Responses of the entomopathogenic nematode,Steinernema riobraveto its insect hosts,Galleria mellonellaandTenebrio molitor

Potential hosts for infective juveniles of entomopathogenic nematodes can vary considerably in quality based on the characteristics of the host species/stage, physiological status (e.g. stress, feeding on toxins), and infection status (heterospecific or conspecific infection). In this study, we investigated responses of the entomopathogenic nematodeSteinernema riobraveto hosts (Galleria mellonellaorTenebrio molitor) that were previously parasitized with conspecifics or injected with the nematode-symbiotic bacterium,Xenorhabdussp., to determine if there is a preference for previously parasitized/injected hosts and when this preference might occur. In no-choice bioassays, the number of juveniles infecting both host species decreased with increasing time post-infection. However, infective juveniles continued to infect previously parasitized hosts up to 72 h. Significant preference was exhibited byS. riobravefor 24 h post-infectionG. mellonellalarvae over uninfected, and by 24 h post-injectionG.mellonellalarvae over 48 h post-injection larvae. No significant preference was exhibited byS. riobraveforT. molitorhosts previously parasitized with conspecifics or those injected with bacteria in any treatment combination. Such preference for, or continued infection of parasitized insects, has the potential to impact nematode efficacy.

Attraction behaviour of three entomopathogenic nematode species towards infected and uninfected hosts

Entomopathogenic nematode infective juveniles are likely to encounter both uninfected and infected insects and host quality depends on the stage of the infection. We hypothesized that nematode response to infected hosts will change over the course of an infection. Here, we tested this hypothesis by focusing on the influence of host infection status on long-range attraction to host volatile cues. The attraction response of 3 nematode species (Steinernema carpocapsae, S. glaseri and S. riobrave) with different foraging strategies to infected and uninfected insects (Galleria mellonella and Tenebrio molitor) was tested at 24 h intervals from start of infection to emergence of infective juveniles from depleted host. As expected, based on their foraging strategies, S. carpocapsae was not very responsive to hosts, S. glaseri was highly responsive and S. riobrave was intermediate. Generally, the level of attraction did not change with time after infection and was similar between infected and uninfected hosts. An exception was S. glaseri infected T. molitor, which tended to be less attractive to S. glaseri than uninfected hosts. These results suggest that any influence of host infection status on infection behaviour is occurring at subsequent steps in the host-infection process than host attraction, or involves non-volatile cues.

Parasitoid-host endocrine relations: self-reliance or co-optation?

High titers of juvenile hormone (JH) maintain developmental arrest in Manduca sexta larvae parasitized by Cotesia congregata. Parasitized hosts exhibit up to 9.5 times greater amounts of total hemolymph JH (from 0.6+/-0.09 to 2.51+/-0.43ng/ml) compared to non-parasitized controls. Elevated titers are observed throughout the fifth instar, even beyond egression of the parasitoids on day 5. GC-MS analysis revealed that in hemolymph of unparasitized control larvae, JH I is the major homolog and levels of JH III are negligible; in parasitized individuals the amounts of JH I, II, and III rise, and JH III predominates. Neck ligation ensured separation of M. sexta's corpora allata from the posterior section, which contained most of the parasitoids in the infected insects. When the posterior region was sampled, JHs were not detected in the non-parasitzed larvae, but in those parasitized, JH III was found (1.98+/-0.29ng/ml, 24 h post-ligation). JH III was the only homolog produced and secreted by the parasitoid in in vitro culture. This is the first report stating that a parasitoid secretes JH III and may contribute, at least in part, to the circulating titer in the host hemocoel, concurrently promoting host production of JH I and II.

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.

Choristoneura fumiferana entomopoxvirus prevents metamorphosis and modulates juvenile hormone and ecdysteroid titers

Larvae of the spruce budworm, Choristoneura fumiferana, infected with C. fumiferana entomopoxvirus (CfEPV) continue to feed and grow without undergoing metamorphosis and die as moribund larvae. The lethal dose (LD(50)) and lethal time (LT(50)) values for fourth instar larvae are 2.4 spheroids and 25.2 days, respectively. One hundred percent of the control fourth instar larvae, which were fed water instead of virus, pupated by 18 days post feeding (PF). Only 30% of the larvae that were fed the LD(50) dose and none of the larvae that were fed the LD(95) dose pupated by 18 days PF. Of the control larvae, 95% became adults by 24 days PF, whereas in the treated group only 2% of larvae that were fed the LD(50) dose and none of the larvae that were fed the LD(95) dose became adults by 24 days PF. Some of the virus-treated larvae died as either larval/pupal or pupal/adult intermediates. These phenotypic effects were similar to the larval/pupal and pupal/adult intermediates, resulting from treating larvae with juvenile hormone (JH) or its analogs, which suggests that EPV may cause such abnormalities by modulating JH and/or ecdysteroid titers. In untreated sixth instar larvae the JH titer decreased to low levels by 24 h after ecdysis and remained low throughout larval life. EPV-fed sixth instar larvae had 2112 pg/ml on day 0, 477 pg/ml on day 1 and 875 pg/ml on day 8 of the sixth instar. Control larvae contained 860 ng of ecdysteroids per ml hemolymph on day 8 of the sixth instar, whereas EPV-treated larvae of the same age (30 days PF) had only 107 ng of ecdysteroids per ml of hemolymph. Thus, EPV infection results in increased JH titer and decreased ecdysteroid titer. Northern hybridization analysis was performed using RNA isolated from control and EPV-fed larvae and cDNA probes for (i) juvenile hormone esterase (JHE), which is JH inducible, (ii) Choristoneura hormone receptor 3 (CHR3), which is ecdysteroid inducible, and (iii) larval specific diapause associated protein 1 (DAP1), whose expression is larval specific. EPV-treated larvae showed higher levels of JHE and DAP1 mRNA and lower levels of CHR3 mRNA, indicating that they had higher levels of JH and lower levels of ecdysteroids. Thus, our data show that EPV prevents metamorphosis by modulating ecdysteroid and JH levels.

Sex pheromones of Callosobruchus subinnotatus and C. maculatus (Coleoptera: Bruchidae): congeneric responses and role of air movement

Females of Callosobruchus spp. are known to produce sex pheromones that attract males. These sex pheromones cannot be adopted for use in pest management without first investigating the responses of the males in the windless conditions of storage environments. Consequently, behavioural bioassays of Callosobruchus subinnotatus Pic males were conducted in an olfactometer in the absence of air-flow. Under these conditions males were found to be able to follow odour trails to the source. However, the latency period was longer in diffusional bioassays than for insects in a Y-tube olfactometer that provided directional wind cues. The highest percentage of males reached the pheromone source when components of the pheromones, (E)-3-methyl-2-heptenoic acid (E32A) and (Z)-3-methyl-2-heptenoic acid (Z32A), were formulated in a 50:50 or 25:75 ratio. Males of C. maculatus (Fabricius) responded to sex pheromone of C. subinnotatus, but males of C. subinnotatus did not respond to that of C. maculatus. The two sex pheromone components of C. subinnotatus are also constituents of C. maculatus sex pheromone. These two components may be potentially useful in monitoring the populations of both species in stored beans. It is postulated that (Z)-3-methyl-3-heptenoic acid (Z33A), the major component of the sex pheromone of C. maculatus, must have acted as an antagonist inhibiting response of C. subinnotatus to the sex pheromone of C. maculatus.

Influence of juvenile hormone and mating on oogenesis and oviposition in the codling moth, cydia pomonella

Oogenesis in the codling moth, Cydia pomonella, and the role of juvenile hormones (JHs) were addressed. Rudimentary ovarian structures were recognisable in day 3-4 pupae, when haemolymph JH was still undetectable by coupled gas chromatography-mass spectrometry in the selected ion mode (GC-MS/SIM). The presence of developing oocytes was observed by light microscopy on day 8, coincident with very low JH titres (0.74 +/- 0.05 ng/ml JH II). Chorionation was only evident upon emergence, following an increase in JH in the pharate adult (0h old: 4.71 +/- 0.34 ng/ml JH II). Analysis of haemolymph from virgin and mated females indicated that JH II was predominant, with approximately equal and lower quantities of JHs I and III (3.3- to 5.0-fold less). When pupae or newly emerged adults were treated with JH homologues, no alteration in ovarian protein content was apparent, but the JH mimetic, fenoxycarb, depressed the number of oocytes filling >/= 50% follicular volume. Chorion deposition was stimulated by JHs I, II, or III (10 &mgr;g), but not by fenoxycarb (0.05 &mgr;g, 10 &mgr;g). Mating provided correct stimuli for enhanced choriogenesis and egg laying, and, since haemolymph JH titres were concomitantly elevated (approximately 2-fold), it was postulated that the rise in JH elicited both these events. Application of JHs to virgin females, however, could not mimic mating; only increases in choriogenesis were induced: JH-treatment of virgins (or mated insects) significantly decreased oviposition rates over 24 and 48 h and markedly reduced the life-time total number of eggs. Arch. Copyright 1999 Wiley-Liss, Inc.

Mating in Heliothis virescens: transfer of juvenile hormone during copulation by male to female and stimulation of biosynthesis of endogenous juvenile hormone

Studies were undertaken to determine whether adult males of Heliothis virescens transfer juvenile hormone (JH) to females during copulation, and an in vitro radiochemical assay was used to determine whether mating causes an allatotropic effect, i.e., stimulation of JH biosynthesis by corpora allata (CA). In vitro, CA from 3-day-old mated females synthesized and released approximately 2.5 times total JH as that of CA from comparably aged virgin females. Of the homologues, JH II exhibited significant increase in mated females; JH I also increased but not significantly. JH III remained similar to that of virgin females. This is the first demonstration of an allatotropic effect of mating in moths. In contrast to the female, CA of virgin males did not produce any JH, but accessory sex glands (ASG) in 3-day-old males synthesized small amounts of JH. Immediately after adult emergence, male ASG contained approximately 1.5 ng JH I and II, which increased by 12 h after emergence and remained at this high level up to 54 h after emergence. JH III was barely detected in ASG. JH in ASG of mated male immediately after uncoupling was depleted almost completely, and 24 h later recovered to levels comparable to that of 54-h-old virgin male. Virgin female bursa copulatrix did not contain any JH, but mated female bursa, immediately after uncoupling, had JH at levels comparable to that observed in virgin male ASG. By 6 h after uncoupling, JH levels decreased dramatically in mated female bursa. These data suggest the transfer of JH to females by the male.

Pheromonotropic and pheromonostatic activity in moths

Pheromone biosynthesis in many species of moths requires a pheromonotropic neurosecretion, the pheromone biosynthesis activating neuropeptide (PBAN), from the brain-subesophageal ganglion-corpora cardiaca complex. Some investigations suggest that PBAN is released into the hemolymph and acts directly on sex pheromone glands (SPG) via a Ca++/calmodulin-dependent adenylate cyclase. Others suggest, however, that PBAN acts via octopamine that is released by nerves from the terminal abdominal ganglion innervating the SPG. These findings suggest that there are controversies on the mode of action of PBAN and other pheromonotropic factors, sometimes even within the same species. Mating in many insects results in temporary or permanent suppression of pheromone production and/or receptivity. Such a suppression may result from physical blockage of the gonopore or deposition of pheromonostatic factor(s) by the male during copulation that result in suppressed pheromone production and/or receptivity in females either directly or by a primer effect. In several species of insects, including moths, a pheromonostatic factor is transferred in the seminal fluid of males. Similar to the controversies associated with the pheromonotropic activity of PBAN, sometimes even within the same species, there appear to be controversies in pheromonostasis in heliothines as well. This paper reviews these conflicting findings and presents some data on pheromonostatic and pheromonotropic activity in Heliothis virescens that support and conflict with current information, raising further questions. Answers to some of the questions are partly available; however, they remain to be answered unequivocally.