Compatibility of the fungus Beauveria bassiana and Trichoplusia ni SNPV against the cabbage looper Trichoplusia ni : crop plant matters

BACKGROUND: Microbial insecticides are an important weapon in insect pest management, but their use is still relatively limited. One approach for increasing their efficacy and use could be to combine different pathogens to increase pest mortality. However, little is known about whether increasing pathogen diversity will improve pest management. Here, we investigated the compatibility of two pathogens for the management of the cabbage looper, Trichoplusia ni; T. ni nucleopolyhedrovirus (TniSNPV) and the entomopathogenic fungus Beauveria bassiana on two crops, tomato and broccoli. The pathogens were applied to individual plants using ultra low volume sprays, alone or in combination, either synchronously or asynchronously. Healthy 3rd instar T. ni larvae were introduced to the plants before application and collected by destructive sampling 24h after the last pathogen application.

RESULTS: Combined applications did not result in an increase in larval mortality compared to TniSNPV alone, although mortality was generally high. Beauveria bassiana was considerably less effective on broccoli compared to tomato. In both the combined treatments, virus-induced mortality was approximately 50% lower when applied together with the fungus, while fungus-induced mortality was not affected by the virus, even when the virus was introduced 24h before the fungus.

CONCLUSION: While our results suggest that applying this combination of entomopathogens would not be beneficial for pest management, this study illustrates the need to consider the target crop as an important driver of the efficacy of both single and mixed pathogen applications in the field.

Methods
Data overview:

date: September and October 2019
location: Simon Fraser University
larvae were reared from neonate in individual 1oz SOLO cups, fed artificial whet-germ based diet and kept at 25C, 16:8 L:D cycle, until 3rd instar
newly 3rd instar larvae were then randomly selected and transferred to one of the two host plants (broccoli and tomato) in a greenhouse
After pathogen application, larvae were collected and reared individually in 1oz SOLO cups, fed artificial whet-germ based diet and kept at 25C, 16:8 L:D cycle

Data collected:

individual date of death or pupation (survival)
cause of death
if death occured, death cause by fungal infection where checked for signs of sporulation on the cadaver