Effects Of Feeding Plants Of Larvae On Oviposition Behaviors Of Adults In Ophraella Communa Lesage(Coleoptera:Chrysomelidae)

An effective way to manage the alien invasive weeds is to introduce its natural enemies. But the challenge is the introduced biological control agent may evolve quickly in the new environment so that its host range may expand to cover some non-target plants and bring unexpected impact to the whole ecosystem. Ophraella communa LeSage (Coleoptera: Chrysomelidae), the natural enemy of the alien invasive ragweed, Ambrosia artemissiifolia (Compositae) has been in China wild for more than ten years. Some researchers have discovered its feeding on Xanthium sibiricum and Helianthus. annuus. In this paper, a series of experiments was carried out to study oviposition behavior of O. communa by using both classic methods and their modifications in choice-and non-choice experiments. The study has tried to discuss the causes of the enlarged host range so that to provided rich evidences for assessment of potential risks of O. communa in biological weed control practices. The following results were obtained.1. Effects of larvae feeding and oviposition plants on oviposition behavior of O. communa in non-choice experimentsTo assess the host specificity of O. communa, a factorial combination of three factors was manipulated. These factors included source plants (A. artemisiifolia and X. sibiricum) on which larvae completed their development, testing plants(A. artemisiifolia and X. sibiricum) and their size (small and large). The eggs deposited over four consecutive ovipositions per female were counted and egg hatching performances recorded. The results showed that number of eggs deposited was affected by source plants, testing plants and their size both independently and in interaction. The female feeding on X. sibiricum at larval stage paid more eggs on A. artemisiifolia (34.3±4.2on large plant,33.3±6.4on small plant). The source plant had a strong effect, as measured by number of eggs, on females when they laid eggs on the plant of the larger size(24.0±7.6on large X. sibiricum and with X. sibiricum feeding experience,18.4±2.3on large X. sibiricum with A. artemisiifolia feeding experience), whereas the testing plant exerted a significant effect on females when they oviposited on the plant of the smaller plant (with A. artemisiifolia feeding experience,34.3±4.2on small A. artemisiifolia,19.0±2.7on small X. sibiricum). Egg size was not influenced by the factors tested. The egg hatching rate, however, was affected by the three factors both independently and in interaction. The egg hatching rate increased when the source plant and testing plant were of same species but decreased when they were not, especially when the testing plant was large in size (0.700±0.065on large A. artemisiifolia,0.603±0.182on large X. sibiricum). The results of this study suggested that O. communa might occasionally use X. sibiricum, but be still highly specific to A. artemisiifolia.2. Effects of larvae feeding and oviposition plants on oviposition behavior of O. Communa in choice experimentsTo further assess the host specificity of O. communa through its oviposition behaviour, a factorial combination of three factors was manipulated the same as the first year’s trial. But hosts of different characters were substituted randomly once the adult female had oviposited. The eggs deposited over four consecutive ovipositions per female were counted and egg hatching performances recorded. The results showed that number of eggs deposited was affected by source plants and testing plants both independently and in interaction. The female feeding on A. artemisiifolia at larval stage paid more eggs on A. artemisiifolia (34.8±3.0on large plant,31.3±3.2on small plant). Egg size was not influenced by the factors tested, neither did the egg hatching rate. Compared with the first year’s trial, the results of this study suggested that O. communa has tended to adjust to the changing oviposition environment with a flexible fecundity and its egg-laying strategy may be directly influenced by host quality.3. Egg distribution and hatching rate on testing plantsTo assess the influence of competition from conspecifics of O. communa, factorial combination of two factors was manipulated including egg-laying plants (A. artemisiifolia and X. sibiricum) and insect number (one and three females). Total eggs deposited over four consecutive ovipositions per treatment were counted and egg hatching performances recorded as well as the distribution of all the eggs. The results showed that number of eggs deposited was more on the upside of A. artemisiifolia, but has no difference among different directions (on A. artemisiifolia, three females deposite20.3±7.2,36.8±9.9,16.5±9.5,22.1±16.8averagely). Density of three females drives more eggs deposited downside (25.7±15.0on A. artemisiifolia and32.9±7.9on X. Sibiricum). More eggs on A. artemisiifolia than on X. sibiricum both upside. The egg hatching rate was higher on the upside four direction of A. artemisiifolia of three females (77±6、56±11、73±10,和68±6%) than one female (59±11、68±12,、52±12和55±10%), but no difference between the upside and the downside. The results of this study suggested that O. communa might prefer to lay eggs on the upside.