Effects Of Parasitism By Meteorus Pulchricornis Wesmeal On Feeding And Predation Risk Of Spodoptera Exigua (Hübner) Larva

The beet armyworm Spodoptera exigua (Hübner) is a polyphagous pest on numerous crops over the world. Its outbreaks have brought about severe damages to agricultural crop production in larger areas in China since 1980s. Meteorus pulchricornis (Braconidae, Hymenoptera) is a common parasitoid attacking larvae of the beet armyworm. Previous studies were conducted on host selection and developmental performances of this parasitoid, but few studies have been carried out on effects on host larvae in food consumption, movement behavior, and in interactions with predaceous insects. To gain insights into evaluation of the parasitoid and other natural enemies in pests' control, we conducted a few experiments in food consumption and utilization, movement behaviors of the beet armyworm larvae that were parasitized, and in prey choice of two predaceous insects between parasitized and healthy larvae.1. Effects of parasitism on food consumption and utilization of Spodoptera exigua Hübner larvaeLaboratory studies were carried out to investigate effects of parasitism by the koinobiontic parasitoid Meteorus pulchricornis, on food consumption, weight, growth rate, and food utilization of the penultimate instar larva of Spodoptera exigua Hübner using the gravimetric method. The parasitism resulted in a reduction in the amount of food consumed, weight, in the growth rate, and in the food utilization performed. The parasitized larva consumed one third and a half less than the unparasitized on the forth and the fifth day after parasitism respectively. The relative growth rate was significantly reduced in the parasitized larvae compared with the unparasitized from the third day on after parasitism. The food utilization of the parasitized larva is significantly less than the unparasitized in the efficiency of conversion of ingested (ECI) and digested (ECD) food,, but the approximate digestibility (AD) and relative metabolism rate (RMR) were increased as expected. However, the above pattern was reversed on the fourth day after parasitism, which was attributed to the effect of a much less food consumption while body weight gained because of the parasitism.2. Effects of parasitism on feeding, movement, and distribution of Spodoptera exigua larvaeUnder laboratory conditions, the fixed-interval time point sampling was used to observe the parasitized and healthy larvae in feeding activity, movement, and distribution on a potted soybean plant. The results indicated that there were significant differences between parasitized and unparasitized larvae in feeding activity, movement, and distribution on the plant in the later period (the last two days to egression of parasitoid larvae from host). Parasitized larvae had a higher chance of appearing at the upper and lower layers of the plant, compared to the healthy larvae. However, more unparasitized larvae located in the middle part of the plant and out of the plant, compared to the parasitized larvae.3. Prey choices of two predaceous insects on Spodoptera exigua larvaeUnder laboratory conditions, experiments were conducted to observe predation selection of predaceous carabid beetle, Paederus fuscipes (Carabidae), and rove beetle, Chaenius naeviger Mor (Staphylinidae), between instars, and between parasitized and unparasitized prey larvae of Spodoptera exigua, which were parasitized by Meteorus pulchricornis. Experimental results showed that P. fuscipes preferred younger to older instars, by the order of preference as L₁ > L₂ > L₃ > L₄ > L₅. But C. naeviger did not show significant preferences between instars. The two predaceous beetles did not exhibit preferences between parasitized and healthy larvae of Spodoptera exigua, which suggests the intraguild predation between the predaceous insects and the parasitoid.