The evaluation of Beauveria isolates for virulence to diamondback moth (Plutella xylostella L.) : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

Beauveria is a genus of entomopathogenic fungi that exhibits traits suitable for the development of commercial biopesticide products to control various arthropod pests, as well as plant pathogens. The literature describes thousands of strains of Beauveria, largely limited to strains of Beauveria bassiana that have been isolated and screened for their ability to control pests around the world. Some of these pests, such as diamondback moth (DBM), the two-spotted spider mite, green peach aphid and housefly, are highly resistant to chemical insecticides. This study was conducted to find at least one New Zealand derived Beauveria isolate able to control DBM. The first question of this study was, “do other Beauveria species have the potential to control DBM”? Thus, 14 isolates from four Beauveria species were assessed for their control of DBM under in vitro conditions. Before the assessment, all 14 isolates were taxonomically identified using both morphological and DNA-based methods. The in vitro bioassays identified three isolates of B. bassiana (Mo1, CTL20, and CTA20) and B. pseudobassiana (FRhp, FW Mana, and I12 Damo) that successfully killed 50% of the DBM larvae (median lethal dose LD50) at a low dosage while isolates from two other species could not achieve LD50 at a higher rate. This is the first report that details B. pseudobassiana as a potential species for DBM control. Combining two or more fungal isolates may improve the efficacy of a biological control agent. The second question examined was, “do combinations of Beauveria isolates result in a synergistic or an antagonistic interaction”? Beauveria isolates were grouped according to their previously assessed virulence towards DBM. Under in vitro conditions, combining the low virulent isolates together resulted in a higher and faster DBM mortality rate than when these isolates were applied separately. Combining the three species of Beauveria together resulted in antagonistic interactions between the fungi, with lower mortality rates than when these isolates were applied separately. Diet can sometimes affect the susceptibility of an insect to entomopathogenic fungi. The third question of this study was “is there any difference in the susceptibility of DBM larvae to fungal infection by selected Beauveria isolates when raised on different brassicas”? DBM larvae were fed on four different brassicas with predicted high (broccoli and cabbage) and low (cauliflower and radish) glucosinolate levels. Glucosinolates have been implicated in the resistance of brassica hosts to plant pathogens. In vitro results showed that DBM larvae fed on broccoli and cabbage were more susceptible to infection by B. pseudobassiana I12 Damo than larvae raised on cauliflower and radish. Conversely, DBM larvae raised on cauliflower and radish were more susceptible to infection by B. bassiana CTL20 than larvae fed on broccoli and cabbage. Secondary metabolites produced by Beauveria spp. have previously been found to have insecticidal properties and can kill a wide range of insects, including DBM. The fourth question investigated was, “was there evidence that an isolate of Beauveria spp. produces toxins that kill DBM larvae directly”? B. pseudobassiana FW Mana was selected as it caused high DBM mortality in previous bioassays but exhibited less sporulation from DBM cadavers compared to the other isolates within the study. The filtered supernatant of B. pseudobassiana FW Mana was used for assays along with toxins from two other less-lethal isolates of Beauveria. As expected, the supernatant from B. pseudobassiana FW Mana caused significantly higher mortality than the supernatant from the other two isolates assessed. Overall, this study found several Beauveria isolates that have the potential to be developed and commercialised as biopesticides to control DBM.