A bodyguard or a tastier meal? Dying caterpillar indirectly protects parasitoid cocoons by offering alternate prey to a generalist predator

In some parasitic Hymenoptera the dying caterpillars remain attached or close to the parasitoid cocoons. It has been suggested that the caterpillars act as ‘bodyguards’ for the vulnerable cocoons and therefore protect them against predators and/or hyperparasitoids (the ‘usurpation hypothesis’). This hypothesis has been demonstrated in associations where the caterpillars remain active and/or aggressive after parasitism. However, in other associations the caterpillars are so physiologically depleted after parasitism that they are unable to physically defend the cocoons and instead sit atop them in a moribund state. In this study a generalist predator, the spined soldier bug, Podisus maculiventris Say (Hemiptera: Pentatomidae), was provided with cocoons of the gregarious endoparasitoid Cotesia glomerata L. and the solitary endoparasitoid Microplitis mediator Haliday (both Hymenoptera: Braconidae), in turn attended by their hosts, Pieris brassicae L. (Lepidoptera: Pieridae) and Mamestra brassicae L. (Lepidoptera: Noctuidae), respectively. Cotesia glomerata produces broods of up to 40 cocoons and the dying caterpillars sit atop the cocoons where they exhibit little response to physical stimuli. Previous studies reported that dying P. brassicae caterpillars were ineffective bodyguards against two species of hyperparasitoids. In both associations, the dying host caterpillars were significantly preferred as food by P. maculiventris over the parasitoid cocoons. However, in absence of caterpillars, the bugs readily attacked the C. glomerata cocoons. Alternatively, the survival of M. mediator was very low, irrespective of whether a caterpillar was present or not. Caterpillars attacked by M. mediator are several times smaller than those attacked by C. glomerata. Consequently, the predators ran out of food much more quickly in the former and switched from one prey to the other. We show that in some host–parasitoid associations the dying caterpillars provide more visually apparent or nutritionally superior prey, rather than acting as bodyguards.     

Midgut proteases from Mamestra configurata (Lepidoptera: Noctuidae) larvae: characterization,cDNA cloning,and expressed sequence tag analysis

The activities of digestive protease within the midgut of Mamestra configurata (bertha armyworm) larvae were examined using specific substrates and protease inhibitors. The bulk of the activity was associated with serine proteases comprising trypsin-, chymotrypsin-, and elastase-like enzymes. At least 10-15 serine protease isozymes were detected using one-dimension gelatin gel electrophoresis. Cysteine or aspartic protease activities were not present; however, amino- and carboxypeptidase activities were associated with the midgut extract. Midgut proteases were active in the pH range of 5.0-12.0 with peaks at pH 7.5 and 11.0. In general, the middle region of the midgut exhibited a higher pH (approximately 8.0) than either the posterior or anterior regions (approximately 7.3-7.7). Moulting larvae possessed a neutral gut pH that was 0.5-1.5 units below that of feeding larvae. Degenerate PCR and expressed sequence tag (EST)-based approaches were used to isolate 30 distinct serine protease encoding cDNAs from a midgut-specific cDNA library including 8 putative trypsins, 9 chymotrypsins, 1 elastase, and 12 whose potential activities could not be determined. cDNAs encoding three amino- and two carboxypeptidases were also identified. Larvae feeding upon artificial diet containing 0.2% soybean trypsin inhibitor experienced a significant delay in development.     

Endocrine changes during diapause development in the cabbage army moth Mamestra brassicae

Many insects undergo diapause to survive adverse seasons. Although the mechanism of diapause induction is the subject of extensive study, that of diapause termination remains poorly understood. In the present study, we show the endocrine processes leading to the termination of pupal diapause in Mamestra brassicae. Diapause of this insect is terminated if the pupae are exposed to a low temperature for several weeks. During this period, the prothoracic glands (PGs) of pupae acquire the potential to secrete sufficient ecdysteroids necessary for inducing adult development. The main endocrine changes observed under the low temperature conditions are: (i) the increase in activity of the PGs in two steps; (ii) the increase in responsiveness of the glands to prothoracicotropic hormone (PTTH); and (iii) two‐step increase in PTTH gene expression in the brain. The timing of the first and second increases in PG activity roughly coincides with that of the two steps of increase in PTTH gene expression, and the timing of the increase in the responsiveness of the PGs to PTTH coincides with the second, larger increase in PTTH gene expression. The ablation of the PGs prior to cooling pupae does not affect the increase in PTTH gene expression, whereas brain removal results in a failure to increase PG activity, strongly suggesting that PTTH is the master regulator of diapause development and termination.     

Development of spermiduct and seminal vesicle during pharate adult of the cabbage armyworm,Mamestra brassicae, (Lepidoptera; Noctuidae)

Summary

Development of the upper vas deferens and seminal vesicle, and the ecdysteroid titers of the cabbage armyworm (Mamestra brassicae) during pharate adult stage were investigated to provide evidence of endogenous control of the sperm movement through the reproductive tract. Apparently, development of the upper vas deferens is initiated when ecdysteroid titers increase after pupation and continues until eclosion. Sperm movement correlates with the decrease of ecdysteroid titers. When ecdysteroid titers remained at low levels, sperm release from the testis was observed one day before eclosion.     

With or without you: Effects of the concurrent range expansion of an herbivore and its natural enemy on native species interactions

Global climatic changes may lead to the arrival of multiple range‐expanding species from different trophic levels into new habitats, either simultaneously or in quick succession, potentially causing the introduction of manifold novel interactions into native food webs. Unraveling the complex biotic interactions between native and range‐expanding species is critical to understand the impact of climate change on community ecology, but experimental evidence is lacking. In a series of laboratory experiments that simulated direct and indirect species interactions, we investigated the effects of the concurrent arrival of a range‐expanding insect herbivore in Europe, Spodoptera littoralis, and its associated parasitoid Microplitis rufiventris, on the native herbivore Mamestra brassicae, and its associated parasitoid Microplitis mediator, when co‐occurring on a native plant, Brassica rapa. Overall, direct interactions between the herbivores were beneficial for the exotic herbivore (higher pupal weight than the native herbivore), and negative for the native herbivore (higher mortality than the exotic herbivore). At the third trophic level, both parasitoids were unable to parasitize the herbivore they did not coexist with, but the presence of the exotic parasitoid still negatively affected the native herbivore (increased mortality) and the native parasitoid (decreased parasitism rate), through failed parasitism attempts and interference effects. Our results suggest different interaction scenarios depending on whether S. littoralis and its parasitoid arrive to the native tritrophic system separately or concurrently, as the negative effects associated with the presence of the parasitoid were dependent on the presence of the exotic herbivore. These findings illustrate the complexity and interconnectedness of multitrophic changes resulting from concurrent species arrival to new environments, and the need for integrating the ecological effects of such arrivals into the general theoretical framework of global invasion patterns driven by climatic change.     

Wipfelkrankheit: modification of host behaviour during baculoviral infection

Infection with an endoparasite frequently alters host behaviour. This study provides the first quantification of larval behaviour in a baculovirus/ Lepidoptera system, and attempts to assess the ecological consequences of behavioural modification during infection. Larvae of the moth Mamestra brassicae (Lepidoptera: Noctuidae) exhibited higher rates of dispersal in the laboratory and field when infected with Mamestra brassicae nuclear polyhedrosis virus (MbNPV) than did uninfected larvae. They adopted positions at death which were not characteristic of healthy larvae, climbing higher on the foodplant and onto the top and edge of leaves. The horizontal and vertical distribution of virus following larval lysis and the effects of rainfall on this distribution were assessed for comparison with the distributions of healthy and infected larvae. Exposure to rainfall increased the infectivity of vegetation in bioassays. Alternative explanations for the evolutionary origins of behavioural modification are considered. I suggest that the behavioural changes observed are most likely to benefit the virus. In particular, climbing prior to death is likely to result in contamination of more foliage with virus particles than would otherwise occur by increasing exposure of cadavers to rainfall. Thus it may profoundly influence horizontal transmission and the dynamics of the host-virus interaction. Received: 12 April 1996 /Accepted: 12 August 1996     

Temporal distribution of PBAN‐like immunoreactivity in the hemolymph of Mamestra brassicae females in relation to sex pheromone production and calling behavior

A direct enzyme‐linked immunosorbent assay has been developed and applied to the analysis of PBAN immunoreactivity in female hemolymph of the cabbage armyworm, Mamestra brassicae. PBAN‐IR determinations have been carried out with third scotophase insects at different times of the photoperiod. The rhythm of calling and the pattern of pheromone production by third scotophase females at different times of the photoperiod have also been determined. PBAN‐IR and calling are well correlated. However, whereas pheromone titers decrease, both PBAN‐IR levels and percentage of calling females remain high in the last hours of the scotophase. These results are discussed in the context of the regulation of sex pheromone biosynthesis in M. brassicae. Arch. Insect Biochem. Physiol. 40:80–87, 1999. © 1999 Wiley‐Liss, Inc.