Predicting thermal performance of Tetrastichus gallerucae (Hymenoptera: Eulophidae): a temperature-dependent functional response model for biological control of the elm leaf beetle in urban green spaces

Abstract

The functional response of the egg parasitoid Tetrastichus gallerucae (Fonscolombe) attacking different densities of elm leaf beetle (Xanthogaleruca luteola Müller) eggs was evaluated at 5 constant temperatures (17, 20, 25, 30, and 32 °C) under laboratory conditions. Host densities of 5, 10, 20, 40, 80, and 160 eggs were exposed to a single mated female for 24 h with 10 replicates per density-temperature combination. Logistic regression was used to determine the functional response type, and Rogers’ random predator equation was fitted to estimate attack rate (a) and handling time (Th). An indicator variable approach was employed for pairwise comparisons of parameters between temperatures, and a temperature-dependent model (quadratic expression for Th) was developed to predict parasitism across the thermal range. A type II functional response was observed at all temperatures. Attack rate varied across temperatures, reaching its highest estimated value at 30 °C (4.566 day−1) and declining sharply at 32 °C (0.453 day−1). Handling time was shortest at 20 °C (0.012 day) and 25 °C (0.013 day) and longest at 32 °C (0.175 day). The maximum theoretical attack rate (T/Th) was highest at 20 °C (83.33 eggs/day). The temperature-dependent model explained about 65% of the variance in parasitism. The 3D surface plot revealed highest predicted parasitism at 25 °C and 160 eggs (60.59 parasitized eggs). These results demonstrate that T. gallerucae performs optimally at 20 to 30 °C, with peak daily parasitism at 20 °C, while efficiency is severely compromised at 32 °C. The parasitoid shows high potential as a biological control agent against elm leaf beetle in temperate urban green spaces, especially at 20 to 25 °C and low to moderate pest densities, but its effectiveness may be limited during heatwaves or in persistently hot regions.