Biological control of imported cabbageworm (Pieris rapae, Lepidoptera : Pieridae) with Steinernema feltiae

Abstract

Imported cabbageworm (Pieris rapae L., Peiridae) is an economically important pest of cabbage throughout temperate and subtropical zones. Damage from P. rapae feeding causes irregular holes on older cabbage leaves. Under severe infestations, larvae skeletonize leaves. Over all, losses can be up to 71% in cabbage. Concerns about effects of pesticides on human health and non-target organisms have led to a reduction in the use of these chemicals. Pieris rapae has many natural enemies, including predators, parasitoids, and pathogens. Entomopathogenic nematodes (EPNs) represent another potential biological agent group for P. rapae. Steinernema carpocapsae and S. feltiae can play a role in the control of P. rapae but face several obstacles to their successful deployment. Insects have an immune response to EPNs and their symbiotic bacteria. However, EPNs react and can be non-recognized (evasion) by the insect host, tolerate insect encapsulation (tolerance), or suppress insect encapsulation (suppression). EPNs are adversely affected by temperature, sunlight, lack of moisture, and UV radiation. Many approaches can be utilized to enhance the application of EPN to control foliage-feeding insects. This thesis research is aimed to determine whether low population densities of EPN infective juveniles (IJs) can work as well as high population densities, whether the efficacy of low EPN population densities on mortality of P. rapae is sufficient, whether S. feltiae infects P. rapae before dying due to lethal environmental conditions, and whether mixing S. feltiae and S. carpocapsae can increase the mortality of P. rapae. Just 1.6 IJs/cm² were able to defend an area and give 92% mortality of P. rapae 4th instar larvae. A 3-hour exposure was sufficient for S. feltiae to infect P. rapae larvae and give 90% mortality. A minimum number of IJs should penetrate the larvae to achieve successful infection and produce a new generation. Not all the cadavers from a 2-hour exposure produced EPNs. Some cadavers apparently were invaded by IJs sufficient to kill the insect but not by enough IJs to escape the insect immunity, survive, and reproduce. Combining EPN species further increases the level of insect mortality over that of a single EPN species. Exposure time is also important to achieve a successful invasion of the target insect, especially for low population densities of EPNs. The basic knowledge for making a decision to use EPNs in the field is to apply during the day avoiding high temperatures and UV radiation. Mixing species of EPNs can enhance EPN efficacy.