Biotic resistance towards Hydrellia egeriae, a biological control agent for the aquatic weed Egeria densa, in South Africa

Egeria densa is a submerged aquatic weed that can grow into dense monocultures in rivers and dams in South Africa, which negatively affects ecosystem functioning and services. The biological control agent Hydrellia egeriae Rodrigues-Júnior (Diptera: Ephydridae) was first released against Egeria densa Planchon (Hydrocharitaceae) in South Africa in 2018. Biotic resistance in an introduced range can have negative impacts on the ability of a biological control agent to establish and exert top-down pressure. Dipteran and lepidopteran species that are used as biological control agents are often susceptible to higher levels of parasitism in their introduced range than biological control agents from other insect orders. In addition, ecological analogues that are present in South Africa, make H. egeriae particularly vulnerable to biotic resistance. Considering this, post-release surveys were conducted to investigate if native parasitoids will extend their host range to include H. egeriae. Chaenusa seminervata van Achterberg, C. anervata van Achterberg (Braconidae: Alysiinae: Dacnusini) and Ademon lagarosiphonae van Achterberg (Braconidae: Opiinae) were reared from field-collected H. egeriae pupae, within a year of its release. These braconid parasitoids were previously recorded from a native herbivore, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae). Parasitism levels of H. egeriae ranged from 50 to 74% in cold months and 0 to 60% in warmer months, with higher levels of parasitism at a site where H. lagarosiphon naturally occurs. This study also found that cumulative release events of the biological control agent increase the probability of parasitism of field populations, by directly increasing the host pool. However, biological control efficacy can potentially be increased by limiting release efforts to a maximum of two release events per site per season, with particular focus on releasing in warm (i.e. spring/summer) months. Continued post-release surveys are necessary to not only monitor H. egeriae’s impact on E. densa, but also to obtain a better understanding of seasonal parasitism levels across E. densa-invaded sites in South Africa.