Niche comparison among two invasive leafminer species and their parasitoid Opius biroi: implications for competitive displacement

Foraging egg parasitoids, Trissolcus vassilievi (Hymenoptera: Platygastridae), respond to host density and conspecific competitors in a patchy laboratory environment

The egg parasitoid Trissolcus vassilievi (Mayr) is a key natural enemy of sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae). This laboratory study examined how the aggregation of parasitoids varied in response to host density and numbers of foraging conspecifics. Five host densities (1, 2, 4, 8, and 15 clutches of E. integriceps eggs), were offered simultaneously to 1, 2, 4, 8, or 16 female wasps in a standardized arena (30 cm diameter) and patch residence times were recorded. Patch residence time was strongly correlated with parasitism, and increased as the number of host clutches increased up to eight, but declined when more clutches were offered. Wasps displayed low mobility and tended to remain in the patch initially encountered, even when it contained few egg masses. At higher wasp densities, patches were occupied more quickly, host exploitation began sooner, and per capita handling time was reduced. However, total patch residence times were similar across all densities. Thus, females responded to conspecifics with increased movement, which increased dispersal and resulted in a more homogeneous distribution of wasps among patches. Pseudo-interference, resulting from wasps remaining in some patches and neglecting others, had greater impact on final levels of host exploitation than did actual interference (patch abandonment following conspecific encounters). These phenomena resulted in decreased parasitoid searching rates and a 2-fold increase in host survival at higher wasp densities.

Using Sentinel-2 Multispectral Images to Map the Occurrence of the Cossid Moth (Coryphodema tristis) in Eucalyptus Nitens Plantations of Mpumalanga, South Africa

Coryphodema tristis is a wood-boring insect, indigenous to South Africa, that has recently been identified as an emerging pest feeding on Eucalyptus nitens, resulting in extensive damage and economic loss. Eucalyptus plantations contributes over 9% to the total exported manufactured goods of South Africa which contributes significantly to the gross domestic product. Currently, the distribution extent of the Coryphodema tristis is unknown and estimated to infest Eucalyptus nitens compartments from less than 1% to nearly 80%, which is certainly a concern for the forestry sector related to the quantity and quality of yield produced. Therefore, the study sought to model the probability of occurrence of Coryphodema tristis on Eucalyptus nitens plantations in Mpumalanga, South Africa, using data from the Sentinel-2 multispectral instrument (MSI). Traditional field surveys were carried out through mass trapping in all compartments (n = 878) of Eucalyptus nitens plantations. Only 371 Eucalyptus nitens compartments were positively identified as infested and were used to generate the Coryphodema tristis presence data. Presence data and spectral features from the area were analysed using the Maxent algorithm. Model performance was evaluated using the receiver operating characteristics (ROC) curve showing the area under the curve (AUC) and True Skill Statistic (TSS) while the performance of predictors was analysed with the jack-knife. Validation of results were conducted using the test data. Using only the occurrence data and Sentinel-2 bands and derived vegetation indices, the Maxent model provided successful results, exhibiting an area under the curve (AUC) of 0.890. The Photosynthetic vigour ratio, Band 5 (Red edge 1), Band 4 (Red), Green NDVI hyper, Band 3 (Green) and Band 12 (SWIR 2) were identified as the most influential predictor variables. Results of this study suggest that remotely sensed derived vegetation indices from cost-effective platforms could play a crucial role in supporting forest pest management strategies and infestation control.