Jiancheng Li

Effects of the Cry1Ac toxin of Bacillus thuringiensis on Microplitis mediator, a parasitoid of the cotton bollworm, Helicoverpa armigera

Interactions between the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), its larval parasitoid Microplitis mediator (Haliday) (Hymenoptera: Braconidae), and the Cry1Ac toxin of Bacillus thuringiensis Berliner were evaluated under laboratory conditions. The growth of H. armigera larvae was delayed and its pupal rate and pupal weight decreased when they were fed on a diet containing Cry1Ac toxin. Due to the lowered growth rate of the host larvae, the time available for parasitization of H. armigera by M. mediator increased when the host larvae were reared on a diet containing Cry1Ac toxin at concentrations of 0.5, 1, 2, and 4 µg g−1. The longevity of female and male parasitoids was not significantly affected when newly emerging wasps fed on honey solutions containing three different concentrations of Cry1Ac toxin (125, 250, and 500 µg ml−1). When female parasitoids were fed on honey solutions containing Cry1Ac, their offsprings’ egg and larval development period, pupal weight, length of pupation, adult weight, and adult longevity did not change significantly in most of the treatments compared with controls. When the female parasitoids parasitized host larvae that had been fed on a diet containing 0.5, 1, 2, 4, and 8 µg g−1 Cry1Ac toxin, their offsprings’ eggs and larvae were significantly delayed. Their pupal weight, adult weight, and adult longevity were also significantly less than controls.

Field Release of the Parasitoid Microplitis mediator (Hymenoptera: Braconidae) for Control of Helicoverpa armigera (Lepidoptera: Noctuidae) in Cotton Fields in Northwestern China’s Xinjiang Province

Abstract Field experiments and surveys were conducted in 2003 and 2004 to evaluate the efficacy of using the parasitoid Microplitis mediator (Haliday) to control populations of Helicoverpa armigera (Hübner) in cotton fields in Northwestern China’s Xinjiang Province. The population levels of the second generation of H. armigera in southern Xinjiang in 2003 and 2004 were ≈20–60% above the economic threshold of this pest. The longevity of field-released female and male parasitoids was 7.6 ± 0.4 and 3.9 ± 0.4 d, respectively. Field tests showed that the number of parasitized H. armigera increased with an increasing number of M. mediator cocoons released in the field. The release of 2,250 or 15,000 laboratory-reared M. mediator cocoons per hectare in cotton fields resulted in 38–61% parasitism of H. armigera in 2003 and 35–66% parasitism in 2004. The efficiency of the parasitoids varied considerably with different numbers of parasitoid cocoons and with the frequency and sites of release. However, the highest damage reduction in these studies was obtained with a single release of cocoons and the highest number of release sites. Large-area releases of M. mediator in cotton fields of Shufu, Shule, and Shache counties resulted in >60% parasitism and an 80% decrease in cotton boll and bud damage compared with the control fields. The above results indicated that field release of M. mediator could be used effectively in managing H. armigera in cotton in Xinjiang.

Role of Photoperiod and Temperature in Diapause Induction of Endoparasitoid Wasp Microplitis mediator (Hymenoptera: Braconidae)

Abstract Diapause in Microplitis mediator Haliday (Hymenoptera: Braconidae) is manifested during the pupal stage and normally occurs during the winter season because of a photoperiodic response that is highly dependent on temperature. No diapause was observed at temperatures above 20°C regardless of the photoperiod. Only a small part of the population entered diapause at 20°C under short daylengths of light:dark 8:16–12:12. In contrast, when larvae were exposed to 16, 18, and 20°C combined with a photoperiod of 10:14, the percentages of parasitoids that entered pupal diapause was 97.9, 87.8, and 26.2%, respectively. Critical photoperiods were determined to be 7.03:16.97 and 12.21:11.79 at 16°C and 6.75:17.25 and 12.03:11.97 at 18°C. The second instar of the parasitoid was the most sensitive to diapause induction for the photoperiods and temperatures tested.

The life parameters of a parasitoid Microplitis mediator (Hymenoptera: Braconidae), reared on cotton bollworm Helicoverpa armigera (Hübner) with Cry1Ac diet

Abstract Microplitis mediator (Haliday) is an important endoparasitoid of the cotton bollworm, Helicoverpa armigera (Hübner) in northern China. Interactions among H. armigera, its larval parasitoid M. mediator, and Cry1Ac over two generations were evaluated under laboratory conditions. The results indicated that the developmental period of M. mediator offsprings eggs and larvae were significantly delayed and pupal and adult weight were significantly less compared to the control when the female parasitoids parasitized H. armigera larvae that fed on diet containing 1, 2, 4 and 8 µg/g Cry1Ac. The female parasitoids emerged from the host fed diet containing 8 µg/g Cry1Ac could oviposit in healthy hosts, and their offsprings biological parameters (egg–larval period, pupal weight and adult weight), parasitism rate, abnormal cocoon rate and adult emergence were not significantly affected. Cry1Ac was detected in larvae and hemolymph of H. armigera, but not in the larvae of M. mediator. The results suggest that the observed significant effects on several fitness parameters of the F1 M. mediator developed from H. armigera fed Cry1Ac diet most likely were host-quality mediated rather than direct effects of Cry1Ac.