Oviposition Tactics In Response To Aphids, Intraspecific Competition And Predation Risks In The Aphidophagous Aphidoletes Aphidimyza (Diptera:Cecidomyiidae)

Aphidoletes aphidimyza (Rondani) is a specialist predator of aphids, and often used in biological control of aphids in greenhouses. At present, most of studies on it focus on biological characteristics and control effect, but less attention has been given toward oviposition tactics in response to conspecific competition and pradtory risks from intragiod predators. To investigate oviposition strategy of female A. aphidimyza, following questions were addressed in this thesis study. (1) Do female midges adjust oviposition behavior measured as number of eggs laid in response to aphid density and nymph stages? (2) Does female A. aphidimyza adjust its oviposition decisions in response to the presence of conspecific eggs and larvae? (3) Does female A. aphidimyza adapt its oviposition decisions in response to the presence of predaceous ladybird beetls? This study would provide noval data to understand A. aphidimyza in foraging behavioral ecology and give us a better understanding in using A. aphidimyza in biocontrol practices of aphids in greenhouses. The main results and conclusions are summarized below.1. Effects of aphid density and age on oviposition behavior of A. aphidimyzaTwo laboratory trials were conducted, manipulating Rhopalosiphum padi density as from 1 to 63 aphids/barley leaf and aphid ages as 1-,3-and 5-day old levels, respectively. Each female midge was separately exposed to a given aphid density or age for oviposition. The number of eggs laid was recorded and egg hatching observed in incubators. The results showed that females laid more eggs on aphid patches of higher densities. Female midges laid fewer eggs on aphid patches composed of the 5-day (10.5±1.28) than 1-day (17.60± 1.84) or 3-day (21.35±2.36) old nymphs. However, there were no significant differences between 1 day and 3-day treatments. Egg hatching rates forthe 3-day old aphids(95.88%± 1.52%) was significantly higher than that for 1-day(86.25%±3.39%)and 5-day old aphids(91.83%±3.36%). The results suggest that female A. aphidimyza can adjust its oviposition decisions in response to aphid density and age.2. Effects of the presence of conspecific eggs and larvae on oviposition behavior of A. aphidimyzaTo examine oviposition behavior of A.aphidimyza in relation to egg densities previously laid by conspecifics and the presence of conspecific larvae we recorded number of eggs laid and measured egg hatching rates.The presence of conspecific eggs significantly influenced the following female midge in its oviposition behavior. The subsequent female laid fewer eggs on aphid patches composing more conspecific eggs in aphid colonies. The presence of midge larvae in aphid patches had significant influence on female midges in number of eggs laid. The female midge laid 3 times more eggs (19.37±2.58) on the aphid patch without a midge larva than with a 2-day old midge larva (6.63±1.70), but same number of eggs as that with a 1-day old larva (15.43±2.07). There were no significant differences in egg hatching rates among treatments.3. Effects of the presence of ladybirds on oviposition behavior of A. aphidimyzaWe tested effects of the presence of Propylaea japonica eggs, larvae and the control with non-choice and choice experiments in greenhouse, on female midges in daily oviposition until death.In the non-choice experiment, the test included another factor aphid density maniputed as 5,10,20,40 and 80 aphis/leaf. The choice trial showed obvious preferencefor the control (16.3±2.1 eggs) as compared to the presence of ladybird beetle eggs(5.4±0.9)and larvae(3.7±0.6), increasing by 77.23% and 66.77% respectively. There were however no significant difference between ladybird egg and larva treatements. The results of this trial suggested that A. aphidimyza prefer aphid patches without ladybird beetles over those with. The non-choice trial showed no significant effect of P. japonica eggs and larvae on A. aphidimyza survival. However, aphid density had significant influence on female midge survival. The survival time of a midge increased by about 1% with each additional aphid in the patch, The presence of P. japonica and aphid density have no significant interactions for A. aphidimyza survival. The presence of P. japonica eggs and larvae and aphid density independently influenced A. aphidimyza oviposition. Under the same aphid density, female midges laid fewer eggs in the presence of ladybug eggs or larvae than without, reducing by about 0.3 times (ladybird eggs) and 1.17 times (ladybug larvae). Disregarding the presence of ladybirds or not, the female laid more eggs on aphid patches at higher densities, with about 1% more eggs laid with each additional aphid present. The presence of P. japonica, aphid density and age of femal midges significantly influenced the number of eggs laid by A. aphidimyza. Furthermore, there was an three-level interaction between the them. The female laid more eggs on aphid patches at higher densities when it was in younger age, increasing laying eggs by about 2% with each additional aphid, but decreasing by about 49% with each additional day in age. However, with the effect of the three-level interaction, the number of eggs laid increased by about 0.35%(in the presence ladybird eggs) and 0.84% (in the presence of ladybug larvae) with each additional aphid in the patch. The female adult laid more eggs when it was 3-day old and in aphids patches with 80 aphids but without ladybird beetles, but laid fewer eggs when less thanlO aphids and ladybird beetles were present. The results of this trial suggested that female midges choose to lay more eggs in aphid patches with more aphids, at earlier ages of aphids, and without ladybird beetles.