Thomas A. Verschut

Sensory mutations in Drosophila melanogaster influence associational effects between resources during oviposition

Neighboring resources can affect insect oviposition behavior when the complexity of sensory information obscures information about host resource availability in heterogeneous resource patches. These effects are referred to as associational effects and are hypothesized to occur through constraints in the sensory processing of the insect during host search, resulting into suboptimal resource use. Because the possibilities to study these constraints on naturally occurring animals are limited, we instead used sensory mutants of Drosophila melanogaster to determine the importance of sensory information in the occurrence of associational effects. We found that oviposition was mainly governed by non-volatile chemical cues and less by volatile cues. Moreover, the loss of gustatory sensilla resulted in random resource selection and eliminated associational effects. In conclusion, our study shows that associational effects do not necessarily depend on constraints in the sensory evaluation of resource quality, but may instead be a direct consequence of distinctive selection behavior between different resources at small scales.

Natal origin affects host preference and larval performance relationships in a tritrophic system

Abstract Many insects face the challenge to select oviposition sites in heterogeneous environments where biotic and abiotic factors can change over time. One way to deal with this complexity is to use sensory experiences made during developmental stages to locate similar habitats or hosts in which larval development can be maximized. While various studies have investigated oviposition preference and larval performance relationships in insects, they have largely overlooked that sensory experiences made during the larval stage can affect such relationships. We addressed this issue by determining the role of natal experience on oviposition preference and larval performance relationships in a tritrophic system consisting of Galerucella sagittariae, feeding on the two host plants Potentilla palustris and Lysimachia thyrsiflora, and its larval parasitoid Asecodes lucens. We firstly determined whether differences in host‐derived olfactory information could lead to divergent host selection, and secondly, whether host preference could result in higher larval performance based on the natal origin of the insects. Our results showed that the natal origin and the quality of the current host are both important aspects in oviposition preference and larval performance relationships. While we found a positive relationship between preference and performance for natal Lysimachia beetles, natal Potentilla larvae showed no such relationship and developed better on L. thyrsiflora. Additionally, the host selection by the parasitoid was mainly affected by the natal origin, while its performance was higher on Lysimachia larvae. With this study, we showed that the relationship between oviposition preference and larval performance depends on the interplay between the natal origin of the female and the quality of the current host. However, without incorporating the full tritrophic context of these interactions, their implication in insect fitness and potential adaptation cannot be fully understood.

Behavioural and physiological evidence for palp detection of the male-specific attractant cuelure in the Queensland fruit fly (Bactrocera tryoni)

The Queensland fruit fly, Bactrocera tryoni, is considered one of the worst horticultural pests in Australia attacking a large variety of fruit crops. To defeat pest insects, olfactory attractants have been developed and widely used in lure and kill strategies. Male B. tryoni are strongly attracted to the compound raspberry ketone and its synthetic analog, cuelure. Despite the strong behavioral response, a recent study failed to show any activation of antennal receptors to cuelure. Therefore, we hypothesized that cuelure may be detected by an accessory olfactory organ, the maxillary palp. Combining behavioral and physiological experiments we clearly demonstrate that male flies, but not female flies, primarily use the maxillary palps and not the antennae to detect and respond to cuelure. Furthermore, regardless of satiety status, male flies always preferred cuelure over a sugar rich source, unless the maxillary palps were excised.

Inter- and intra-specific variation in movement behaviour of benthic macroinvertebrates from a transitional habitat: a laboratory experiment

The movement behaviour of vagile organisms has long been acknowledged as a key determinant of species distribution and biodiversity patterns. Yet, scant information is available for aquatic invertebrates; in addition, the effects of inter-specific differences in morphology and intra-specific variations in body size have been inadequately investigated. In the present study, the movement behaviour of three crustacean species [Proasellus coxalis and Lekanesphaera hookeri (Isopoda) and Gammarus aequicauda (Amphipoda)] and two gastropod species (Ecrobia ventrosa and Bithynia leachii) dominating the macrobenthic assemblage of a Mediterranean transitional habitat (Giammatteo channel estuary, SE Italy) was analysed under resource-free laboratory conditions. The average path length and speed were determined for individuals encompassing a 16-fold range in body sizes. The scale-independent fractal dimension D was used to quantify the tortuosity of their trajectories. In general, significant differences were observed in movement metrics between crustaceans and gastropods; however, species-specific effects were obscured within each taxonomic group by a strong intra-specific variability. All crustacean species were characterized by a significant negative relationship between body size and path tortuosity; specifically, breaks in the slopes of the regression curve were observed, coinciding with sexual maturation. In contrast, negligible relationships were observed for gastropod species. The results of the study suggest that body size may represent a good predictor of movement behaviour for crustaceans but not for gastropods. Specifically, crucial steps of the biological cycle of amphipods and isopods may reflect on considerable variations in movement metrics. The metabolic and ecological implications of these findings are discussed.

Selection by parasitoid females among closely related hosts based on volatiles: Identifying relevant chemical cues

Abstract Parasitoid fitness is influenced by the ability to overcome host defense strategies and by the ability of parasitoid females to select high‐quality host individuals. When females are unable to differentiate among hosts, their fitness will decrease with an increasing abundance of resistant hosts. To understand the effect of mixed host populations on female fitness, it is therefore necessary to investigate the ability of female parasitoids to select among hosts. Here, we used behavioral assays, headspace volatile collection, and electrophysiology to study the ability of Asecodes parviclava to use olfactory cues to select between a susceptible host (Galerucella calmariensis) and a resistant host (Galerucella pusilla) from a distance. Our studies show that parasitoid females have the capacity to distinguish the two hosts and that the selection behavior is acquired through experiences during earlier life stages. Further, we identified two volatiles (α‐terpinolene and [E]‐β‐ocimene) which amounts differ between the two plant–herbivore systems and that caused behavioral and electrophysiological responses. The consequence of this selection behavior is that females have the capacity to avoid laying eggs in G. pusilla, where the egg mortality is higher due to much stronger immune responses toward A. parviclava than in larvae of G. calmariensis.

Sensory deficiencies affect resource selection and associational effects at two spatial scales

Abstract Many insect species have limited sensory abilities and may not be able to perceive the quality of different resource types while approaching patchily distributed resources. These restrictions may lead to differences in selection rates between separate patches and between different resource types within a patch, which may have consequences for associational effects between resources. In this study, we used an oviposition assay containing different frequencies of apple and banana substrates divided over two patches to compare resource selection rates of wild‐type Drosophila melanogaster at the between‐ and within‐patch scales. Next, we compared the wild‐type behavior with that of the olfactory‐deficient strain Orco 2 and the gustatory‐deficient strain Poxn ΔM22‐B5 and found comparable responses to patch heterogeneity and similarly strong selection rates for apple at both scales for the wild‐type and olfactory‐deficient flies. Their oviposition behavior translated into associational susceptibility for apple and associational resistance for banana. The gustatory‐deficient flies, on the other hand, no longer had a strong selection rate for apple, strongly differed in between‐ and within‐patch selection rates from the wild‐type flies, and caused no associational effects between the resources. Our study suggests that differences in sensory capabilities can affect resource selection at different search behavior scales in different ways and in turn underlie associational effects between resources at different spatial scales.

A random survival forest illustrates the importance of natural enemies compared to host plant quality on leaf beetle survival rates

BackgroundWetlands are habitats where variation in soil moisture content and associated environmental conditions can strongly affect the survival of herbivorous insects by changing host plant quality and natural enemy densities. In this study, we combined natural enemy exclusion experiments with random survival forest analyses to study the importance of local variation in host plant quality and predation by natural enemies on the egg and larval survival of the leaf beetle Galerucella sagittariae along a soil moisture gradient.ResultsOur results showed that the exclusion of natural enemies substantially increased the survival probability of G. sagittariae eggs and larvae. Interestingly, the egg survival probability decreased with soil moisture content, while the larval survival probability instead increased with soil moisture content. For both the egg and larval survival, we found that host plant height, the number of eggs or larvae, and vegetation height explained more of the variation than the soil moisture gradient by itself. Moreover, host plant quality related variables, such as leaf nitrogen, carbon and phosphorus content did not influence the survival of G. sagittariae eggs and larvae.ConclusionOur results suggest that the soil moisture content is not an overarching factor that determines the interplay between factors related to host plant quality and factors relating to natural enemies on the survival of G. sagittariae in different microhabitats. Moreover, the natural enemy exclusion experiments and the random survival forest analysis suggest that natural enemies have a stronger indirect impact on the survival of G. sagittariae offspring than host plant quality.