Characterization of detoxification enzymes in Japanese beetle, Popillia japonica Newman (Coleoptera: Scarabaeidae)

Abstract

Japanese beetle, Popillia japonica Newman, is a highly polyphagous scarab with a host range of over 300 plant species spreading over 79 families. It is a cosmopolitan pest with agricultural, landscape and horticultural importance, over 460 million USD is spent every year to control both the larva and adult stage. Currently, it is unclear how this polyphagous scarab is able to utilize an array of host plants with very dissimilar chemistry. Thus, the goal of our research is to understand the biochemical defense of this generalist herbivore that confers on it the ability to utilize diverse host plants. This study examines the roles of the widely studied multiple enzymes systems cytochrome P450 (P450), glutathione S-transferase (GST) and carboxylesterase (CoE) in the physiology and host plant utilization by Japanese beetle. In the first objective (Chapter 2) we studied the induction of P450, GST and CoE activities in midguts of adult Japanese beetle after it had consumed single-plant or mixed-plant diets of preferred (rose, Virginia creeper, crape myrtle and sassafras), or non-preferred hosts(boxelder, riverbirch and red oak). Non-preferred hosts were only sparingly fed upon yet induced higher activities of P450, GST and CoE than did feeding on preferred hosts. Similarly, beetles tended to have greater enzyme activities after feeding on a mixture of plants compared to a single plant type. Consequences of higher induced activities of detoxification enzymes by nonpreferred hosts on the physiology and feeding ecology of Japanese beetles are discussed. In Chapter 3 (i.e. objective 2), we studied the connection between geranium (quisqualic acid) intoxication and the activities of P450, GST and CoE in the midgut of adult Japanese beetles. Beetles fed varying quantities of toxic petals of geranium flowers or agar plugs containing quisqualic acid were assayed for midgut enzyme activities. Contrary to expectation, activities of P450, GST and CoE were induced in Japanese beetles that consumed both geranium petals and quisqualic acid. Peak enzyme activity occurred after 24 h of feeding but peak paralysis (intoxication) occurred after just 12 h. Our study suggests that consumption toxic geranium and quisqualic acid do induce of active detoxification enzymes, however this induction does not completely protect Japanese beetle from being paralyzed by geranium and quisqualic acid. In the last objective (Chapter 4), we characterized the expression profiles of P450, GST and CoE in the immature life stages of Japanese beetles using multiple model substrates and also examined the influence of feeding duration and starvation on the activities of P450, GST and CoE in the midgut of adult Japanese beetle using a preferred host plant Virginia creeper. Our results show that activities of these enzymes are developmentally expressed in the different life stages of Japanese beetles, with enzymes from each life stage having varying affinity to the substrate and reaction speed. Furthermore, a feeding bout of 24 h was enough to induce optimal activities of P450, GST and CoE in the midgut of adult beetles. The consequences of the observed pattern of enzyme activities in the physiology and ecology of the Japanese beetle are discussed.