Peter C Ellsworth

Abstract:

A general description of the life stages of Chelonus sp. nr. curvimaculatus, an egg-larval parasitoid of pink bollworm, Pectinophora gossypiella (Saunders), is presented. Pink bollworms were reared in the laboratory on a wheat germ diet. At 29°C, parasitoid eggs (0.12-0.18 mm) begin to eclose ∼22 h after oviposition. Three instars occur in this species. The 1st instar is endoparasitic and ranged in length from 0.14 mm (neonate) to 1.25 mm (∼9 d old. end of the 1st stadium). The 2nd instar also is endoparasitic and is 1.89-3.04 mm long. This stadium lasts ∼2-3 d. The 3rd instar is an average of 3.82 mm long and is endoparasitic early in its development but becomes ectoparasitic toward the completion of its development. This stadium lasts ∼3 d. The pupal stage lasts ∼6-7 d. Adult parasitoids begin to emerge ∼21 d after oviposition. Parasitized and unparasitized pink bollworm larvae deceloped through 4 stadia. Larval head capsule widths. body lengths, and weights of parasitized pink bollworms are significantly smaller than those of unparasitized larvae during the 3rd and 4th stadia. Parasitized 4th-instar pink bollworms have a mean head width of 0.8687 mm, body length of 6.28 mm, and weight of 6.9 mg. Fourth-instar unparasitized pink bollworm measurements were 1.0743 mm, 9.31 mm, and 17.7 mg, respectively.

Abstract:

Detailed biological information is lacking on Chelonus sp. nr. curvimaculatus, an egg-larval parasitoid of Pectinophora gossypiella (Saunders). We conducted laboratory studies to gain new understanding of the biology of C. sp. nr. curvimaculatus reared on this economically important host. Developmental time, adult longevity, and fecundity of C. sp. nr. curvimaculatus were studied under 5 constant temperatures and 3 photoperiods. At 20°C, parasitoid development, from egg to adult, was longer for females (53.6 d) than for males (49.5 d), but at 35°C both sexes developed in a similar period (19.9 d for females and 18.8 d for males). The thermal constant for female and male C. sp. nr. curvimaculatus, from egg to adult, was 366 and 353 above a base temperature of 12,95°C and 12.47°C, respectively. Adult male (≈16.5 d) and female (≈20 d) parasitoids lived longest at 20°C, and at 35°C both lived ≈6.5 d. True and realized fecundity was highest at 25°C (≈1,034 and 420 eggs per female, respectively) and lowest at 35°C (≈119 and 67, respectively). The percentage of superparasitized host eggs was greatest at 25 and 30°C (≈55%), and lowest at 35°C (≈29%). Net reproductive rates (Ro) were variable across all conditions. Based on the realized fecundity, Ro was highest at 20°C (103.37) and lowest at 35°C (32.79). Temperature and the age of the parasitoid had the greatest influence on fecundity. Photoperiod played a minor role in influencing developmental rates, but not adult longevity or fecundity of C. sp. nr. curvimaculatus. This life history and rearing information should be useful in field release studies and the development of future biological control programs for pink bollworm.

Abstract:

This study examined the impact of irrigation water on certain aspects of an insect-plant relationship in the field including the assessment of plant-mediated water effects on an herbivore's development, survival, and behavior, and plant damage parameters and host tissue water status. Maize (Zea mays L.) plants were arranged in a randomized complete block design in the field over two years in North Carolina (NC). Four blocks were subjected to three different irrigation treatments initiated ca. one week before anthesis: optimal, intermediate, deficit water supply. Each plant was infested with one (1986) or two (1987) black head stage, E-race European corn borer [Ostrinia nubilalis (Hübn.)] (ECB) egg masses at tasselling. ECB development, tunnelling site, and survival as well as plant tissue water status (tissue % water contents [θ] & leaf water potentials [Ψ]) were recorded through July. The irrigation effect on ECB parameters was slight and variable. Internal stalk temperatures of optimal plants were consistently cooler than their deficit counterparts (1 day-degree/day). With degree-days included as an explanatory variable in the analyses, there were no significant irrigation effects on the ECB parameters, except for total proportion of ECB's bored into maize plant parts. More ECB's bored into drier plants than in optimal plants; however, this trend was not significant in 1987. Plant water indices showed that though Ψ responded to irrigation, there were only minor changes in tissue θ, particularly in view of the larger diurnal tissue changes observed and the relatively high, sustained stalk θ levels seen over all treatments. Examination of ECB pupal θ confirmed that dietary water changes were minor or non-limiting to the insects' developmental physiology, because pupal θ was not sensitive to the irrigation treatments. Though water supply changes have drastic developmental and agronomic consequences for the maize plant, little or no changes were seen in the ECB feeding environment. Furthermore, a plant damage model was developed whereby the total % of ECB's tunnelled into maize was related to the mean larval age. The implications of this model on the understanding of ECB tunnelling behavior, damage potential, and pest management is noted. © 1992 Kluwer Academic Publishers.

Abstract:

A large-scale study was conducted in 1996 to evaluate and demonstrate strategies for pest management of Bemisia tabaci (Gennadius) in cotton involving different insecticide regimes, application methods, and action thresholds. Here we examined the effects of the various management systems on the abundance and activity of native natural enemies. Population densities of 18 out of 20 taxa of arthropod predators were significantly higher in regimes initiated with the insect growth regulators (IGRs) buprofezin (chitin inhibitor) or pyriproxyfen (juvenile hormone analog) compared with a regime dependent on a rotation of conventional, broad-spectrum insecticides. There were no differences in predator density between the two IGR regimes, and generally no effects due to application method or action threshold level. Predator to prey ratios were significantly higher in regimes utilizing the two IGRs compared with the conventional regime, but were unaffected by application method or threshold level. Rates of parasitism by Eretmocerus eremicus Rose and Zolnerowich and Encarsia meritoria Gahan were higher in the IGR regimes compared with the conventional regime, but were unaffected by insecticide application method, or the action threshold used to initiate applications of the IGRs. Results demonstrate the selective action of these two IGRs and suggest that their use may enhance opportunities for conservation biological control in cotton systems affected by B. tabaci, especially relative to conventional insecticide alternatives.

We used computer simulations to examine evolution of resistance to the insect growth regulator (IGR) pyriproxyfen by the sweetpotato whitefly, Bemisia tabaci (Gennadius), biotype B [=Bemisia argentifolii (Bellows & Perring)]. Consistent with trends seen in cotton (Gossipyium spp.) fields in Arizona and Israel, results suggest that evolution of resistance to pyriproxyfen may occur rapidly in this haplodiploid insect. Similar to results from models of diploid insects, resistance evolved faster with increases in toxin concentration, dominance of resistance in females, the initial frequency of the resistance allele, and the proportion of the region treated with pyriproxyfen. Resistance was delayed by fitness costs associated with resistance. Movement between treated and untreated cotton fields had little effect, probably because untreated cotton leaves provided internal refuges in treated fields and whiteflies were controlled with other insecticides in external refuges. Resistance evolved faster when susceptibility to pyriproxyfen was greater in susceptible males than susceptible females. In contrast, resistance evolved slower when susceptibility to pyriproxyfen was greater in resistant males than resistant females. Results suggest that growers may be able to prolong the usefulness of pyriproxyfen by applying lower toxin concentrations and promoting susceptible populations in refuges.