Thomas F. Leigh

Plant compensation, natural biological control, and herbivory by Aphis gossypii on pre‐reproductive cotton: the anatomy of a non‐pest

Crop plant compensation for herbivory and the population dynamics of herbivores are two key elements in defining an herbivores pest status. We studied the dynamics of natural, unmanipulated populations of the aphid Aphis gossypii on seedling plantings of cotton, Gossypium hirsutum and Gossypium barbadense, over a 4‐year period in Californias Central Valley. Aphid populations colonized all plantings, but reached densities in excess of 0.5 aphids/leaf during only one year (1991), when outbreaks occurred. Outbreak populations were, however, ephemeral; predation and parasitism suppressed aphid populations prior to the initiation of flower bud production, when cotton plant growth may become photosynthate‐limited. Effective natural biological control was observed despite the action of hyperparasitoids and the heavy mortality of immature parasitoids that occurred when predators consumed parasitized aphids.

The use of life tables to assess varietal resistance of cotton to spider mites

Life tables of Tetranychus urticae Koch were used to assess resistance to spider mites in five varieties of cotton in the southern San Joaquin Valley of California. Experiments were conducted during midseason and again in mid‐ to late season. The most resistant variety was ‘Pima S‐5’ and the most susceptible was ‘Acala SJ‐2’. Resistance of S‐5 was expressed primarily as lower mite survivorship and fecundity. Developmental rates were similar on all 5 varieties. Although differences in the net reproductive rate (R0) were large, there were only slight differences in the intrinsic rate of increase (r). Comparisons of the data in the first and second experiment, showed that resistance to spider mites increased as the plants matured. It is hypothesized that increased resistance associated with plant maturation results from a decrease in available nutrients in the leaves of older plants.

The Developing Program of Integrated Control of Cotton Pests in California

During the years immediately following World War II, cotton pest control in California’s San Joaquin Valley was dominated by the organochlorine insecticides (e.g., DDT, toxaphene, endrin). The persistence and high toxicity of these materials made them effective killers of the two key pests, Lygus hesperus Knight and Heliothis zea (Boddie). As a result, normally only one or two treatments per season were needed to effect satisfactory pest control at relatively low cost.

Interplanting alfalfa as a source of Metaseiulus occidentalis (Acari: Phytoseiidae) for managing spider mites in cotton

Abstract Cotton, Gossypium hirsutum L. (Malvaceae), plots were established in 1988 and 1989 with an alfalfa, Medicago sativa L. (Fabaceae), strip at one edge to determine whether alfalfa could serve as a source of the western predatory mite, Metaseiulus (= Typhlodromus = Galandromus) occidentalis Nesbitt (Acari: Phytoseiidae). Western predatory mites were released into the alfalfa early in the season. Spider mite (Acari: Tetranychidae) and western predatory mite density was monitored in alfalfa and in cotton at varying distances from alfalfa. Western predatory mite successfully established populations in alfalfa in both 1988 and 1989. Western predatory mite density was significantly higher close to alfalfa early in the season, indicating that alfalfa was the source. Average seasonal spider mite density was significantly lower close to the alfalfa. Spider mites were effectively controlled and cotton yield was higher adjacent to alfalfa in 1989 but not in 1988. Our results indicate that there is potential in the use of alfalfa as a nursery crop for enhancing western predatory mites in agricultural systems.

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.

Insecticide resistance of greenhouse whitefly (Hom., Aleyrodidae) and fitness on plant hosts relative to the San Joaquin Valley (California) cotton agroecosystem

The relationship between insecticide resistance and fitness parameters in greenhouse whitefly was assessed by examining longevity and fecundity in seven organophosphate‐resistant and susceptible colonies on six plant hosts representing common cultivated plants and weeds in the San Joaquin Valley (SJV), Calif., cotton system. All colonies had higher fecundity and longevity on common bean (cultivar Sal) and cheeseweed than on cotton cultivars, whereas on tree tobacco longevity and fecundity were lowest. Within cotton cultivars, Pima S‐6 was a better host than Acala SJ‐2 and L‐3940. There was no indication of host races in greenhouse whitefly as suggested by the lack of significant colony by host interactions. Longevity was equivalent in organophosphate‐susceptible and resistant colonies. Fecundity in the resistant colonies was higher than in the susceptible colonies. Thus, there was no obvious fitness disadvantage associated with resistance to insecticides in greenhouse whitefly. Implications of cultivated and weed hosts with respect to resistance amplification and resistance management for greenhouse whitefly in the SJV cotton system are discussed.

Bionomics of Cotton Thrips: A Review

This is a review of selected world literature on thrips of cotton, with emphasis on the major pest species. Thrips inhabiting cotton plants include bud (grass and flower) and leaf feeders, and predatory species. Bud feeding thrips inhabit the growing tips of pre-flowering cotton causing distorted growth. This group may be most apparent on seedling cotton, but persists through the growing season. Both adult and immature stages of leaf feeding thrips inhabit older leaves and frequently defoliate plants. Predatory thrips attack infestations of spider mites, thrips, and whitefly and usually become abundant in late summer. Bud thrips are most common on cotton in arid to semiarid regions. Abundance is strongly influenced by availability and condition of alternate hosts, and infestations are suppressed by frequent or timely rainfall. Several of the bud dwelling species are facultative predators on spider mite eggs, hence seasonal time of occurrence may make this group of thrips a major element in the biological control fauna of cotton fields. Management decisions for cotton should consider thrips abundance, projected heat unit accumulation during the seedling stage of plant growth, potential benefit of spider mite predation, and availability of effective insecticides.

Sublethal effects of acephate and biphenate on fecundity, longevity, and egg viability in greenhouse whitefly (Hom., Aleyrodidae)

Sublethal effects of acephate (an organophosphate) and biphenate (a pyrethroid) on greenhouse whitefly (Trialeurodes vaporariorum Westw.) were investigated by measuring male and female longevity, daily rate and lifetime fecundity per female, and egg viability for cohorts exposed to LC50s when Acala SJ‐2 cotton was a host. Males T. vaporariorum were more sensitive to sublethal concentrations than females. Significant reductions in male longevity were caused by exposure to acephate and biphenate, whereas female longevity was not significantly affected by exposure. Exposure to biphenate and acephate treatments reduced lifetime fecundity by 29% and 37%, respectively. Viability of eggs laid by treated females was not affected. Analysis of patterns and heterogeneity of reproduction suggests that timing and magnitude of oviposition, average level of daily oviposition and reproductive decline were influenced by sublethal concentrations of insecticides. Results of repeated‐measures analysis of variance confirmed reductions in fecundity of females T. vaporariorum exposed to sublethal concentrations.