Parasitoid load affects plant fitness in a tritrophic system

Plants attacked by herbivorous insects emit volatile compounds that attract predators or parasitoids of the herbivores. Plant fitness increases when these herbivorous insects are parasitized by solitary parasitoids, but whether gregarious koinobiont parasitoids also confer a benefit to plant fitness has been disputed. We investigated the relationship between parasitoid load of the gregarious Cotesia glomerata (L.) (Hymenoptera: Braconidae), food consumption by larvae of their host Pieris brassicae L. (Lepidoptera: Pieridae), and seed production in a host plant, Brassica nigra L. (Brassicaceae), in a greenhouse experiment. Plants damaged by caterpillars containing single parasitoid broods produced a similar amount of seeds as undamaged control plants and produced significantly more seeds than plants with unparasitized caterpillars feeding on them. Increasing the parasitoid load to levels likely resulting from superparasitization, feeding by parasitized caterpillars was significantly negatively correlated with plant seed production. Higher parasitoid brood sizes were negatively correlated with pupal weight of Cotesia glomerata, revealing scramble competition leading to a fitness trade-off for the parasitoid. Our results suggest that in this tritrophic system plant fitness is higher when the gregarious parasitoid deposits a single brood into its herbivorous host. A prediction following from these results is that plants benefit from recruiting parasitoids when superparasitization is prevented. This is supported by our previous results on down-regulation of synomone production when Brassica oleracea was fed on by parasitized caterpillars of P. brassicae. We conclude that variable parasitoid loads in gregarious koinobiont parasitoids largely explain existing controversies about the putative benefit of recruiting these parasitoids for plant reproduction

Contrasting effects of heat pulses on different trophic levels, an experiment with a herbivore-parasitoid model system

Under predicted global climate change, species will be gradually exposed to warmer temperatures, and to a more variable climate including more intense and more frequent heatwaves. Increased climatic variability is expected to have different effects on species and ecosystems than gradual warming. A key challenge to predict the impact of climate change is to understand how temperature changes will affect species interactions. Herbivorous insects and their natural enemies belong to some of the largest groups of terrestrial animals, and thus they have a great impact on the functioning of ecosystems and on the services these ecosystems provide. Here we studied the life history traits of the plant-feeding insect Plutella xylostella and its specialist endoparasitoid Diadegma semiclausum, when exposed to a daily heat pulse of 5 or 10°C temperature increase during their entire immature phase. Growth and developmental responses differed with the amplitude of the heat pulse and they were different between host and parasitoid, indicating different thermal sensitivity of the two trophic levels. With a +5°C heat pulse, the adult parasitoids were larger which may result in a higher fitness, whereas a +10°C heat pulse retarded parasitoid development. These results show that the parasitoid is more sensitive than its host to brief intervals of temperature change, and this results in either positive or negative effects on life history traits, depending on the amplitude of the heat pulse. These findings suggest that more extreme fluctuations may disrupt host-parasitoid synchrony, whereas moderate fluctuations may improve parasitoid fitness. (Résumé d'auteur

Micro-organismen beschermen planten tegen rupsenvraat

Samenvatting van de voordracht te houden op 30 november 2005 tijdens de Najaarsvergadering van de KNPV (Koninklijke Nederlandse Plantenziektekundige Vereniging). Onderzoek naar inductie van resistentie in Arabidopsis tegen vraat van rupse

Chemical Mediation of Oviposition by Anopheles Mosquitoes : a Push-Pull System Driven by Volatiles Associated with Larval Stages

The oviposition behavior of mosquitoes is mediated by chemical cues. In the malaria mosquito Anopheles gambiae, conspecific larvae produce infochemicals that affect this behavior. Emanations from first instar larvae proved strongly attractive to gravid females, while those from fourth instars caused oviposition deterrence, suggesting that larval developmental stage affected the oviposition choice of the female mosquito. We examined the nature of these chemicals by headspace collection of emanations of water in which larvae of different stages were developing. Four chemicals with putative effects on oviposition behavior were identified: dimethyldisulfide (DMDS) and dimethyltrisulfide (DMTS) were identified in emanations from water containing fourth instars; nonane and 2,4-pentanedione (2,4-PD) were identified in emanations from water containing both first and fourth instars. Dual-choice oviposition studies with these compounds were done in the laboratory and in semi-field experiments in Tanzania. In the laboratory, DMDS and DMTS were associated with oviposition-deterrent effects, while results with nonane and 2,4-PD were inconclusive. In further studies DMDS and DMTS evoked egg retention, while with nonane and 2,4-PD 88% and 100% of female mosquitoes, respectively, laid eggs. In dual-choice semi-field trials DMDS and DMTS caused oviposition deterrence, while nonane and 2,4-PD evoked attraction, inducing females to lay more eggs in bowls containing these compounds compared to the controls. We conclude that oviposition of An. gambiae is mediated by these four infochemicals associated with conspecific larvae, eliciting either attraction or deterrence. High levels of egg retention occurred when females were exposed to chemicals associated with fourth instar larvae.</p

Role of Large Cabbage White butterfly male-derived compounds in elicitation of direct and indirect egg-killing defenses in the black mustard

To successfully exert defenses against herbivores and pathogens plants need to recognize reliable cues produced by their attackers. Up to now, few elicitors associated with herbivorous insects have been identified. We have previously shown that accessory reproductive gland secretions associated with eggs of Cabbage White butterflies (Pieris spp.) induce chemical changes in Brussels sprouts plants recruiting egg-killing parasitoids. Only secretions of mated female butterflies contain minute amounts of male-derived anti-aphrodisiac compounds that elicit this indirect plant defense. Here, we used the black mustard (Brassica nigra) to investigate how eggs of the Large Cabbage White butterfly (Pieris brassicae) induce, either an egg-killing direct [i.e., hypersensitive response (HR)-like necrosis] or indirect defense (i.e., oviposition-induced plant volatiles attracting Trichogramma egg parasitoids). Plants induced by P. brassicae egg-associated secretions expressed both traits and previous mating enhanced elicitation. Treatment with the anti-aphrodisiac compound of P. brassicae, benzyl cyanide (BC), induced stronger HR when compared to controls. Expression of the salicylic (SA) pathway- and HR-marker PATHOGENESIS-RELATED GENE1 was induced only in plants showing an HR-like necrosis. Trichogramma wasps were attracted to volatiles induced by secretion of mated P. brassicae females but application of BC did not elicit the parasitoid-attracting volatiles. We conclude that egg-associated secretions of Pieris butterflies contain specific elicitors of the different plant defense traits against eggs in Brassica plants. While in Brussels sprouts plants anti- aphrodisiac compounds in Pieris egg-associated secretions were clearly shown to elicit indirect defense, the wild relative B. nigra, recognizes different herbivore cues that mediate the defensive responses. These results add another level of specificity to the mechanisms by which plants recognize their attackers

The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity

We present the results of our survey of 1612 MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud. We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from IR stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust driven winds: vexp / ZL0:4. We compare the sub-solar metallicity LMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic Bulge. For 8 of the Bulge stars we derive pulsation periods for the first time, using near-IR photometry from the VVV survey. We have modeled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass loss rates by scaling the models to the results from maser profiles. We have done this also for samples in the Galactic Centre and Bulge and derived a new mass loss prescription that includes luminosity, pulsation period, and gas-to-dust ratio _M = 1:06+3:

Plant feeding by Nesidiocoris tenuis: Quantifying its behavioral and mechanical components

Zoophytophagous predators play an important, though sometimes controversial, role in pest management programs in different crops. In tomato crops, damage caused by phytophagy of the mirid Nesidiocoris tenuis has mainly been reported at high predator population levels or when prey is scarce. Previous research has focused on predator/prey ratios, stylet morphology and saliva composition to explain plant damage by N. tenuis. In this study, we investigated the behavioral and mechanical components of the damage. For this, we compared the feeding behaviors of males, females and fifth-instar nymphs of N. tenuis. Additionally, we investigated the type of stylet activities performed by each stage while probing in plant tissue, using the electrical penetration graph technique (EPG). Furthermore, stylectomy was performed and plant histology studied with the aim to correlate the feeding activities observed in the EPG recordings with stylet tip positions in specific tissues of the leaf petioles. Behavioral observations during a 30-min period showed that nymphs probed more frequently (38.6 ± 1.5 probes) than males and females (25.3 ± 1.1 and 24.3 ± 1.1 probes, respectively). Similarly, nymphs spent a higher proportion of time (656.0 ± 67.6 s) feeding on tomato apical sections compared to males and females (403.0 ± 48.8 s and 356.0 ± 43.7 s, respectively). The EPG recordings during 5 h indicated that cell-rupturing was the main stylet activity for all insect stages, and that fifth-instar nymphs spent a higher proportion of time on cell-rupturing events compared to adults. The histological studies revealed a trend of N. tenuis for the tissues within the vascular semi-ring. The stylet tips were found both in the vascular bundles and in the parenchyma of the interfascicular region. The findings of this study confirm an important role of fifth-instar nymphs feeding behavior in the damage potential of N. tenuis. Moreover, the increased time spent on cell rupturing behaviour suggests that stylet laceration and enzymatic maceration of the saliva occurring during this event might greatly contribute to the inflicted damage. A comprehensive understanding of the interactions of N. tenuis with the plant, at both the behavioral and mechanical levels, might shed light on new approaches to minimize its damage potential to tomato while maintaining its benefits as biocontrol agent