Foraging Behaviors Of Meteorus Pulchricornis For Spodoptera Exigua Larvae

As the fittness of parasitoids depend on hosts they could find and oviposit, research of foraging behavior has been in focus for study of life history parasitoids. Owing to convenience to study parasitoid behaviours in the lab but difficulty in the field, most of foraging behaviors of parasitoids are studied in the lab. However, what are observed in the lab may not reflections of that performed in the field. Host prefenece of parasitoids are usually measured by parasitism rate, but parasitization of parasitoids can be influenced by both environmental factors and physiological states of parasitoits. Therefore, variables other than parasitism rate are needed for the evaluation of host preference in parasitoids, which are the chanllenges facing behavioral ecologists. The thesis study was conducted to observe foraging behaviors of Meteorus pulchricornis (Wesmael) in a large walk-in cage in the outdoor as compared to those in small cages in the lab. In addition to the traditional variable (parasitism rate) for measuring host selection, the host recognition time was used to measure selection of parasitoids by fitting to Cox proportional hazard model. The main results obtained are summarized as followed below.1. Foraging behaviors of Meteorus pulchricornis in the laboratory(1) Non-choice experimentsTo assess the parasitization hazard of different instar larvae of Spodoptera exigua by Meteorus pulchricornis, the host recognition time of first and subsequent launches were recorded under non-choice condition, and fitted by Cox proportional hazard model of survival analysis. The results showed that L1 host was not susceptible to parasitism, but L3 to L5 hosts were more prone to be attacked than L2 ones by 2.6-4.0 fold. However, survival curves, measuring the fraction of host larvae unparasitized, were compared for the first and subsequent attacks, which indicated that the host recognition time for the first attack on L2-L4 was much less than for the subsequent attack. The study suggests that higher instar larvae are more likely to be attacked by naive parasitoids. (2) Choice experimentsIn order to study the host-instar selection of Meteorus pulchricornis, a choice-experiment was conducted in the lab, and host selection frequencis were observed and estimated by Manly's selection ratio. Under the multiple-choice condition, the parasitoids preferred higher instar larvae at both first and subsequent attacks as measured by oviposition, where the relative selection ratio in terms of Manly standardized selection ratio was 70% for L4 plus L5 hosts and only 10% for L2 host in the first attack, and 73% for L4 plus L5 hosts and 3.0% for L1 host in the successive attack.2. Foraging behavior of Meteorus pulchricornis in the open-field(1) Foraging behavior for host instars with same densityIn order to study the difference of parasitoids behaviors between the lab and field, half-field experiment was conducted in a cage (2.5×2.5×1.8) with random patches. Foraging behaviors of parasitoids to different host instars were observed instantaneously, and the parasitism rates were caculated. The results showed that parasitism rate was higher in the old patches than in the young ones, the selection ratio of host patches was correlative with the host instar positively. The sting ratio of L1 and L2 were 13% and 16%, but it was up to 70% from L3 to L5, the parasitism ratio was lower on L1-L2 host patches(18%), but it was higher on L3-L5 host patches(82%). Cox proportional hazard model fitted by the host recognition time showed that L3 were more prone to be attacked than L1. However, there were no significant differences among L1, L2, L4 and L5.(2) Foraging behavior of parasitoid to host instars with different densityIn the open-field, the number of hosts often decreased with host instars. To research into the host preference of parasitoids in the field and the influence of host density, the semi-field experiment with different levels of host density was conducted. The results showed that parasitism rate was positively correlated with host instars. The detection rate of L1-2-host patches and L3-4-host patches were 23% and 28% respectively, but was up to 49% for L5-host patch. The parasitism rate was higher in old-host patches than the younger by three-fold. Cox-proportional hazard model fitted by the host recognition time showed that there was no difference among L1-5 host larvae, but the older hosts were preferred to the younger.