The Elm Leaf Beetle, Pyrrhalta luteola in Southern California: Its pattern of increase and its control by introduced parasites

Abstract

A 2-yr study of elm leaf beetle (ELB), Pyrrhalta luteola (Müller), populations was conducted at 2 locations in southern California during 1973 and 1974 to determine its phenology and seasonal pattern of increase, its seasonal pattern of defoliation, and the degree of biological control achieved by 2 parasites, Erynniopsis antennata Rondani (= E. rondanii Towns.) (= Erynnia nitida Rondani) and Tetrastichus brevistigma Gahan, introduced against the beetle 3 decades ago. ELB populations rapidly increased in the mid- to late-summer period and in most cases by late summer, severely defoliated Siberian elm, Ulmus pumila L., the major host in southern California. Although parasitization by E. antennata reached high levels (>50%) during this period, it failed to prevent defoliation. T. brevistigma was found at 1 location but never parasitized more than 10% of the ELB pupae. Limited samplings suggested that E. antennata parasitized a high proportion (65-88%) of overwintering adult ELB but that asynchrony between the emergence of adult E. antennata in the following spring (April) and the period of larval availability (1st generation) in the late spring (May) apparently resulted in low levels of parasitization of 1st (spring) generation ELB larvae. It was the survivors of this 1st generation as adult females which caused the rapid population increase in mid-summer.