Metabolism or behavior: explaining the performance of aphids on alkaloid-producing fungal endophytes in annual ryegrass (Lolium multiflorum).

Plant-herbivore interactions are often mediated by plant microorganisms, and the "defensive mutualism" of epichloid fungal endophytes of grasses is an example. These endophytes synthesize bioactive alkaloids that generally have detrimental effects on the performance of insect herbivores, but the underlying mechanisms are not well understood. Our objective was to determine whether changes in the physiology and/or behavior of aphids explain the changes in performance of insects feeding on endophytic plants. We studied the interaction between the aphid Rhopalosiphum padi and the annual ryegrass Lolium multiflorum symbiotic (E+) or not symbiotic (E-) with the fungus Epichloë occultans that can synthesize loline alkaloids. We hypothesized that aphids feeding on E+ plants have higher energetic demands for detoxification of fungal alkaloids, thereby negatively impacting the individual performance, population growth, and structure. Aphids growing on E+ plants had lower values in morphometric and functional variables of individual performance, displayed lower birth rate, smaller population size, and dramatic structural changes. However, aphids exhibited lower values of standard metabolic rate (SMR) on E+ plants, which suggests no high costs of detoxification. Behavioral variables during the first 8 h of feeding showed that aphids did not change the phloem sap ingestion with the presence of fungal endophytes. We hypothesize that aphids may maintain phloem sap ingestion according to their fungal alkaloid tolerance capacity. In other words, when alkaloid concentrations overcome tolerance threshold, ingestion of phloem should decrease, which may explain the observed lower values of SMR in E+ feeding aphids.

Feeding Behavior and Virus-Transmission Ability of Insect Vectors Exposed to Systemic Insecticides.

The majority of plant viruses depend on Hemipteran vectors for their survival and spread. Effective management of these insect vectors is crucial to minimize the spread of vector-borne diseases, and to reduce crop damage. The aim of the present study was to evaluate the effect of various systemic insecticides on the feeding behavior of Bemisia tabaci and Myzus persicae, as well as their ability to interfere with the transmission of circulative viruses. The obtained results indicated that some systemic insecticides have antifeeding properties that disrupt virus transmission by their insect vectors. We found that some of the tested insecticides significantly reduced phloem contact and sap ingestion by aphids and whiteflies, activities that are closely linked to the transmission of phloem-limited viruses. These systemic insecticides may play an important role in reducing the primary and secondary spread of tomato yellow leaf curl virus (TYLCV) and turnip yellows virus (TuYV), transmitted by B. tabaci and M. persicae, respectively.

Probing behavior of aposymbiotic green peach aphid (Myzus persicae) on susceptible Solanum tuberosum and resistant Solanum stoloniferum plants.

The green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae) is one of the potato important pests; it is the most efficient vector of potato viruses. Myzus persicae harbors the endosymbiotic bacteria Buchnera aphidicola which supplements their diet. There is increasing evidence that B. aphidicola is involved in plant-aphid interactions and we previously demonstrated that B. aphidicola disruption (aposymbiosis) affected the probing behavior of M. persicae on radish plants, delaying host plant acceptance. In this work, we evaluated the effect of aposymbiosis on the probing behavior of M. persicae on 2 Solanum species with different compatibility with M. persicae, Solanum tuberosum (susceptible) and Solanum stoloniferum (resistant) with the electrical penetration graph technique (EPG). To disrupt B. aphidicola, rifampicin was administered to aphids through artificial diets. Aposymbiotic aphids, on both plant species, showed increased pathway activities, mechanical problems with the stylets, and delayed salivation in the phloem. The extended time in derailed stylet mechanics affected the occurrence of most other probing activities; it delayed the time to the first phloem phase and prevented ingestion from the phloem. The effect of aposymbiosis was more evident in the compatible interaction of M. persicae-S. tuberosum, than in the incompatible interaction with S. stoloniferum, which generated the M. persicae-S. tuberosum interaction to become incompatible. These results confirm that B. aphidicola is involved in the plant-aphid interaction in relation to plant acceptance, presumably through a role in stylets penetration in the plant.