Microplitis similis Lyle (Hymenoptera: Braconidae) is a solitary endoparasitic braconid that generally parasitizes larvae of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) and many other noctuid species. To understand host preference, fitness, and the effects of M. similis on the hosts, we compared percentage parasitism, development periods, and the effects on host growth in candidate noctuid species. We found high levels of parasitism of S. exigua and Spodoptera litura (F.) larvae but not of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae. The parasitoid wasp larvae took similar amounts of time for development on S. exigua and S. litura larvae, i.e., 13.87 ± 0.15 and 13.69 ± 0.42 d, respectively. Compared with the control larvae, the growth and development of the hosts were severely affected. The hosts were able to molt to 4th instars after being parasitized as early 3rd instars, but were unable to develop to the 5th instar. The body weight was similar between parasitized and non-parasitized larvae within the first 4 d (3 d in S. litura) but later began to show a significant difference from the 5th day on (4th day in S. litura). The host larvae eventually weighed up to 50 to 80% less than the non-parasitized larvae. Furthermore, the host larvae lived for an extended period in the same instar after egression of the parasitoid, but the body mass did not increase.
Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) and Spodoptera litura (F.) are economically important pest species. Both of them have a wide distribution, broadly overlapping generations, and broad host ranges, and cause significant crop losses. Traditionally, management of S. exigua and S. litura relies mainly on chemical pesticides, which tend to be highly efficient in reducing pest populations in the field. However, the abuse of chemical pesticides and factitious interventions to increase crop yield have led to many negative effects (Luttrell et al. 1994; Chau 1995). As the concept of Integrated Pest Management (IPM) has become generally accepted, natural enemies of the insect pests, such as parasitoid wasps, have been used as alternatives (Xu et al. 2001; Beckage & Gelman 2004).
The biological characteristics of certain parasitoid wasps enable them to strongly suppress insect pest species that are their hosts. The development of parasitoid wasps has been shown to have dramatic impacts on host larval growth, development, and even metamorphosis (Webb & Dahlman 1985; Beckage & Gelman 2004). As soon as the egg is laid in the host body, the development of the egg induces a series of major physiological changes in the host (Brodeur & Boivin 2004; Zhang et al. 2012). Most of these changes are likely to be caused by the venom — which contains various functional proteins — injected into the host during oviposition (Goecks et al. 2013). Also, during the development time of the immature parasitoids, the synthesis of proteins favorable to the parasitoids themselves has effects on host larval growth (Kaeslin et al. 2005). Therefore, the host larvae cannot gain weight, and often feed and molt less than non-parasitized hosts (Liu & Li 2006).
By means of field research and surveys conducted for many years, a number of parasitoid wasp species have been discovered, and the Natural Enemy of Pests database has been enriched greatly in China. Spodoptera exigua was shown to be vulnerable to 33 parasitoid wasp species (He et al. 2002a) and S. litura to 40 species (He et al. 2002b), but more studies on parasitoid wasp species are still in needed to fill knowledge gaps.
Microplitis similis Lyle (Hymenoptera: Braconidae) — with Agrotis ypsilon (Rottemberg) (Lepidoptera: Noctuidae) as its original host (Lyle 1921) — was one of the major larval endoparasitoids in the Indo-Australlan Region (Wilkinson 1930; Shepard & Barrion 1998). We studied the biological features and geographical distribution of M. similis. Recently, we discovered that this species was first recorded in Changsha City, Hunan Province, China. In addition, the interactions between M. similis and its hosts remained unrevealed, and its effects on its hosts have not been studied systematically. These questions are theoretically important because their answers underlie the potential of biological control of insect pests.
In this study, we examined the effects of M. similis on the hosts S. exigua and S.