As part of a search for natural enemies of the gypsy moth (Lymantria dispar), virus-infected samples were collected near Toulouse, France. Light and electron microscope studies confirmed that the French strain is a multinucleocapsid nuclear polyhedrosis virus (MNPV). In vivo bioassays using the New Jersey strain of L. dispar, and comparing L. dispar MNPV (LdMNPV) strains from France, North America and Korea, showed that the French strain was the least active, whereas the North American strain had the highest activity. The viral efficacy of all strains was enhanced 200 to 1300-fold in the presence of 1% fluorescent brightener. The enhancement was highest in the American strain and lowest in the French strain. French LdMNPV (LdMNPVF) DNA cut with four restriction enzymes (BamHI, EcoRI, HindIII, and NotI) revealed minor fragment size differences, but many similarities when compared to the North American and the Korean strain. PCR amplification of a LdMNPV early gene (G22) was detected in the North American and the Korean strain, but not in the French strain.
After high population densities of insect defoliators there is often a dramatic decrease in their abundance due to various limiting factors. Here, we compared gypsy moth larvae (Lymantria dispar L.) reared singly and in crowded conditions. We compared a number of physiological parameters of these insects and the effect of L. dispar population density on the activation of covert baculovirus infections in the larvae. It was found that the population density of gypsy moth larvae did not affect the mortality due to the activation of the covert virus infection or the total mortality. On the other hand, solitary-reared larvae were heavier, took longer to develop, and showed a four-fold higher concentration of dopamine in their haemolymph than larvae reared in groups. Thus, we demonstrated that an increase in the population density of larvae per se facilitates some changes in fitness and innate immunity traits but is not related to the activation of covert baculovirus infection. We suggest that an increase in population density does not increase the risk of epizootics triggered by the activation of covert baculovirus infection and that researchers should pay more attention to studying density-associated factors, such as starvation.