Oviposition biology and seed damage by Smicronyx lutulentus Dietz (Coleoptera: Curculionidae), a biological control agent of Parthenium hysterophorus L. (Asteraceae) in South Africa

Abstract

The seed-feeding weevil Smicronyx lutulentus Dietz (Curculionidae) was released in South Africa in 2015 to supplement the biological control programme against the invasive annual weed Parthenium hysterophorus L. (Asteraceae). Larval feeding within young, developing ovules and larval-induced abortion of adjacent seeds within the flowerheads causes seed destruction, augmenting low levels of natural seed abortion. We studied female fecundity in a glasshouse trial, to identify critical periods in oviposition and inform mass-rearing and field release protocols. Newly emerged mated females displayed a pre-oviposition period of 1–2 weeks, a peak in oviposition after 7–8 weeks and a lifetime fecundity of 324–446 eggs. We also studied the relationship between weevil density and seed damage during two separate glasshouse trials, to facilitate predictions of seed damage under field conditions. Plants exposed to 15 mating pairs of weevils harboured substantially higher proportions of larval-damaged flower buds (75 %) than plants with five pairs (45–56 %) or a single pair (19 %). Larval progeny from five adult pairs caused significantly higher seed inviability (38 %) than progeny from a single pair (14 %), versus the weevil-free control (5 %). Progeny from 15 pairs damaged 30 % of developing seeds, potentially causing up to 60 % seed inviability due to enhanced abortion of neighbouring seeds. Although ≥5 females per plant caused appreciable seed damage, the proportions of unexploited new buds (25–55 %) versus buds with multiple occupancy (≥2 larvae; 35–47 %) indicate the importance of floral phenology in oviposition site selection, as flowerbuds of 2–3 mm are preferred. Although progeny from five females per plant could cause up to 30 % seed reduction, within a week of oviposition, higher densities are required to limit the entry of viable seeds into the soil seed banks.