Efficacy, host-finding ability and application methods of entomopathogenic nematodes to control pupae of Gonipterus sp. 2 (Coleoptera: Curculionidae)

Gonipterus sp. 2 is a pest of economic importance to South Africa as it threatens the sustainability of eucalypt plantations. The adult beetles and larvae cause damage to Eucalyptus spp. through defoliation. There are currently no insecticides registered to control Gonipterus sp. 2 and only limited control is achieved by using an egg parasitoid, Anaphes nitens. It is thus necessary to explore other alternatives such as entomopathogenic nematodes (EPNs). Entomopathogenic nematodes of the families Steinernematidae and Heterorhabditidae have a symbiotic relationship with bacteria from the genus Xenorhabdus and Photorhabdus, respectively. They are obligate parasites of many soil-dwelling insect pests of economic importance. EPNs are mass-produced for commercial purposes and distributed around the world. Despite the documented efficacy however, newly identified EPN species should be tested against economically important pests. In this study, the efficacy, host-finding ability and application methods of five local EPN species, namely Heterorhabditis noenieputensis, H. safricana, Steinernema fabii, S. jeffreyense, and S. yirgalemense, to control pupae of Gonipterus sp. 2 were investigated. Results from laboratory bioassays indicated that S. yirgalemense provided the highest pupal mortality and the lethal concentration of this EPN species was determined, namely LC50 = 45.7 IJs/pupa and LC90 = 150 IJs/pupa. Steinernema yirgalemense was also confirmed to kill pupae within their pupal cases and provided high mortality of the two pupal age groups tested, namely 1-3 days old and 9-12 days old. The host-foraging strategies of the five local EPN species were investigated by studying their movement behaviour in the presence of the host and comparing the distance they travel on a rough substrate and a smooth substrate. Heterorhabditis noenieputensis, H. safricana, S. jeffreyense, and S. yirgalemense were able to locate larvae of G. mellonella buried at a depth of 10 cm, thus causing high mortality. This suggested that the four EPN species use a cruiser foraging strategy, although this result was not supported when comparing the movement of those EPNs on the xvi two substrates. Steinernema fabii was not effective at locating the larvae of G. mellonella, resulted in low larval mortality, and showed a reduced movement on a rough substrate, suggesting that it uses an ambusher foraging strategy. The infective juveniles of S. yirgalemense applied with both nematode-infected cadavers and aqueous suspension application methods were equally and highly effective at locating and infecting Gonipterus sp. 2 pupae buried at depths of 10 cm. This is the first study in South Africa to report the potential of five local EPN species as biological control agents of pupae of Gonipterus sp. 2.