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Insect thermal tolerance affects survival and distribution of species but can vary within and between individuals due to the environmental conditions they experience. The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), recently invaded Africa and its local hotspots are associated with weather and crop phenology. We investigated the effects of larval diet (maize plants, wheat plants or chickpea‐based artificial diet), life stages and acclimation temperature on thermal tolerance traits of FAW to explore how these variables might contribute to its presence in the field. First and sixth instar larvae and adults reared on each diet at 25°C were acclimated for 24 h at 20, 25 or 30°C. We then recorded the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of individuals. Sixth instars had the highest CTmin but the effects of acclimation and diet on this trait depended on the life stage being tested. CTmin of first instars increased with acclimation temperature when fed on an artificial diet, but sixth instars and adults were not affected by acclimation or larval diet. CTmax was lowest among adults but acclimation and diet again had effects that differed between life stages. CTmax of first instars and adults increased with acclimation temperature but not in sixth instars. Sixth instars and adults reared on the artificial diet had the highest CTmax but diet had no effect on first instar CTmax. Our results show the complexity of thermal tolerance across FAW life stages and suggest the need to consider local temperature variation and available dietary resources when predicting their potential distribution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]