Response to Tobacco Alkaloids and Neonicotinoid Pesticides in Green Peach Aphid (Myzus persicae).

MERCHÁN, HECTOR ALEJANDRO. Response to Tobacco Alkaloids and Neonicotinoid Pesticides in Green Peach Aphid (Myzus persicae). (Under the direction of Hannah J. Burrack and Fred Gould.) In this dissertation I analyzed different aspects about the relationship between the green peach aphid (Myzus persicae) and its control in cultivated tobacco (Nicotiana tabacum) using the neonicotinoid pesticide imidacloprid. I focused on this system because imidacloprid targets the nicotinic acetylcholine receptor in insects and this is the same target affected by the alkaloid nicotine, present in tobacco. This relationships offers very interesting possibilities in the study of evolution of pesticide tolerance or resistance and adaptation. Also, tobacco is a very important commodity for North Carolina and I was also interested in possible improvements to pest management practices that could come from our experiments and benefit stakeholders. For the first chapter I studied the effectiveness and longevity of imidacloprid under field conditions using bioassays with M. persicae. It is clear that the time period in which a systemic pesticide is effective under field conditions has wide implications for pest management. Pesticide concentration should be lethal when the pest attacks the crop but low when the crop is marketed. Our aim in this study was to determine the duration of insecticidal activity of systemic pesticides commonly used in tobacco, by measuring the levels of insect infestations on field plots, as well as the effect of the pesticide on reproduction and survival of a target pest in controlled bioassays. Plants were treated with different concentration of two systemic, neonicotinoid pesticides, imidacloprid and thiamethoxam, planted and managed using standard agronomic techniques, except for insect control. I used the green peach aphid as the target pest (Myzus persicae, Sulzer), because it is a common pest in tobacco in the United States, and is readily reared under laboratory conditions. Our results show that the systemic pesticides used are very effective under field conditions against the green peach aphid for at least 13 weeks after transplant. Pesticides also affected the reproductive output of adult aphids and the survival of small nymphs in bioassays, but these results were not always statistically significant, and some aphids survived on pesticide-treated leaves. Our results also suggest that physical and chemical aspects of leaf maturation seem to affect survival in aphids. I showed that current management techniques using systemic neonicotinoid pesticides are very effective against the green peach aphid, but that pesticide longevity is less important for aphid management in the second half of the growing season. For the second chapter I analyzed how the systemic concentration of imidacloprid present within the leaves affect establishment and reproduction in Myzus persicae. One of our aims in these experiments was to understand how aphids are exposed to the insecticide. Imidacloprid is applied systemically and moves from the root to the leaves through the xylem. It appears that imidacloprid is the stored in the leaf and concentrations can increase with leaf age. Aphids, on the other hand, feed on the phloem sap, so it is not completely clear how they are exposed to imidacloprid. One possibility is that aphids are exposed through the cuticle, as trichomes in tobacco release small amounts of nicotine and other alkaloids in their exudates. To test this hypothesis I put a barrier between treated and untreated leaves and the aphid that prevented contact with the trichomes, but allowed feeding. I demonstrated that feeding is the most likely route of exposure. Another interesting aspect of imidacloprid is that it elicits a feeding deterrence response when present in low concentrations in the leaf. I tested if aphids are able to locate untreated plants in an environment where different concentrations of the insecticide are present. I developed bioassays with four, equidistant leaf discs. In one set of experiments, I tested the response when all discs had the same concentration of insecticide and in another set of experiments all four discs had different concentrations. I used behavioral observations from the no-choice experiments to predict the distribution of live adult aphids in the choice arenas. I found that a proportion of aphids are able to locate the untreated leaf and settle on it. I also found that the leaf disc with an intermediate concentration of insecticide shows higher levels of settled adults than expected, suggesting that aphids eventually stop responding to the insecticide and can feed on treated plants. I also analyzed the role of age and starvation in aphid establishment a found that aphids show signs of senescence and produce a lower number of nymphs when they are older. Low levels of starvation can induce a hormetic response that increases fecundity in Myzus persicae.Together these experiments allow us to improve our understanding of the colonization process of tobacco treated with imidacloprid. For the third chapter I tested levels of tolerance to imidacloprid in different matrilines collected from four research stations in North Carolina. I also compared the level of tolerance to nicotine and imidacloprid between aphid clones that feed on tobacco and clones that do not. I collected 12 lines in each of the four research stations and tested their level of tolerance to imidacloprid by measuring the fecundity of two adults exposed to leaves treated with different concentrations of imidacloprid. I also measured the survival of the nymphs the adults produced in the bioassays. I found evidence of higher tolerance on populations from Oxford and Clayton. I then tested 16 of those lines with more adults per assay and a larger number of insecticide concentrations and did not find any evidence of significant difference in tolerance between the different populations. I also compared adult responses to different concentrations of technical grade imidacloprid in an artificial diet. These experiments found a relatively higher level of tolerance to imidacloprid in adults from the Oxford populations, but I did not find that difference when looking at survival of the nymphs. Lastly, I compared levels of tolerance between two non-tobacco aphid clones and four tobacco clones. Our results suggest that there is a correlation between high tolerance to nicotine and high tolerance to imidacloprid. Tobacco aphids, which are adapted to nicotine, are able to tolerate higher levels of nicotine when compared to the non-tobacco clones. The results of these experiments show that there was not a high amount of variation in the levels of tolerance to imidacloprid in our sampled populations. While I did find some significant differences in tolerance in aphids coming from Oxford, the results were not consistent in all experiments. I believe that tobacco aphids are more likely to develop resistance to imidacloprid and they are already pre-adapted to tolerate some level of the insecticide. These results suggest that I need to keep monitoring for the possible evolution of resistance to imidacloprid in our tobacco fields. Response to Tobacco Alkaloids and Neonicotinoid Pesticides in Green Peach Aphid (Myzus persicae) by Hector Alejandro Merchán A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy