Spore yield of the microsporidium Nosema pyrausta (Paillot) Weiser, 1961 is influenced the insect host rearing temperature and diet composition

Abstract

The microsporidium Nosema pyrausta (Paillot) Weiser, 1961 is an important mortality factor of the stem borers of the genus Ostrinia and may be used to control other lepidopteran pests. Rearing conditions (temperature regime and diet composition) of the laboratory host Ostrinia furnacalis (Guenée, 1854) (Pyraloidea: Crambidae) were compared in terms of their influence on N. pyrausta spore production, estimated as the mean spore number per host pupa. At lowered temperature (+20 °C), most of the experimental insects died at the larval stage, and in survived pupae, the microsporidium spore yield was below 8 × 10 6 spores/pupa. At the temperature of +24 °C, which is supposed to be the optimal for the insect laboratory culture exploited, N. pyrausta spore yield was about 50 × 10 6 spores/pupa. At increased temperature (+28 °C), the parasite spore yield was not higher than 28 × 10 6 spores/pupa. The mean spore yield values were significantly different between the variants of the three thermal regimes at 99% confidence level. The microsporidium spore production was further compared in insects fed with a standard corn flour-based diet vs modified diet (corn flour mixed with soybean flour at an equal proportion), reared at + 24 °C. Insect feeding with the modified diet caused an increase of the average spore yield by 35% (70 × 10 6 vs 52 × 10 6 spores/pupa obtained using the standard diet). Similarly, the weight of the pupae reared on the modified diet was 33% higher (0.0721 g vs 0.0542 g on the standard diet). Both the spore yield and the pupal weight values were significantly different between the two diet types at 95% confidence level. This provides one logical explanation of the parasite spore production increase as conditioned by the insect host weight increment. These observations indicate that +24 °C is the optimal thermal regime for N. pyrausta spore production which can further be augmented by supplementing diet of infected Ostrinia furnacalis larvae with soybean flour.