Nosema whitei, a microsporidan pathogen of some species of Tribolium: III. Effect on T. castaneum

Abstract

A dose-mortality experiment was set up with 50 newly hatched Tribolium castaneum larvae per dose, incubated at 30°C. The LD 50 was 5.4 × 10 7 spores per gram of medium after 20 days, and 2.2 × 10 5 after 48 days. The slopes were 1.2 and 1.7, respectively. A total of 8 similar assays gave a mean LD 50 ± standard deviation of (1.8 ± 21.1) × 10 7 for mortality after 20 days. The LT 50 ranged from 18.6 days at a dose of 1.0 × 10 8 to 27.5 days at 8.0 × 10 5. A dose of about 10 6 is shown to be critical; above this dose the insects generally died as larvae, whereas below this dose they generally emerged as adults. Infected larvae grew more slowly than healthy larvae. Pupal weights were only significantly lighter at doses above 8.0 × 10 5, but adults from all doses were significantly lighter than the controls. At 8.0 × 10 5, 6 of the 15 adults formed were unable to fold the hind pair of wings correctly. A second experiment showed that at 1.0 × 10 7 all but the first two larval molts were delayed. At this dose, no larvae molted more than five out of a normal seven times. The mean weights at the first four molts were similar in both infected and control groups, but at the fifth molt infected larvae were much lighter. In the final experiment infection was shown to reduce adult fecundity, though the viability of the eggs was not affected and no trans-ovum transmission of infection occurred.