Phlyctinus callosus taxonomy and field management

This project aims to assess the taxonomic status of P. callosus complex, confirm the laboratory efficacy of EPF and EPN strains against the different species of P. callosus, as well as elaborate semi-field trials to test efficacy of several products/strains of EPF and EPN that were found to be effective previously. The EPF semi-field trials will test targeted application of EPF under field conditions as an alternative to direct spray application. This targeted application has the advantage of placing EPF in specific microhabitats where BFW naturally aggregates and incorporating microbial ‘insecticide’ application with standard management and monitoring practices to save time and cost of materials. Concentrating the number of infective propagules of EPF in sheltered microhabitats where weevils occur will potentially prolong the active life of the product and potentially lead to increased levels of control relative to general spray application

Tissues of mounted reference specimens of the six Phlyctinus species described in Haran et al. (2020), as well as other Western Cape pest weevils, were sequenced for the standard barcode (COI) gene region.

Adult Phlyctinus callosus and Phlyctinus xerophilus were screened against the EPN and EPF species/strains found to be effective in previous studies using 24 well bio-assay trays.

Larvae of both species were tested in small scale, caged field experiments, and assessed 96 hours after application on a field site in Ceres during March 2022.

Barcoding can be integrated as a tool to identify Western Cape pest weevils. Barcoding accuracy is decreased by the potential presence of unsubscribed cryptic species in some taxa. More basic taxonomic work is needed.

Differences were observed in the susceptibility of P. callosus and P. xerophilus adults to EPNs and EPF. P. callosus seems to be more susceptible to EPF, while P. xerophilus seems more susceptible to EPNs. Adults of both species are very resistant even to virulent EPNs and EPF.

Caged field trials resulted in mortality of P. callosus of 22 % and P. xerophilus of 45%. Control mortality was below 5%, and average % of pupae recovered was 77%.

Molecular identification can play an important role in pest weevil ID. Barcoding accuracy decreases due to the potential presence of as yet unsubscribed cryptic species, and genetic diversity within species, indicating the importance of basic taxonomic research.

Different cryptic species in the BFW complex may react differently to biological control agents. Accurate identification of BFW species in a specific farm/orchard is therefore important when applying biological products. Adult weevils are very resistant to EPNs and EPF-larvae and pupae may present a better target for biological control through EPNs.

Further research should focus on larger field trials, in order to improve current mortality rates, but manipulating application time, different EPN species and concentration.

An increased focus on basic taxonomic research underpins any applied research, as the presence of cryptic species plays a role in accuracy of molecular identification and potential susceptibility to biological control products. This was seen most notably in the small scale field trial, where mortality of P. xerophilus was more than double that of P. callosus using EPNs.