Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2000
Publication Date: 10/20/2000
Citation: James, R.R., Jaronski, S.T. 2000. Effect of low viability on infectivity of beauveria bassiana conidia towards silverleaf whitefly. Journal of Invertebrate Pathology 76(3):227-228. Interpretive Summary: For many fungi that are pathogenic to insects, such as Beauveria bassiana, it is the spores that are most infectious and responsible for the spread of disease. These fungi can be very important as biological pesticides of serious insect pests, such as the silverleaf whitefly. However, the manner in which the fungus is grown and stored can affect spore survival. To standardize bioassays and field trials, researchers sometimes use the number of live spores as a measure of active ingredient. We speculate that conditions which reduce survivorship might also reduce the ability of remaining spores to infect an insect. Using silverleaf whitefly nymphs as a host, we tested the infectivity of three preparations of B. bassiana that had high (95%) and low (55 and 51%) survivorship. In the laboratory, we tested a range of spore application rates and found that it took nearly twice as many live spores to cause infection in whiteflies for the preparations with low survivorship than in the one with high survivorship. Thus, when the spore survivorship was reduced by half, the infectivity of remaining spores was also reduced by the same amount. In other words, if a spore preparation of B. bassiana, and possibly other fungal pesticides, has been stored such that many of the spores have died, researchers and farmers cannot fully make up for the loss of pathogenic activity by just increasing the amount used until they have the same number of live spores. Considerably more spores may be needed.
Technical Abstract: Viability of conidia from deuteromycetous fungi that infect insects, such as Beauveria bassiana, can vary depending on growth and storage conditions. When conidia are the infective unit being tested in bioassays and field trials, researchers sometimes compensate for low viability in a fungal preparation by increasing the concentration so that they have the desired number of viable conidia. We speculate that conditions which reduce viability might also reduce infectivity. Using 3rd instars of the silverleaf whitefly (Homoptera: Aleyrodidae) as a host, we tested here the infectivity of three preparations of B. bassiana that had high (95%) and low (55 and 51%) viability. Using a dose response bioassay, we found the LC50 for the preparation with high viability to be 306 viable spores mm-2; whereas, the LC50 for the low viability preparations was nearly twice as high, at 589 and 593 viable spores mm-2, respectively. Thus, when the viability was reduced by half, the infectivity was also reduced by the same magnitude (i.e. the LC50 was nearly twice as high). The implication is that if a spore preparation of B. bassiana, and possibly other deuteromycetes, has low viability (<95%), significantly more viable spores may be needed to obtain the same pathogenic activity as a preparation of fresh spores with >95% viability.