Effects of larval host plants on over-wintering preparedness and survival of the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae)

Abstract

Laboratory colonies of cotton bollworm larvae, Helicoverpa armigera, kept at 20 °C under a photoperiod of L:D=10:14 were fed on five host plants (cotton, corn, kidney bean, tobacco and tomato) and an artificial diet (control) to determine the effects of larval host quality on survival and pupal over-wintering preparedness. A separate experiment showed that diapausing pupae weighed more and contained greater nutrient stores than did non-diapausing pupae. Diapausing pupae reared on different host plants showed significant differences in terms of over-wintering reserve storage, and degree of cold-hardiness (extent of low-molecular-weight substances and SCPs), and survivorship. The more nutrients the host plant had, the more the pupae weighed and the higher the levels of total lipids and glycogen. Body water content was also significantly affected by larval food quality. The mean pupal super-cooling capacities varied significantly from −16.7 to −18.9 °C according to host plants the larvae feed on, and these significantly related to water content, pupal weight, lipid and glycogen content, and the levels of glycerol. Levels of trehalose, glycerol, and inositol, which were mainly low-molecular-weight substances, showed no significant differences among different host plants, except for trehalose. Pupal mortality varied from 39.7% on corn to 3.3% on the artificial diet, which was significantly related to pupal weight, total lipid content, trehalose levels, and super-cooling points. These results suggest that larval food quality can affect survival and influence the over-wintering preparedness of the cotton bollworm.

Introduction

The cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is one of the most serious insect pests in China, Australia and India. This species has a wide range of host plants, including both cultivated and wild species, about one hundred and seventy two species of host plants from 40 families in Australia (Zalucki et al., 1994) and about 200 species from 30 families in China (Xu et al., 1958). Every year, the larvae of this species cause substantial damage to cotton, corn, tomato, kidney bean and other vegetable crops. In China alone, this insect caused several billion RMB losses per year during the 1990s, about 10 billion RMB of which (about 1.25 billion USA dollars) was a direct loss of plants in 1992 (www.stdaily.com/gb/stdaily/2005-10/20/content_444994.htm), before Bt plants had been planted widely. Over most of its range in China, this species produces four or five generations a year and over-winters as a diapausing pupa.

Diapause is an important strategy by which insects avoid unfavorable environmental conditions (Tauber et al., 1986; Danks, 1987). The main factors associated with diapause in insects are photoperiod and temperature (Hodkova and Socha, 1995; Nakai and Takeda, 1995). Other environmental factors, such as food quality (Tzanakakis et al., 1992), have received limited attention. Research on cold hardiness has emphasized the contribution of diapause (Tauber et al., 1986; Jiang and Zhang, 1997; Slachta et al., 2002), to the exclusion of possible host plant effects. Nonetheless, food has been shown to be a major factor regulating diapause for a few species of insects (Tauber et al., 1986), influencing the accumulation of energy needed for over-wintering (Zvereva, 2002). Host plant quality affects larval growth rate, the sensitive stage for diapause induction, and cold hardiness (Hunter and McNeil, 1997), which are particularly obvious in polyphagous insects. For example, Zvereva (2002) reported that host plant quality can disturb over-wintering preparedness and thus affect the over-wintering success of the leaf beetle, Chrysomela lapponica.

It is well known that insects must accumulate reserves of glycogen before they enter winter diapause (Sakurai et al., 1992). Moreover the accumulated low-molecular-weight sugars and/or sugar-alcohols during over-wintering (Storey and Storey, 1991) are closely correlated with the nutritive quality of host plants (Zvereva, 2002). In this paper, we confirmed that diapausing cotton bollworm pupae weighed more than non-diapausing pupae that had been raised under the same environmental conditions. Clearly, the potential for accumulating reserves is related to the nutritional quality of host plants. Our previous research, which agrees with results reported by Kidd and Orr (2001), suggested that host plants had a significant effect on the development of the immature stage and body weight of pupae of the cotton bollworm (Liu et al., 2004).

Host plants of the cotton bollworm have different nutritive values, which may affect the rate of development of larvae that feed on them and therefore influence the population dynamics of this pest (Ruan and Wu, 2001). The contributions of host plants to developing generations of the cotton bollworm are clear. The suitability of host plants can be ranked as follows: cotton > corn > legume > tobacco > tomato > hot pepper (Liu et al., 2004). To date, the influences of host plants on pupal survival and the cold-hardiness of over-wintering populations remain unclear.

In China, under natural conditions, the fourth or fifth over-wintering generation of the cotton bollworm has a wide range of potential host plants. Changes in cultivation practices have resulted in summer corn being planted on a large scale; corn and concurrently grown cotton, tobacco, and tomato crops supply food of differing quality for the larvae of the over-wintering generation (Lu and Xu, 1998). Differences in the nutritional levels of host plants in the field may affect the over-wintering potential, the over-wintering survival, and the intensity of outbreaks of this pest in the subsequent years. Diapause in the cotton bollworm, Helicoverpa armigera, is induced by the short photoperiod and low temperatures of autumn (Li and Xie, 1981; Wu and Guo, 1995) and is terminated by either high or low temperatures (Wu and Guo, 1995). In China, only diapausing pupae can successfully over-winter in the temperate zone. In the field, the cotton bollworm enters winter diapause in October and emerges from this state in late December, after which temperatures below the lowest threshold for pupal development prevent any activity until April of the following year (Jiang and Zhang, 1997). For insect herbivores, growth and development are often intimately linked to host plant quality (Bernays, 1990). Thus differences in the quality of host plants for the cotton bollworm might be expected to influence diapause occurrence and cold-hardiness of over-wintering pupae.

In this study, we reared newly hatched H. armigera larvae on five host plants, using artificial diet as a control. We measured the effects of larvae host plants on the over-wintering preparedness of pupae at the beginning of over-wintering (November). This information is essential to the development of a theoretical foundation for managing over-wintering populations and for improving forecasts of the population dynamics of this pest.