Effects of the Larix gmelinii grown under different light intensities on the development and defensive enzyme activities of Lymantria dispar larvae

Abstract

Larix gmelinii is among the most ecologically and economically important timber species in northeastern China due to its cold hardiness, drought resistance and rapid growth. Lymantria dispar L. is a defoliating insect pest which seriously harms larch during outbreaks. Plants, including trees, employ an array of physical and chemical constitutive defense mechanisms that play an important role in protection from insect herbivory. Constitutive defenses are always present, and because they are maintained even in undamaged plants, they are affected by environmental factors to some degree. In many organisms light is a crucial environmental signal influencing natural physiological and developmental processes. Different light intensities could induce changes in plant defense systems. However, pest insects can detoxify ingested plant secondary metabolites with their biotransformation system. We studied the developmental and physiological responses of Lymantria dispar larvae to changes in the constitutive defenses of Larix gmelinii under three sunlight intensities (100%, 50% and 25% of natural sunlight intensity) mimicking the light conditions of forest edges, forest gaps and understories. The results showed that the detoxification enzyme activities of both fourth and fifth instar L. dispar larvae were significantly affected by their host L. gmelinii trees grown under the three different sunlight intensities. Compared with the control (100% emergence rate) of L. dispar was significantly inhibited after being fed on the needles of L. gmelinii grown under shaded conditions (P<0.05). The activities of the protective enzymes SOD, POD, CAT and detoxification enzymes ACP, AKP, CarE, GSTs, MFO in L. dispar larvae after fed on the needles of L. gmelinii grown under the shaded conditions (25% and 50% sunlight intensity) were significantly lower (P<0.05) than those in the larvae fed on the needles of L. gmelinii grown under the control (100% sunlight intensity) light condition. The activities of all the tested protective and detoxification enzymes excepting CAT were the most significantly inhibited after the larvae being fed on the needles of L.gmelinii grown under 50% sunlight intensity (P <0.05). These larval enzyme activities were significantly different (P <0.05) between fourth and fifth instar larvae while fed on the needles grown under the same light condition. The SOD, POD, CarE, GSTs and MFO activities in the fourth instar larvae were significantly higher than those of fifth instar larvae (P<0.05), while the ACP and AKP activities in the fifth instar larvae were significantly higher than those of fourth instar larvae (P<0.05). This result indicated that L. dispar might use different enzymes or enzyme complex for defense and/ or detoxification during their larval development stages. However, light intensities did not show any significant direct effects on the growth development and defensive enzyme activities of L. dispar larvae on the artificial diets, indicating that the changes observed in L. dispar larval development and defensive enzyme activities were mainly caused by the larch resistance but not by the light intensity. Our results suggested that adjusting the light conditions rationally via silviculture approaches in the larch forests might not only increase the tree growth, but also increase the overall tree resistance to certain serious pest insects including L. dispar.