Published online by Cambridge University Press: 10 July 2009
The termites most injurious to crops and trees in West Africa are subterranean or mound-building species of the Termitidae, mainly Macrotermitinae with some Amitermitinae and Nasutitermitinae. Insecticides for their control may be applied generally to the soil, locally around the plant or directly to the colony.
In investigations in Northern Nigeria, dusts containing aldrin or dieldrin were mixed with the top six inches of soil of a type commonly cultivated (a slightly humic, brown, loamy sand), exposed to weathering in the field and tested for persistence of the insecticides by bioassay at intervals, using workers of Trinervitermes ebenerianus Sjöst., a locally common surface-foraging species. The concentration of the insecticide in the soil was measured in terms of the time in days taken for 50 per cent. of the insects to be killed (T50). After 33–34 months, between one-third and one-fifth of the insecticide remained in soil originally treated with 0·5, 2 and 5 lb. active ingredient (a.i.) per acre.
T. ebenerianus proved very sensitive to dieldrin; the T50 value was 1·48–4·10 days for single samples, from each of five colonies, exposed to filter paper containing 0·0018 parts per million, as compared with 10–24 days for the controls. It is suggested that general soil treatments should be used with caution until more is known of their effects on termite populations, which are important in facilitating aeration, and penetration of water, in tropical soils.
Application of dieldrin emulsion, at a dose equivalent to 1 lb. a.i. divided among the planting holes for one acre (1,225), during planting of one-year-old, root-pruned seedlings of Eucalyptus camalduensis, resulted in a mean survival after 2½ years of 60 per cent. of the young trees, as compared with 17 per cent. in untreated controls. Pot-grown seedlings of Eucalyptus spp. were similarly treated at 8 oz. a.i. dieldrin per 700 pots prior to setting out in the field, when they showed very low mortality due to termites over the next 1½ years, attack only occurring where too short a pot allowed access by Macrotermes natalensis (Hav.) to the tap root. Four hundred pot-grown cacao seedlings similarly treated with 4 oz. a.i. dieldrin showed only four deaths due to termites one year after planting out. Pre-treatment of potting soil for Eucalyptus seedlings at 5–10 oz. of 2 per cent, dieldrin dust per cubic yard (sufficient for 500 pots) has given promising preliminary results.
Colonies of M. natalensis, which constructs large mounds, were successfully poisoned with 2½ fl. oz. of aldrin 40 per cent, emulsifiable concentrate in six gallons of water applied through three auger holes made into the central ‘ hive ’, containing the queen cell and associated chambers. It is considered that this dose could safely be reduced.