Alfalfa (Medicago sativa L.) hosts several species of aphid, Acyrthosiphon pisum (Harris), Aphis craccivora Koch and Therioaphis trifolii (Monell). The preference of the aphids of alfalfa plants for dense assemblies or individual plants, as well as for healthy or infested plants, was investigated in the field as in the laboratory. Years of field research have revealed the specific preferences of all three species of aphid. A. pisum and T. trifolii are most commonly found in alfalfa crops, while A. craccivora is mostly found on alfalfa weeds. Also, a single species of aphid alone is usually present on a plant. In order to determine the reason for this clear preference and to establish whether at the very beginning, i.e. at the stage of choosing a host, aphid species distance themselves from each other, we tested the effect of the volatiles of healthy and infested plants on their attractiveness to aphids. A. craccivora is repelled by the volatiles of dense crops and plants previously infested with one of the other two species. A. pisum and T. trifolii choose a dense assembly of plants, repelled by the volatiles of plants previously infested with A. craccivora. A. pisum displays the weakest competitive traits, and A. craccivora the strongest. This research showed that competition between aphid species does not occur only when they find themselves on the same plant at the same time, fighting for resources, but also in the choice of plant, in order to avoid later competition.

For insect herbivores, the quality of the larval host plant is a key determinant of fitness. Therefore, insect populations are supposed to be positively correlated with the nutritional quality of their host plant. This study aimed to determine if and how different varieties of grapes (including the wild grape Lambrusque) affect both larval and adult performance of the polyphagous European grapevine moth Lobesia botrana (Denis & Schiffermüller). Significant differences were found in larval development time, but not in pupal mass, adult emergence rate, or sex ratio. Although the fecundity of females is not different among varieties, females fed on some varieties produced eggs of different sizes which are correlated to their fertility. Thus, females adapt resource allocation to eggs depending on their diet as larvae. Using a fitness index, the average reproductive output was found to be highest for females reared on cv. Chardonnay. Females reared on wild grape produced a fitness index identical to the cultivated grapes. However, Lambrusque and Gewurztraminer separate themselves from the cultivated varieties according to our discriminant analyses. It is emphasized, through this study, that cultivars fed on by larvae should be considered in the population dynamics of L. botrana and that egg number is insufficient to determine host plant quality.

The sensory receptors on the antennal flagellar segments and on the rostral tip of four adult hemipteran insects were identified and compared using scanning electron microscopy in order to better understand the role of sensillae in the process of host plant selection and the structural adaptations of feeding organs for phytophagy. The species studied are Riptortus pedestris Fabricius (Alydidae), Elasmolomus sordidus (Fabricius) (Lygaeidae), Cyclopelta siccifolia Westwood (Pentatomidae) and Chrysocoris purpurea (Westwood) (Pentatomidae). Differences in the distribution and arrangement of sensory receptors in the hemipteran families were noted, especially in the shape of the antennal apices, which are of taxonomical significance. A large array of trichoid basiconic and coeloconic sensillae is distributed on the antennal surface. Distal antennal segments have more sensillae and more types of sensillae than the proximal segments, indicating their prominent role in host plant surface exploration. There is only a slight variation in the shape of the stylets owing to the similarity in feeding habits. The mandibular tips are straight and sharp and do not contain any hooks or barbs as in carnivorous species.