Effects Of Host Body Size And Age On Preference And Development Performance In Brachymeria Lasus (Hymenoptera: Chalcididae)

Relationship between parasitization preference and progeny performance is one of central themes in evolutionary ecology. For most parasitoids, host body size is usually regarded as the main accordance in oviposition strategy, which is known in "host size-quality"hypothesis. However, host age also has impact on oviposition strategy of parasitoids, for its changes will be along with physiological transformation which will affect the developmental performance of offspring parasitoids. So far, there have been two methods to test "host size-quality" hypothesis, one is using different host species with wide body size variation; the other is using the same host specie with broad range in body size. However, the former had unconquerable problems in inheritance differences; the latter relyed on the variation in body size in the same host specie. So in this thesis study, to test the "host size-quality" hypothesis,4th instar larvae of P. rapae were manipulated by starvation whose pupae were exposed to parasitism by B. lasus. Then the oviposition behaviors of mother parasitoids; the development performances of offspring parasitoids were recorded. Meanwhile, host age was tested as another impact factor. The main results are as followed below.1. Influence of host pupa weight on parasitization selection and offspring developmental performance of Brachymeria lasusTo test the "host size-quality" hypothesis,4th instar larvae of P. rapae were manipulated by starvation regime to obtain a wide range of host body size, and these pupae were individually exposed to parasitism by the solitary parasitoid B. lasus. Then the behaviors of parasitization selection of mother parasitoids; development time, sex ratio and body size of offspring parasitoids were recorded. The results showed that (ⅰ) searching time and preening time were noncorrelated with host body size; ovipositioning time was correlated with host size positively while the resting and creeping time was correlated with that negatively; (ii) the results of non-choice and choice experiment suggested that parasitoids prefer to lay eggs in larger host pupae, and more females are produced from larger host pupae. The body size of male and female offsprings were increased with host body size in a linear relationship, yet the development time remained constant regardless of host body size. The results indicate that the influence of host pupa weight on parasitization decision and offspring developmental performance of B. lasus are in accordance with the predictions of "host size-quality" hypothesis.2. Effects of host pupa age on parasitization decision and offspring developmental performance of Brachymeria lasusTo investigate the effects of host pupa age on parasitization decision and offspring developmental performances of the parasitoids, larvae of P. rapae were manipulated by starvation regime to obtain a wide range of host body size. Those pupae at age of either 1-day,3-day, and 5-day old were individually exposed to parasitism. The results showed that (i)there was no interaction between host body size and host age; (ii) host age did not have impact on probability of female offsprings, which was only affected by host body size,; (iii) there was a weak yet significant relationship between female development time and host age, but other developmental performances (including male development time,offspring lifespan and body size)were not influenced by host age. The results suggest that host pupa age does not affect the parasitization decision of B. lasus, but it has impact on offspring development to some extent.3. The effect of learning in parasitization selection of B. lasusTo assess whether parasitization experience would affect the latter oviposition behaviors of B. lasus or not, two observation values—selection frequency and host recognition time were selected to observe and analyze. Under choice condition, two host pupae with starvation treated and another two without were put on a potted cabbage to observe the first and succecive ovipostion of a single parasitoid. The results were followed (i)the Chiqs-test of selection frequency showed that those parasitoids with parasitization experience would prefer to lay eggs on larger host pupae significantly; (ii)however, parasitization experience did not influence the parasitzation preference of B. lasus when it was evaluated by host recognition time.