An oviposition stimulant binding protein in a butterfly: Immunohistochemical localization and electrophysiological responses to plant compounds

Mixtures of Milkweed Cardenolides Protect Monarch Butterflies against Parasites

Plants have evolved a diverse arsenal of defensive secondary metabolites in their evolutionary arms race with insect herbivores. In addition to the bottom-up forces created by plant chemicals, herbivores face top-down pressure from natural enemies, such as predators, parasitoids and parasites. This has led to the evolution of specialist herbivores that do not only tolerate plant secondary metabolites but even use them to fight natural enemies. Monarch butterflies (Danaus plexippus) are known for their use of milkweed chemicals (cardenolides) as protection against vertebrate predators. Recent studies have shown that milkweeds with high cardenolide concentrations can also provide protection against a virulent protozoan parasite. However, whether cardenolides are directly responsible for these effects, and whether individual cardenolides or mixtures of these chemicals are needed to reduce infection, remains unknown. We fed monarch larvae the four most abundant cardenolides found in the anti-parasitic-milkweed Asclepias curassavica at varying concentrations and compositions to determine which provided the highest resistance to parasite infection. Measuring infection rates and infection intensities, we found that resistance is dependent on both concentration and composition of cardenolides, with mixtures of cardenolides performing significantly better than individual compounds, even when mixtures included lower concentrations of individual compounds. These results suggest that cardenolides function synergistically to provide resistance against parasite infection and help explain why only milkweed species that produce diverse cardenolide compounds provide measurable parasite resistance. More broadly, our results suggest that herbivores can benefit from consuming plants with diverse defensive chemical compounds through release from parasitism.

Electrophysiological and Morphological Characterization of Contact Chemosensilla in Adults and Larvae of the Butterfly, Atrophaneura alcinous

Distribution and electrophysiological responses of contact chemosensilla were examined in the Aristolochiaceae-feeding butterfly Atrophaneuraalcinous. In adult butterflies, tarsal contact chemosensilla of the foreleg were classified into two groups based on length: long- and short-type sensilla. Long-type sensilla were distributed much more widely in females than in males, whereas short-type sensilla were found at the edge of the tarsi in a similar manner in both sexes. Taste responses of the long- and short-type sensilla to methanol extracts of Aristolochia debilis and Citrus spp. were recorded. Aristolochia debilis extracts evoked spikes with different amplitudes, whereas Citrus spp. extracts evoked spikes with a single amplitude in the long-type sensilla. Short-type sensilla did not respond to either extract. Moreover, we recorded responses to different concentrations of sucrose and NaCl. Results suggest that adult butterflies can discriminate the taste of host plant components from other chemicals using long-type sensilla during oviposition and may recognize diets containing sugar and salts during feeding using short-type sensilla. In the larval mouthparts, there were lateral and medial styloconic sensilla on the maxillary galea and epipharyngeal sensillum on the epipharynx. Electrophysiological responses of these sensilla suggest that larvae can discriminate between host plant compounds.

Floral Volatiles from Vigna unguiculata Are Olfactory and Gustatory Stimulants for Oviposition by the Bean Pod Borer Moth Maruca vitrata

We investigated the role of floral odors from cowpea, Vigna unguiculata (L.), in mediating oviposition of the bean pod borer moth, Maruca vitrata, a serious pest of grain legumes that flies to host plants at the flowering stage and oviposits onto flowers and buds. The flower of the host plant V. unguiculata was a stimulus for egg-laying by M. vitrata in an oviposition bioassay. Commercial longifolene, β-caryophyllene, linalool, geraniol, and (Z)-3-hexenyl acetate were used as stimulus. Each one elicited dose-dependent electroantennogram responses in female M. vitrata, and all but longifolene stimulated oviposition, when presented singly. Beta-caryophyllene was the most active stimulant, similar to that of the flower of V. unguiculata, and eliciting a dose-dependent oviposition response. Either olfaction or gustation was sufficient to mediate an oviposition response to V. unguiculata floral volatiles: intact M. vitrata responded to β-caryophyllene whether or not they could contact the source of the volatiles, and females with amputated antennae responded if allowed to contact the source. We believe this is the first demonstration in a moth where β-caryophyllene from the host plant was able to mediate an oviposition response. As β-caryophyllene is widely expressed by non-host plants, we suggest that its role in stimulating oviposition could be exploited as part of a push–pull strategy for pest management in which β-caryophyllene-expressing non-host plants provide a population sink for M. vitrata.

The role of proboscis of the malaria vector mosquito Anopheles stephensi in host-seeking behavior

BACKGROUND

The proboscis is an essential head appendage in insects that processes gustatory code during food intake, particularly useful considering that blood-sucking arthropods routinely reach vessels under the host skin using this proboscis as a probe.

RESULTS

Here, using an automated device able to quantify CO(2)-activated thermo (35°C)-sensing behavior of the malaria vector Anopheles stephensi, we uncovered that the protruding proboscis of mosquitoes contributes unexpectedly to host identification from a distance. Ablation experiments indicated that not only antennae and maxillary palps, but also proboscis were required for the identification of pseudo-thermo targets. Furthermore, the function of the proboscis during this behavior can be segregated from CO(2) detection required to evoke mosquito activation, suggesting that the proboscis of mosquitoes divide the proboscis into a "thermo-antenna" in addition to a "thermo-probe".

CONCLUSIONS

Our findings support an emerging view with a possible role of proboscis as important equipment during host-seeking, and give us an insight into how these appendages likely evolved from a common origin in order to function as antenna organs.