An Entomopathogenic Fungus, Hirsutella Thohpsonii Fisher (Fungi Imperfect) as a Potential Biological Control Agent of Hononychellus Tanajoa. Bondar (Acari: Tetranychidae)

Abstract

The infectivity of H. thompsonii on cassava green mite, (CGM), H. tanajoa was determined in laboratory, cage and field experiments. In the laboratory, eggs, larvae and adult females were sprayed with suspensions of the fungus at concentrations ranging between 9.8x10^5 to 1x10^8 conidia per ml (CPM). Between 3 to 21%egg hatch reduction was recorded from H. thompsonii. treatment compared with the control. Phase contrast micrographs did not show that the fungus grew onto the surface of, or, penetrated into t he eggs. Significant mortality (up to 34%) was recorded from H. thompsonii treated CGM females. The number of eggs laid per CGM female treated with the fungus was significantly lower than the number laid by the control batch. Death of infected female mites mainly occurred between 3 to 6 days after infection in the laboratory. Within this time, mite cadavers were observed to undergo some morphological changes, e.g body coloration from creamy appearence through brown, dark-brown and development of fungal growth; body became turgid, and then broke open and rapidly shrank to disappearence; they were invariably attached to the substratum; fungal aycellal penetration into the aite tissue and their condiation could be demonstrated from 72 hours after treatment. CGM larvae were not appreciably killed by H. thompsonii. In potted experiments, the number of eggs and live mites on the leaves were reduced following application of the fungus. The protection given was slightly better than those from Aablyseius teke (a phytoseiid mite predating on CGM) introduction. In the field aqueous suspensions of H. thoapsonii conidia at a concentration of 8.0x10^5 and 3.7x10^5 CPM were applied to M. tanajoa pest on field cassava at the peak of pest infestation, which occurred at the driest and hottest period of the season. The treatment reduced the number of egg and live CGM by 63 and 77% respectively. The damage symptoms caused by the pest was also reduced. This was particularly evident from the second week after treatment application. Rainfall was a density independent mortality factor. Heavy precipitation led to a sharp reduction crash in mite numbers and subsequent levels of pest damage. It is therefore shown here that timely application of ff. thompsonii in coordination with other control or mortality factors, e.g under integrated pest management practices, could greatly help reduce the level of pest damage. Pest distribution was most concentrated on the upper, younger leaves between leaves 1 to 5. The numbers fell with increasing age of the leaves. The effects of temperature, rainfall totals and number of rainy days per month on pest population and damage symptom indices are discussed.