Jeremy N. McNeil

Host-plant quality influences diapause and voltinism in a polyphagous insect herbivore

Facultative diapause is a strategy that allows insects to initiate an additional generation when conditions are favorable, or to enter diapause when they are not. The occurrence of additional generations within one year can have a profound effect on the ecology and evolution of a species. Most previous studies of the cues that induce facultative diapause in insects have concentrated on photoperiod and temperature. In contrast, we studied the effects of host-plant quality on the induction of larval diapause and subsequent voltinism in a polyphagous insect herbivore, Choristoneura rosaceana (Lepidoptera: Tortricidae). Field experiments demonstrated that the proportion of larvae entering diapause differed among four host-plant species. Larvae feeding on chokecherry were more likely to continue development and produce a second generation than larvae feeding on red maple or black ash. Paper birch was intermediate in diapause induction. Laboratory experiments with excised foliage supported the results of the field experiments and demonstrated that the induction of diapause depended on host species under constant conditions of photoperiod and temperature. A further study with low- and high-quality artificial diet demonstrated that the nutritional quality of larval diet has a direct effect on diapause induction: low-quality diet favored diapause induction. Dispersal of neonate larvae from host foliage did not necessarily reflect host quality and the potential for bivoltinism. We argue that the effect of plant quality on diapause induction in natural populations of insect herbivores merits more attention.

Seasonal microhabitat selection by an endoparasitoid through adaptive modification of host behavior

Differences in the distribution of parasitized and unparasitized hosts has been used to infer modification of host behavior by insect parasitoids. Data are presented showing that not only do parasitized hosts behave differently from unparasitized ones, but that the behavior of parasitized hosts varies in function of the physiological state of the parasitoid. Aphids containing nondiapausing parasitoids leave the aphid colony and mummify on the upper surface of the leaves, whereas those containing diapausing parasitoids leave the host plant and mummify in concealed sites. Modification of host behavior by diapausing parasitoids results in the selection of a suitable microhabitat that reduces the incidence of hyperparasitism and should decrease the action of adverse climatic conditions during the lengthy dormant period.

Puddling in butterflies: sodium affects reproductive success in Thymelicus lineola*

ABSTRACT. Adults of many species of Lepidoptera, principally the males, frequent mud puddles, edges of streams, carrion and animal excreta where they imbibe moisture, an activity referred to as ‘puddling’ Sodium ions are the only known stimulus present which cause males of at least two lepidopteran species to drink for extended periods. In the European skipper Thymelicus lineola (Ochsenheimer) (Lepidoptera: Hesperiidae), only males puddle, even though they have concentrations of abdominal sodium 2–3 times that of females at emergence. During their first mating, males transfer 32% of their abdominal sodium to females. This could be of considerable importance given that an average egg complement contains >50% of the total body sodium of females at emergence. Virgin females, as well as having reduced fecundity, have reduced longevity. This is attributed to virgins not obtaining important nutrients which males transfer to females during mating. Access to sodium ions increases the total number of matings by c. 50% for males living >15 days. Access to sodium ions by once‐mated males increases the percentage of males which re‐mate on the day following first mating; the percentage of females, mated to the twice‐mated males, which lay >50% fertile eggs; and the drought resistance of eggs laid by those females.

Host behaviour modification by the endoparasitoid Aphidius nigripes: a strategy to reduce hyperparasitism

Abstract. 1. One possible component of successful parasitism by insect parasitoids is the reduction of predation and hyperparasitism through the modification of host behaviour.

Biosynthesis and release of juvenile hormone and its precursors in insects and crustaceans: The search for a unifying arthropod endocrinology

It now appears that arthropods produce and release a wider variety of juvenile hormones (JH) and related compounds than previously thought. For instance, in the adult crayfish, Procambarus clarkii, the mandibular organs, the homologous structure to insect corpora allata (CA), release both farnesoic acid (FA) and methyl farnesoate (MF), the immediate precursors of JH III, but not JH III itself. In larvae of the cockroach Diploptera punctata, JH III production ceases during the last half of the 4th stadium, but the CA continue to produce and release FA throughout this period. The embryos of the same species also release JH III and a product that coelutes with MF on HPLC. In adult blowfly, Calliphora vomitoria, the CA release JH III bisepoxide and possibly the 6,7-epoxide, in addition to JH III. In the lepidopteran species Pseudaletia unipuncta, male CA produce and release JH acids I, II, and III as well as a product which we have tentatively identified as homo-(and/or) dihomo-FA. In the females, CA produce and release the three common JH homologues and a product that we believe is the esterified version of the male compound, homo/dihomo-MF. Although the release of JH precursors from their sites of synthesis might result in their conversion to the active hormone in peripheral tissues, there is only limited evidence for such a process. Studies on biological activities of these compounds and on the developmental changes in biosynthesis and its regulation should provide information necessary for the defining of these compounds as hormones or otherwise and should improve our understanding of the evolution of the JH biosynthetic pathway in the phylum Arthropoda.

CALLING BEHAVIOUR OF THE ARMYWORM, PSEUDALETIA UNIPUNCTA

The calling behaviour of virgin female armyworms, Pseudaletia unipuncta (Haw.), was studied at 25° ± 0.5° and 65 ± 5% RH under a 16L: 8D photoperiod. The age at which females started calling following emergence varied from 2 to 10 days, with a mean of 3.9 days. Due to these differences comparisons of calling behaviour on successive days were made using the “calling” age rather than age from emergence. The calling pattern of individual females, observed every 10 min, changed on successive days of calling. Over 7 days, the mean onset calling time advanced from 396 to 301 min after the onset of the scotophase, the total time spent calling per day increased from 62 to 182 min. the number of calling bouts decreased from 1.8 to 1.4 while the duration of a calling bout increased from 34 to 125 min. Entrained females held under 56 hr of darkness exhibited an endogenous circadian rhythm of calling. Observations taken every 10 min significantly underestimated the number of calling bouts per day (1.8 instead of 16 on the first day of calling) and overestimated the duration of each bout (34 min instead of 2 min on the first day of calling) when compared with the data collected from continuous observations. Values for the mean onset calling time and the time spent calling each day did not differ significantly using the two observation schedules.

Molecular characterization of a cDNA from Pseudaletia unipuncta encoding the Manduca sexta allatostatin peptide (Mas-AST)

A 15-residue neuropeptide, Manduca sexta allatostatin (Mas-AST), strongly inhibits juvenile hormone (JH) biosynthesis in vitro by corpora allata (CA) from Manduca fifth-stadium larvae and adult females as well as Helicoverpa zea adult females (Kramer et al., 1991 Proc. Natl. Acad. Sci (USA) 88, 9458-9462). In contrast, this study found that 1.0 microM Mas-AST has no JH biosynthesis inhibitory activity in Pseudaletia unipuncta sixth instar larvae or newly-emerged (day 0) adults but inhibited CA of 5-day-old adult females by 60%. From a P. unipuncta brain cDNA library, was isolated a cDNA that encodes a 125 amino acid polypeptide containing the Mas-AST sequence. Within the precursor, Mas-AST is situated at the carboxy terminus and is flanked by different dibasic proteolytic cleavage signals. The Pseudaletia gene specifying the Mas-AST peptide is present as a single copy per haploid genome. Expression of this gene was low in Pseudaletia sixth instar larvae, prepupae and early pupae but was relatively high in late pupae, and day 1 and 3 adults of both sexes. In day 5 adults, the relative transcript level appears to be maintained in females but declines in males. This pattern of Mas-AST expression does not correlate well with the profile of JH biosynthesis in Pseudaletia, which increases during the first 5 days of adult life, suggesting additional or alternative functions for this peptide.

Effects of nectar concentration on butterfly feeding: measured feeding rates for Thymelicus lineola (Lepidoptera: Hesperiidae) and a general feeding model for adult Lepidoptera

SummaryField observations of the adult European skipper, Thymelicus lineola (Ochs), feeding on concentrated nectars (40–65% sucrose) from a variety of flower species led us to question recent literature stating that butterflies feed primarily, and most effectively, on dilute nectars. Rate of sucrose solution intake, volume consumed and feeding duration were measured for males and females at 25 and 35°C under laboratory conditions. As sucrose concentration increased, the volume of solution ingested per meal first increased and then decreased gradually, while sucrose intake was highest at concentrations ≧40%. Females fed more than males at all concentrations >10% while temperature had no significant effect on meal size. Feeding duration increased with concentration, was shorter at 35 than at 25°C, and was longer for females than males.The rate of volume intake decreased as concentration incresed, but not nearly as rapidly as predicted by earlier models. Rates did not differ between the sexes but were faster at 35 than 25°C. This increase was contributed to equally by a reduction in viscosity and an increase in power output of the cibarial pump. The form of the relations was similar, with maximum rate of sucrose intake occurring at 40% sucrose.A new mathematical model was developed to describe the rate — concentration relation based on the Hagen-Poiseuille equation for laminar fluid flow through pipes. Our model differs from previous models principally in that the power output of the insects cibarial pump remains relatively constant while the pressure drop created by the pump to induce suction is highly variable. This change results in a very different feeding rate — sucrose concentration function with the optimal rate of sucrose intake at a concentration of approximately 40%. The model indicates that the same relation should hold for a wide range of proboscis shape and size and type of suction pump, and should therefore be applicable to all other nectar feeders with sucking mouth parts. Independent verifications of the model were carried out by measuring the rate of uptake of sucrose solutions of the adult common armyworm, Pseudaletia unipuncta (Haw.), and of human subjects using a volumetric pipette, both of which gave an excellent fit.Nectar concentrations which correspond to optimal rates of sucrose intake should be highly preferred by insects with high feeding costs, those which are time-limited, or which are very vulnerable while feeding. High transport costs and severe water stress may shift preferences to higher and lower concentrations respectively.

Host instar, body size and fitness in the koinobiotic parasitoid Aphidius nigripes

In aphidiine parasitoids, resources for growth and adult body size increase with host instar used by ovipositing females, but the fitness consequences of body size on fitness are poorly documented. We compared the fitness of male and female A. nigripes adults that varied in size as a consequence of developing in different instars of their host Macrosiphum euphorbiae. When reproductive fitness was measured without considering time, female wasps from small and large hosts performed similarly, contributing 125–175 foundresses plus 100–180 sons to the next generation. However, when expressed as the innate capacity for increase (rm), female fitness correlated with host‐induced variation of wasp size, indicating that micropopulations initiated by large wasps would increase faster. In a wind‐tunnel, a sex pheromone plume from large female wasps induced more males to fly upwind when released at a distance of 50 cm downwind than small females, indicating that large females were sexually more attractive. With respect to male body size effects on fitness, large individuals performed similar to small ones, whether fitness was measured by lifetime mating frequency, fertile inseminations, or proportion of daughters among progeny born from their mates. When young naive males of unequal size were directly competing for mating with a virgin female, small and large males had equal mating success, and large individuals were no more successful than small ones at displacing a competitor already positioned on a receptive female. In a wind‐tunnel test where males were scored on their ability to reach a female pheromone source, small and large males were equally affected by wind speed but reached the source located 50 cm downwind in equal proportions, suggesting similar capacity for finding mates by flying upwind. Our results indicate that despite host resources not being fixed at the time of attack for the koinobiont A. nigripes, fitness consequences of resource limitation by the mother may be perceived to be greater for daughters than sons, which would explain male‐biased sex ratio in early‐instar hosts.

Calling behaviour and pheromone titre of the true armyworm Pseudaletia unipuncta (Haw.) (Lepidoptera: Noctuidae) under different temperature and photoperiodic conditions

Abstract True armyworm females that were exposed to temperatures of 20, 15 or 10°C following their first night of calling at 25°C significantly advanced their mean onset time of calling compared to those maintained continuously at 25°C. The mean time spent calling also increased significantly following a 10 or 15°C decrease from 25°C, while a 5°C decrease had no effect. Only temperature conditions during the scotophase affected the overall expression of the overt calling behaviour, with those during the photophase significantly modifying neither the onset nor the duration of calling. Regardless of the time at which a 15°C decrease in temperature occurred during the first half of the scotophase, more than 45% of the females were already calling by the 5th hour of the scotophase, compared to 0% in the 25°C controls. This suggests that the absolute ambient temperature determines the time at which calling will be expressed, with the calling gate opening earlier under low night temperatures. However, when the temperature was increased from 10 to 25°C at 2 and 4 h after lights off (following a decrease from 25°C the first night of calling), some females started calling much earlier than would be expected if the expression of calling behaviour was uniquely related to ambient temperature. Data from other experiments indicated that temperature conditions during the previous calling period may, under certain conditions, modify calling behaviour. Both the proportion of glands containing pheromone and the pheromone titre/female varied in a periodic fashion during the photoperiodic cycle immediately following the first night of calling at 25°C, 16 h light:8 h dark. Both values decreased during the photophase, falling to zero by the onset of the scotophase, before increasing again during the latter half of the scotophase when overt calling occurred. Results obtained from females held under continuous darkness following their first night of calling confirmed that the periodicity of pheromone titre, like that of calling behaviour, is circadian. A good correspondence between the pheromone titre and calling behaviour was also observed under 12 h light:12 h dark at 25°C, even though the mean onset time of calling was significantly different than that at 16 h light:8 h dark, 25°C. Furthermore, pheromone-gland analyses of females subjected to a decrease in temperature from 25 to 10°C at different times during the photoperiod following the first night of calling confirmed that overt calling behaviour and increases in pheromone titre are closely linked.