George N. Mbata

Effects of Temperature and Exposure Time on Mortality of Stored-Product Insects Exposed to Low Pressure

Abstract The prospects of using low pressure that creates a low-oxygen atmosphere to control stored-product insects were investigated in the laboratory. Eggs, larvae, and pupae of Tribolium castaneum (Herbst), Plodia interpunctella (Hübner), and Rhyzopertha dominica (F.) were exposed to 32.5 mmHg in glass chambers at 25, 33, 37, and 40°C for times ranging from 30 min to 144 h. Time-mortality data were subjected to probit analyses and lethal dose ratios were computed to determine differences in lethal time (LT) values among all species-life stage combinations across the four temperatures. Eggs of each species were the life stage most tolerant to low pressure. Pupae of T. castaneum and R. dominica were more tolerant to low pressure than larvae. In all life stages, mortality increased with increasing exposure time to low pressure and also with increasing temperature. Immature stages of R. dominica were more tolerant to low pressure than immature stages of the other two species. The LT99 for R. dominica eggs was 176.32 h at 25°C and that for P. interpunctella eggs was 28.35 h at the same temperature. An increase in temperature to 33°C resulted in a LT99 of 85.98 h for R. dominica and 6.21 h for P. interpunctella. Higher temperatures resulted in further significant reduction in lethal time values. Low pressure represents a simple, nonchemical alternative to fumigants such as methyl bromide and phosphine for controlling pests of stored-products or other commodities.

Sex pheromone production in Callosobruchus maculatus (Coleoptera: Bruchidae): electroantennographic and behavioral responses.

Behavioral and electroantennogram (EAG) assays were used to determine rhythmicity and dynamics of the sex pheromone biology of the bruchid beetle, Callosobruchus maculatus (F.). Beetles were sexually active throughout the photophase. Virgin females entrained to a 16:8 L:D light cycle emitted more sex pheromone during the first half of photophase than during the second half of photophase or during scotophase. Sex pheromone production increased from day 0 to day 3 of adult life, and then decreased as females grew older. Availability of garbonzo beans, one of the larval food and adult oviposition hosts, to females did not stimulate sex pheromone production. Mating resulted in a reduction in post-coital sex pheromone production. Mated females, however, regained attractiveness to males and this attractiveness increased from day 1 to day 3 after first mating when females had been mated at day 0 after emergence. (Z)-3-Methyl-2-heptenoic acid (Z32A) and (Z)-3-methyl-3-heptenoic acid (Z33A), two compounds implicated as sex pheromone components for C. maculatus, both elicited male EAG and behavioral responses in the beetles; however, Z33A elicited larger responses than Z32A. Binary mixtures of the two compounds were more effective than either compound alone.

Mortality of Life Stages of Cowpea Weevil (Coleoptera: Bruchidae) Exposed to Low Pressure at Different Temperatures

Abstract Previous studies have shown that low pressure creates a low oxygen controlled atmosphere that can kill stored-product insects. The current study was conducted to determine the mortality of life stages of the cowpea weevil, Callosbruchus maculatus (F.) (Coleoptera: Bruchidae), exposed to different low pressures and temperatures for various exposure periods. The adults were the most susceptible life stage to low pressure; 99% mortality was achieved within 0.8 h at 32.5 mmHg, 30°C. The pupae were the most tolerant life stage to low pressure, requiring exposure periods between 28.98 and 153.20 h at temperatures of 20–35°C to achieve 99% mortality. Mortality increased with exposure time and also with increasing temperature in all life stages. Early stage eggs (3 h old) and late stage eggs (48 h old) experienced higher mortality (values for LT99 of 42.331 and 46.652 h, respectively) compared with intermediate aged eggs (24 h old; LT99 of 74.735 h) under the same conditions of low pressure and temperature. Dried beans, including cowpea, Vigna unguiculata (Walp.), are currently protected with fumigants. Application of low pressure as a pest management tool represents a potential nonchemical alternative to fumigants such as methyl bromide and phosphine for controlling the cowpea weevil and related bruchids.

Some physical and biological factors affecting oviposition by Plodia interpunctella (Hubner) (Lepidoptera: Phycitidae)

Some factors affecting oviposition by Plodia interpunctella (Hubner) were studied under various controlled conditions in the laboratory. The effect of temperature, relative humidity, light and mating on oviposition were investigated.Temperatures of 25 and 30°C were optimal for oviposition. The best condition was 30°C and 80% r.h. at which an average of 315.3 eggs were laid per female during its lifetime. The least favourable temperatures investigated were 15 and 35°C. Relative humidity did not have significant effect on the number of eggs laid at 15 and 25°C but at higher temperatures of 30 and 35°C, high r.h. increased the number of eggs laid by individual moths.Light regimes of 12 hr light and 12hr darkness (LD) and 24 hr darkness (DD) were more favourable than 24 hr light (LL) for oviposition.Virgin females laid fewer eggs than mated females while prolonged virginity reduced the number of eggs laid and caused resorption of immature oocytes and retention of ripe eggs in the ovaries.RésuméQuelques facteurs affectant l’oviposition par les Plodia interpunctella (Hubner) ont été étudiés dans diverses conditions contrôlées au laboratoire. Les effets sur l’oviposition température, de l’humidité relative, de la lumière et de l’accouplement ont été examinés.Les températures de 25°C et de 30°C étaient les plus favorables à l’oviposition. La meilleure condition était celle de 30°C 80% r.h. dans laquelle on a obtenu la moyenne de 315, 3 ouefs par femelle le long de sa vie. Les températures les moins favorables constatées étaient de 15 et 35°C. L’humidité relative n’a pas eu un effet significatif sur le nombre d’oeufs pondus à 15 et 25°C, mais aux températures plus élevées de 30 et 35°C une humidité relative élevée a augmenté le nombre d’oeufs pondus par chaque mite.Les régimes d’illumination de 12 hr de lumière plus 12 hr d’obscurité (LD) et de 24 hr d’obscurité (DD) étaient plus favorables a’ l’oviposition que celui de 24 hr de lumière (LL).Les femelles vierges ont pondu moins d’oeufs que les femelles accouplées tandis que la virginité prolongée a réduit le nombre d’oeufs pondus et a causé la résorption d’oocytes non mûris et la retention d’oeufs mûrs dans les ovaries.

Rhythmicity of sex pheromone content in female Heliothis virescens : impact of mating

Abstract Pheromone production in virgin females of Heliothis virescens (F.) (Lepidoptera: Noctuidae) peaked between the fourth and seventh hours of scotophase on the second, third, fourth and fifth days following eclosion. The highest peak (186 ng) occurred on day 3 after eclosion. Z‐11‐Hexadecenal comprised the highest proportion of seven components in the pheromone glands. Disproportionately higher amounts of hexadecanal and Z‐11‐hexadecenol occurred during photo‐phase and other periods when low quantities of total pheromone were recorded.

Evaluation of susceptibility of varieties of cowpea to Callosobruchus maculatus (F.) and Callosobruchus subinnotatus (Pic.) (Coleoptera: Bruchidae)

Abstract Twenty-six varieties of cowpea some of which are among those distributed for international trials by the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria were evaluated in comparison with Ife Brown, a known susceptible variety, for susceptibility to Callosobruchus maculatus (F.) and Callosobruchus subinnotatus (Pic.). The most susceptible varieties to C. maculatus were TVu 310, TVu 2896, TVu 9944 and TVu 9836. Some varieties were found to be moderately susceptible to C. subinnotatus and these included TVu 18, TVu 36, TVu 2896 and IT81D-1020. Most varieties were found to be more susceptible to C. maculatus than to C. subinnotatus. However, the median developmental periods of the bruchids on the cowpea varieties were comparable in most cases. Some varieties were found to be consistently less susceptible to attack by the two bruchids and these included TVu 2027, IT81D-1064, IT81D-1157 and IT86-498. The weight losses due to infestation by the bruchids correlated very strongly to the susceptibility indices (SI) of the varieties to the bruchids. The losses in weight caused by C. subinnotatus were comparable with those caused by C. maculatus even though the number of F1 adults of C. maculatus that emerged from the varieties was higher than those of C. subinnotatus.

Necessity of juvenile hormone for choriogenesis in the moth, Heliothis virescens (Noctuidae).

Summary Hormones such as ecdysone, juvenile hormone and neurosecretions have been shown to regulate vitellogenesis, the process of yolk synthesis and deposition, in insects. Surprisingly, although choriogenesis, the process of eggshell formation, has been studied from biochemical, morphological, and genetic viewpoints, little is known of its hormonal regulation. We report here that juvenile hormone (JH) is essential for choriogenesis in the tobacco budworm moth, Heliothis virescens. Decapitated adult females are unable to deposit the chorion, or eggshell, while injection of JH III into decapitated females results in production of chorionated eggs comparable to that in normal females 24 h later. This suggests that JH alone is sufficient to restore choriogenic capabilities in decapitated moths. Treatment with 20-hydroxyecdysone does not restore choriogenesis in decapitated insects. Choriogenesis in virgin H. virescens females is inhibited in a dose-dependent manner by precocene II but not by precocene I.

Tolerance of pupae and pharate adults of Callosobruchus subinnotatus Pic. (Coleoptera: Bruchidae) to modified atmospheres: a function of metabolic rate.

Three developmental stages (pupae, early pharate and late pharate adults) of Callosobruchus subinnotatus (Pic.) were investigated for their tolerance or susceptibility to four modified atmospheres. Two of these atmospheres were hypercarbic and two were hypoxic. The hypercarbic atmospheres were found to cause mortality earlier than hypoxic atmospheres. Late pharate adults died earlier than pupae or early pharate adults. Late pharate adults that survived the exposure took a longer time to eclose than the pupae or early pharate adult.Using high resolution microrespirometric techniques, it was possible to record the oxygen consumption rate and CO(2) output of different developmental stages in air. The metabolic rate was determined manometrically as the oxygen uptake rate at an ambient temperature of 25 degrees C. The oxygen uptake rate differed significantly between groups of 20 individuals of different stages (p<0.01; t-test). The lowest rate of oxygen uptake (510.6+/-52.2 &mgr;l g(-1) h(-1)) was recorded in pupae. Higher oxygen uptake rates were found in early pharate adults (668.4+/-45.6 &mgr;l g(-1) h(-1)) and late pharate adults (1171.2+/-45.0 &mgr;l g(-1) h(-1)), and adult beetles (1310.4+/-53.4 &mgr;l g(-1) h(-1)). The patterns of CO(2) release were similar to those of oxygen uptake. CO(2) release was highest in eclosed adults and late pharate adults followed by early pharate adults, and lowest in pupae. The mode of CO(2) release ranged from continuous CO(2) release in pupae to discontinuous CO(2) release in late pharate and eclosed adults. Thus, high metabolic rates, and perhaps, in conjunction with discontinuous CO(2) of late pharate adults are responsible for their higher susceptibility to modified atmospheres than pupae and early pharate adults.

Rhythmicity of mating and oviposition inCallosobruchus subinnotatus (Pic) (Coleoptera: Bruchidae)

Mating and oviposition behaviors were studied inCallosobruchus subinnotatus. Copulation was most frequent during the late scotophase, 2–3 h before onset of photophase. The females were less willing to mate during photophase, which increased the time to initiate mating while decreasing the duration of mating. Females exhibited increased movement prior to mating, resting immediately after mating, and remained stationary for 6 h when oviposition commenced. Multiple mating by both males and females affected the number of eggs laid, duration of mating, and uncoupling time at the end of mating. Females that mated two or three times laid more eggs than females that mated once or more than three times. Females that remainedin copula for less than 18 min showed greater readiness to remate than those that remainedin copula longer. There was a gradual decrease in the number of eggs females could lay with an increase in the number of previous matings by males.

Role of juvenile hormone-esterase in mating-stimulated egg development in the moth Heliothis virescens.

Juvenile hormone (JH) titer in virgin females of Heliothis virescens is significantly lower than that in mated females of the same age. The JH titer in virgin females follows a diel pattern in which it begins to increase towards the end of photophase, remains high around the onset of scotophase, and declines during scotophase. The titer reaches its lowest levels at the onset of photophase, and remains low during the first half of photophase. In mated females, the diel pattern of JH titers is not as pronounced. JH-esterase (JHE) activity in mated females is significantly lower than that of virgin females during photophase; JHE levels in the former are similar to levels seen in newly emerged females. JHE activity in mated females also exhibits a diel pattern, in which activity is low during photophase and high at the onset of scotophase. Evidence for the indirect involvement of JHE in the mating-stimulated egg development is provided by the effect of selected JHE inhibitors in inhibiting JHE activity and stimulating egg production in virgin females.