Gerrit Jacobs

Wolbachia" intracellular manipulators of mite reproduction

Cytoplasmically transmitted Wolbachia (alpha-Proteobacteria) are a group of closely related intracellular microorganisms that alter reproduction in arthropods. They are found in a few isopods and are widespread in insects. Wolbachia are implicated as the cause of parthenogenesis in parasitic wasps, feminization in isopods and reproductive (cytoplasmic) incompatibility in many insects. Here we report on the widespread occurrence of Wolbachia in spider mites and predatory mites based on a PCR assay for a 730 bp fragment of the ftsZ gene with primers that are specific for Wolbachia. An additional PCR, using two primer pairs that amplify a 259 bp region of the ftsZ gene that are diagnostic for the two Wolbachia subdivisions A and B, showed that infected mites only carried type B and not type A Wolbachia. The fact that some species tested negative for Wolbachia does not mean that the entire species is uninfected. We found that natural populations of Tetranychus urticae are polymorphic for the infection. The possible effects of Wolbachia on mite reproduction and post-zygotic reproductive isolation are discussed.

Predators use volatiles to avoid prey patches with conspecifics

1. Simple models of optimal foraging, such as ideal free distribution models, are based on the assumption that foragers are omniscient with respect to the quality of all patches in the environment; they know how much food and how many competitors are present in each patch. 2. In contrast, simple population dynamic models treat predator-prey distributions in a phenomenological way, and do not take fitness consequences for individual foragers into account. Yet, the precise way in which these distributions come into being is what really matters to population dynamics. It is therefore necessary to study the behavioural mechanisms underlying the distributions of foragers over patches. 3. We studied the behaviour of a predatory mite, Phytoseiulus persimilis, in response to prey patches occupied by conspecifics. It is well known that high predator densities in prey patches promote dispersal of these predatory mites. Our question was to what extent predators can assess the presence of conspecifics from a distance. 4. Experiments with a Y-tube olfactometer showed that predatory mites avoid patches occupied by conspecifics. 5. This avoidance cannot be attributed to odours of conspecific predators, or of prey damaged by predation, as these odour sources both appear to be attractive. 6. Separating the prey patch from the conspecific predators in the odour source led to the avoidance response only when the predators in the odour source were positioned upwind from the prey patch, and not when they were positioned downwind. This suggests that predators release an odour that elicits the production of yet another odour by the prey. This was supported by the observation that removal of adult prey led to a quick disappearance of the avoidance response. 7. We argue that distant discrimination between patches with and without competing conspecifics may be quite common among predators and parasitoids, and that the use of odours instead of physical inspection of patches allows predators to instantaneously integrate information on the distribution of food and competitors. 8. This behavioural mechanism may bring predators and parasitoids closer to behaving as ideal free foragers than was previously thought possible.

How Predatory Mites Learn to Cope with Variability in Volatile Plant Signals in the Environment of their Herbivorous Prey

When the chemical cues co-occurring with prey vary in time and space, foraging predators profit from an ability to repeatedly associate chemical cues with the presence of their prey. We demonstrate the ability of a predatory arthropod (the plant-inhabiting mite, Phytoseiulus persimilis) to learn the association of a positive stimulus (herbivorous prey, Tetranychus urticae) or a negative stimulus (hunger) with a chemical cue (herbivore-induced plant volatiles or green leaf volatiles). It has been suggested that the rate at which the integration of information becomes manifest as a change in behaviour, differs between categories of natural enemies (parasitoids versus insect predators; specialist versus generalist predators). We argue that these differences do not necessarily reflect differential learning ability, but rather relate to the ecologically relevant time scale at which the biotic environment changes.

Diet–dependent effects of gut bacteria on their insect host: the symbiosis of Erwinia sp. and western flower thrips

Studies on bacteria in the gut of insect species are numerous, but their focus is hardly ever on the impact on host performance. We showed earlier that Erwinia bacteria occur in the gut of western flower thrips, most probably acquired during feeding. Here, we investigate whether thrips gain a net benefit or pay a net cost because of these gut bacteria. On a diet of cucumber leaves, the time to maturity is shorter and the oviposition rate is higher in thrips with bacteria than in thrips without (aposymbionts). When fed on cucumber leaves and pollen, aposymbionts develop faster and lay more eggs. So Erwinia bacteria benefit or parasitize their thrips hosts depending on the diet, which is in accordance with theoretical predictions for fitness of organisms engaged in symbiotic interactions. Possibly, the transmission of gut bacteria has not become strictly vertical because of this diet–dependent fitness variability.

Onion Thrips, Thrips tabaci, Have Gut Bacteria That are Closely Related to the Symbionts of the Western Flower Thrips, Frankliniella occidentalis

Abstract It has been shown that many insects have Enterobacteriaceae bacteria in their gut system. The western flower thrips, Frankliniella occidentalis Pergande [Thysanoptera: Thripidae], has a symbiotic relation with Erwinia species gut bacteria. To determine if other Thripidae species have similar bacterial symbionts, the onion thrips, Thrips tabaci, was studied because, like F. occidentalis, it is phytophagous. Contrary to F. occidentalis, T. tabaci is endemic in Europe and biotypes have been described. Bacteria were isolated from the majority of populations and biotypes of T. tabaci examined. Bacteria were present in high numbers in most individuals of the populations studied. Like F. occidentalis, T. tabaci contained one type of bacterium that clearly outnumbered all other types present in the gut. This bacterium was identified as an Erwinia species, as was also the case for F. occidentalis. However, its biochemical characteristics and 16S rDNA sequence differed from the bacteria present in F. occidentalis.