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Title: Density-dependent prophylaxis in crowded beet webworm, Loxostege sticticalis (Lepidoptera: Pyralidae) larvae to a parasitoid and a fungal pathogen

Author
item KONG, HAILONG - Yangzhou University
item LUO, LIZHI - Chinese Academy Of Agricultural Sciences
item Sappington, Thomas
item JIANG, XING FU - Chinese Academy Of Agricultural Sciences
item ZHANG, LEI - Chinese Academy Of Agricultural Sciences
item CHENG, YUNXIA - Chinese Academy Of Agricultural Sciences

Submitted to: International Journal of Pest Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2013
Publication Date: 6/25/2013
Citation: Kong, H., Luo, L., Sappington, T.W., Jiang, X., Zhang, L., Cheng, Y. 2013. Density-dependent prophylaxis in crowded beet webworm, Loxostege sticticalis (Lepidoptera: Pyralidae) larvae to a parasitoid and a fungal pathogen. International Journal of Pest Management. DOI:10.1080/09670874.2013.807957.

Interpretive Summary: The beet webworm is a serious agricultural pest of several crops and pastures in Asia. It overwinters in southern regions and migrates north in the summer where outbreak populations can develop quickly in certain areas. Populations of this moth can be held in check by natural enemies such as fungal pathogens and parasitic flies. When the larvae of this insect are crowded during a population outbreak, a situation that normally favors spread of disease and parasitism, we found that they actually become less susceptible to infection. Experiments showed that the immune system apparently is activated to higher levels than normal in response to crowding, and is perhaps responsible for the observed protection against these natural enemies. This positive feedback loop may contribute to population outbreaks characteristic of this pest. Other crop pests, including some in North America, probably use the same strategy. This information will be used by university and government scientists in the U.S., China, and throughout the world to better understand the mechanisms controlling explosive growth of outbreak insect populations, and thus lead to better methods for their prediction and management.

Technical Abstract: Transmission of parasites and pathogens is generally positively density-dependent. Thus, as an insect population's density increases, the risk of an individual becoming attacked or infected increases. In some insect species, individuals experiencing crowded conditions are more resistant to natural enemies than those experiencing low-density conditions, and are predicted to divert resources to increase resistance. This phenomenon is called density-dependent prophylaxis. Here, possible expression of prophylaxis in fifth-instar larvae of beet webworm, Loxostege sticticalis, to biocontrol agents was investigated under rearing densities of 1, 10, and 30 larvae per jar (650-ml). Larvae reared at the moderate density and those reared in isolation displayed the greatest and lowest resistance, respectively, to an entomopathogenic fungus and a parasitoid. Moreover, larvae from the moderate density treatment exhibited elevated phenoloxidase, total hemocyte count and antibacterial activity in the hemolymph, whereas phenoloxidase levels in the midgut was not affected. The results suggest that larval rearing density significantly affects the immune system, and provide evidence for density-dependent prophylaxis of larval L. sticticalis to its biocontrol agents. These results have implications for understanding the population dynamics and biocontrol of beet webworm.