity was not uniform across bioassay months for either year; however, in the four out of 21 assays where there was a significant difference it favored detached roots by twofold. Fruit harvest resulted in higher survival in the July assay but not for assays in August and September; however, neither developmental rate nor fecundity was affected by harvest in any of the assays. We conclude that mortality rather than nutritional factors are most limiting for field populations on susceptible vines. This work suggests that detachment of roots as occurs with root girdling by root pathogens may increase grape phylloxera populations on infested, susceptible vines. These results imply that excised root bioassays over-estimate grape phylloxera virulence and underestimate rootstock resistance.

Effect of attack intensity on host utilization in grape phylloxera

Abstract The ability of grape phylloxera, Daktulosphaira vitifoliae (Fitch), to utilize different Vitis hosts was tested with laboratory and greenhouse assays at three different attack intensities using a susceptible (Cabernet Sauvignon) and a weakly resistant (AXR#1) and two strongly resistant (3309C and 5C) rootstocks. Performance of grape phylloxera increased as numbers of attacking phylloxera increased on the susceptible and weakly resistant but not on the strongly resistant rootstocks. Development rate and fecundity of phylloxera were significantly greater at high attack intensity in comparison with low and medium attack intensities. Phylloxera performance was positively correlated with the size of tuberosities induced by phylloxera at the feeding sites. Resistance may not be overcome by high numbers of attacking phylloxera in strongly resistant rootstocks. The implications of this study with respect to phylloxera management are discussed.

Inheritance of dicrotophos resistance in greenhouse whitefly

We studied inheritance of resistance to dicrotophos in greenhouse whitefly, Trialeurodes vaporariorum Westwood (Homoptera, Aleyrodidae). Compared with females from a field‐collected susceptible strains (S), females from a resistant strain (R) of T. vaporariorum derived from heavily treated cotton fields had a 28‐fold greater LC50 to dicrotophos in laboratory bioassays. Concentration‐mortality lines obtained from female progeny of reciprocal F1 crosses (R♀ XS♂ and S♀ XR♂) were similar, suggesting that inheritance of dicrotophos resistance was autosomal and not influenced by maternal effects. Responses of F1 female progeny were similar to those of the parental S strain, indicating that the resistance was partially recessive (degree of dominance, D, was −0.61). Mortality observed in female progeny obtained from a backcross (F1♀ XR♂) corresponded more closely to expectations derived from polygenic models than to expectations from a monogenic model. The estimated number of effective factors (sensu Lande, 1981) contributing equally to resistance was three.