Non-crop habitats may enhance diversity of natural enemy species in simplificated crop fields and benefit the reduction of pest population density. Hedgerow is a type of non-crop habitat which has its specific characteristics in vegetation structure. Much research in other countries indicates that hedgerows can attract natural enemies or become their habitats because they can provide shelter and alternative food, so that hedgerows have become important sources of natural enemies in crop fields. In China, however, the effect of hedgerows on the population distribution of predatory insects in agricultural fields has not been reported. This paper constitutes a study on the effect of hedgerows on the population distribution of predatory insects in wheat fields. Its purpose is to provide certain scientific evidence that hedgerow can provide shelter and alternative food for the survival and development of natural enemy population and to utilize these non-crop habitats. This paper reports a study on the habitat selection of Harmonia axyridis Pallas as the main predator in hedgerows, including Hippophae rhamnoides L. and coppice shoot poplar (Populus sp.), in wheat fields, and on the effect of hedgerows on the distribution of Harmonia axyridis Pallas in the agroforestry system of Daxing, Beijing, from 2009 to 2010. The result shows that: firstly, in the early period of wheat growth (from regreening to heading or anthesis), the population density of H. axyridis on hedgerows is 3.25 to 9.57 times more than that in wheat fields, so that hedgerows are the main habitat for H. axyridis when transiting from overwintering sites to wheat fields and the species pool of H. axyridis in the agroforestry system. Secondly, in different periods of wheat growth, H. axyridis is inclined to the habitat of aphides substantive distribution, and there is apparent movement of H. axyridis among these habitats. More concretely, H. axyridis appears to favour hedgerow in the early stages of wheat growth (from seedling establishment to heading stage or anthesis), because on the one hand there are many other aphides in the hedgerow which can provide alternative food for H. axyridis to reproduce when there are not many wheat aphides in early stages of wheat growth, and on the other hand the overwintering generation of H. axyridis prefers to perch on high plants after excited insects, while H. axyridis prefers the wheat field in the later period of wheat growth (anthesis or pustulation to full ripe stage), because wheat aphides increase largely but there are few aphides in the hedgerow. Thirdly, after wheat harvesting, the population density of H. axyridis on coppice shoot poplar (Populus sp.) increases by 42% more than that in the full ripe stage, when the hedgerow can supply an important refuge for H. axyridis.
This paper concludes that population density of H. axyridis rises and falls in different periods and different habitats, which on the one hand has to do with spatio-temporal distribution of aphides and on the other hand hedgerows that are stable and in natural habitat provide an important refuge for H. axyridis when agricultural landscape changes sharply due to crop harvest. It implies that habitat having above-mentioned functions is the survival foundation of natural enemies in the agro-ecological system and is the requirement for realizing ecological regulation and management of pests. Therefore agricultural protection forest may play an important role of ecological stability in agroforestry systems.