Effect of Pore Size and Gap Width of Artificial Oviposition Substrates on the Egg-Laying Behavior in Agrilus planipennis: Implications for Larval Host Selection

Abstract

Phytophagous insects select for oviposition sites that improve offspring survival outcomes. Many woodboring insects, such as the invasive emerald ash borer (EAB), Agrilus planipennis, select for oviposition sites underneath bark crevices of host trees. Although the preference for rougher bark has been demonstrated, the limits of EAB oviposition site selection are unknown. Here we determined the effective bark crevice size that EAB females oviposit under through trials with coffee filter paper covered with mesh screening in a no-choice experiment. We then conducted a choice experiment where we wrapped an ash log with polypropylene ribbons at different tightness treatments as oviposition stimulants. We found that EAB females oviposited onto coffee filter paper through mesh pores ranging from 0.6 × 0.6 mm (0.36 mm²) to 1.4 × 1.4 mm (1.96 mm²), but not 0.2 × 0.2 mm (0.04 mm²). The width of ovipositors on a sample of 30 EAB females ranged from 0.270 to 0.689 mm, suggesting the smallest pores prevented ovipositor insertion. Females also oviposited more on logs wrapped with ribbon at a moderate gap width (0.5 mm) than on logs with loose gaps (1 mm) or logs with tight gaps (0.06 mm), which was smaller than our measured ovipositor widths. Our findings suggest that female EAB prefer gap sizes that are relatively tight so long as they are large enough to allow insertion of the ovipositor. These results have implications for larval host selection in field settings as the attribute of bark crevices limiting EAB oviposition may vary drastically with host plant.