Temperature Affects Biological Control Efficacy: A Microcosm Study of Trichogramma achaeae

Current quality control of mass-reared biological control agents (BCAs) is usually performed in the laboratory and often fails to include behavioural aspects of the BCAs. As a result, the use of efficacy measurements determined solely under laboratory conditions to predict field efficacy can be questioned. In this study, microcosms were designed to estimate biological control efficacy (realised parasitisation efficiency) of Trichogramma achaeae Nagaraja and Nagarkatti (Hymenoptera: Trichogrammatidae) parasitising Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) eggs across the operational temperature range (15–30 °C). Temperature greatly affected the success of females in finding and parasitising E. kuehniella eggs, with parasitisation being reduced at 15 and 20 °C, as both the percentage of parasitised host eggs and the percentage of leaves per plant with parasitised host eggs decreased sharply compared with higher temperatures. Graphing previous data on laboratory fecundity against parasitisation efficiency shows that the laboratory-measured fecundity of T. achaeae was unlikely to predict field efficacy across temperatures. Results also showed that leaf side had no effect on the preference of T. achaeae in parasitising E. kuehniella eggs; however, T. achaeae preferred to lay their eggs on the top tier of plants. These findings suggest that more complex assays, which include behavioural responses, might be developed for optimised quality control of BCAs intended for field application.

Clutch variation and egg rejection in three hosts of the pallid cuckoo, Cuculus pallidus

In theory, hosts of avian brood parasites would benefit by modifying their egg appearance in two ways to help identify mimetic foreign eggs: (i) by laying clutches that are more uniform in appearance and (ii) by laying clutches that differ from those of other females in the population. Support for these theories is inconsistent, and few studies have used objective measures of clutch variation. Here we used reflectance spectrophotometry to quantify within-clutch and between-clutch variation of three host species of an Australian brood parasite, the pallid cuckoo (Cuculus pallidus). We used egg-swapping experiments in which subjects were presented with either a conspecific egg or a heterospecific egg to compare the egg rejection responses of a frequently parasitised host, the white-plumed honeyeater (Lichenostomus penicillatus), with two less frequently parasitised hosts, dusky woodswallows (Artamus cyanopterus) and willie wagtails (Rhipidura leucophrys). As predicted, rejection rate increased as contrast between foreign egg and host clutch increased. Further, the major host showed greater between-clutch variation than the occasional hosts, and also rejected more similar-looking eggs. Contrary to predictions however, within-clutch variation was not lower in the major host, nor was it important in predicting the rejection rate of foreign eggs by the three host species.

Effects of Abscisic Acid on the Transfer of Sucrose From Host, Pelargonium zonale (L.) Aiton, to a Phanerogamic Parasite, Cuscuta reflexa Roxb

Using isolated Pelargonium leaves parasitised by Cuscuta, we have established by means of autoradiographs that, in vivo, abscisic acid (ABA) is exported from the host leaf petiole to the attached parasite shoot. Experiments with simplified models (parasitised host stem slices, unparasitised host stem slices, isolated haustoria and parasitised host stem after removal of haustoria) showed that, in vitro, [2-14C]ABA is rapidly accumulated in haustorium tissues, and the internal/external ratio of ABA becomes greater than 1 within 45 min. The very fast ABA uptake, strongly dependent on the pH, may involve facilitated diffusion in addition to the diffusion of the undissociated acid. The high level of ABA concentration in Cuscuta, especially in haustoria, might be explained by ABA import from host tissues. In Cuscuta stem and haustoria, the hormone enhances the uptake and decreases the efflux of sucrose. In host tissues, ABA enhances phloem unloading of sucrose and decreases sucrose uptake. Thus, ABA may have a central role in the host-parasite relationship by enhancing sucrose transfer.