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The potential for using synthetic sex pheromone traps as a simple and practical method of monitoring population densities of insect pests has been investigated in many crop systems. Yet, factors enabling the forecast of infestations based on pheromone trap catches are not fully understood. This study tested the prediction that high survival of immature stages of the target pest is a pre‐requisite for trap catches to correlate well with future infestations on the crop. The influence of parasitoids, as an important natural mortality factor of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), larvae and pupae in South Africa, on the ability of pheromone traps to forecast infestations was investigated continuously at weekly intervals over 6 years on unsprayed cabbage. During late October–May, when parasitism levels were high (≥50%), infestations and trap catches were significantly lower than during low parasitism (<50%) observed during June–early October. Because infestations were negatively related to parasitism level, trap catches correlated with infestations better when observations were made in the same week during periods of high parasitism. Conversely, when survival of P. xylostella immature stages was high due to low parasitism levels, trap catches correlated with future infestations well for up to 2 weeks. Thus, trap catches can be used to forecast infestations during September–October, a period that coincides with high P. xylostella infestations as a consequence of low natural control by parasitoids. This is the first study to show that the ability of pheromone trap catches to forecast infestations depends on survival of the immature stages of the target pest.

Oviposition decisions (i.e., host selection and sex allocation) of female parasitoids are expected to correspond with host quality, as their offspring fitness is dependent on the amount and quality of resources provided by a single host. The host size model assumes that host quality is a linear function of host size, with larger hosts believed to contain a greater quantity of resources, and thus be more profitable than smaller hosts. We tested this assertion in the laboratory on a solitary larval-pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) developing on three instars (second-fourth) of one of its hosts, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). In a no-choice test, parasitism levels and sex ratio (i.e., proportion of female progeny) were significantly high in hosts attacked in the second instar followed by third then fourth instars. However, the few parasitoids that completed a generation from the fourth instars did so significantly faster than conspecifics that started development in the other two instars. In direct observations, however, the parasitoids (i) randomly attacked the various host instars, (ii) spent a similar period examining the various host instars with their ovipositors, (iii) subdued all three host instars with about the same effort, and (iv) no statistical differences were observed in the attack rates on the three host instars. In a choice test, the females parasitized significantly more third instars followed by second then fourth instars. However, total parasitism in this experiment was 43% lower compared to parasitism of only second instars in the no-choice test. No significant differences were detected in progeny sex ratios. In both choice and no-choice tests, significantly more fourth instars died during the course of the experiments than second instars, while third instars were intermediate. The higher parasitism of third than second instars in the choice test indicates that the females perceived larger hosts as higher quality than smaller hosts, despite their lower suitability for larval development.

Highlights-Cotesia vestalis was a dominant primary parasitoid of Plutella xylostella.-It was also a secondary host to three obligate hyperparasitoid species.-At low hyperparasitism, C. vestalis limited hosts available to competitors. -As C. vestalis population declined, competitors parasitized more of available hosts.-Species that are invulnerable to hyperparasitism took over role of the vulnerable. Graphical abstractAbstract : Obligate hyperparasitoids are widely considered an important ecological disturbance to biological control of insect pests, as they develop at the expense of primary parasitoids. However, supporting evidence is largely derived from direct trophic interactions in simple food webs. Yet, a multitude of insect pest populations simultaneously support development of several primary parasitoid species in horticultural and natural systems. Since primary parasitoid species in a community can differ in vulnerability to obligate hyperparasitoids, it is desirable to establish if the invulnerable primary parasitoids can take advantage of reduced competition from affected species by increasing their contribution to total primary parasitism levels thereby mitigating effects of hyperparasitism on biological control. To investigate this question, populations of the diamondback moth, Plutella xylostella (Linnaeus) (Plutellidae), its primary parasitoids and hyperparasitoids were monitored on unsprayed cabbage plots at weekly intervals over six consecutive years. Cotesia vestalis (Haliday) (Braconidae), a dominant primary parasitoid in this system, was a secondary host to three obligate hyperparasitoids: Mesochorus sp. (Ichneumonidae), Eurytoma sp. (Eurytomidae) and Pteromalus sp. (Pteromalidae). The higher efficiency of C. vestalis in utilizing younger host larvae at lower hyperparasitism levels limited host availability to other major primary parasitoids. But, as hyperparasitism levels increased and its populations declined, populations of Oomyzus sokolowskii (Kurdjumov) (Eulophidae) and Diadromus collaris (Gravenhorst) (Ichneumonidae) increased significantly as they parasitized a greater proportion of available hosts. As a consequence, the impact of hyperparasitoids did not result in trophic cascades, as their impact on total primary parasitism levels and infestation levels was insignificant. This study shows that primary parasitoid species that are invulnerable to hyper-parasitism can take over the function of vulnerable ones in communities where interspecific interactions among species are strong. Thus, an approach that considers both direct and indirect effects of hyperparasitoids in primary parasitoid communities improves our understanding of the net impact of hyperparasitism on biological control of insect pests.