Observations on the bionomics of Laspeyresia ptychora Meyr. (Lepidoptera, Eucosmidae) infesting cowpea in Nigeria

Extract

One of the principal pests of cowpea (Vigna unguiculata) in southern Nigeria is Laspeyresia ptychora Meyr., the larvae of which attack the seeds within mature pods and cause considerable loss of yield. It has not so far been recorded from northern Nigeria. Details of the life-history of this Eucosmid are described from laboratory and field observations made in Nigeria in 1962–63. Eggs are laid on mature pods, which the larvae enter very soon after hatching, feeding on the seeds within. Pupation normally takes place in a cocoon in the soil, but pupal development can be completed in the absence of soil and of a cocoon. Four generations normally occur on the two crops of cowpea that can be grown each year in southern Nigeria, a complete first and a partial or complete second generation on each; but five generations may sometimes occur. The pest survives through the dry season and between the two crops in the rainy season by means of generations that develop on stand-over and volunteer cowpea, on leguminous plants grown in market gardens, and possibly on some yet undiscovered wild plant. No diapause in any stage was observed.

Annual fluctuations in larval populations and levels of infestation are described in relation to the cowpea crops and to the damage caused. In a field trial in 1963, 35 per cent, (by weight) of seed from the early crop and 20 per cent, of that from the late crop was damaged. These figures take no account of seed that was completely destroyed, so that the true losses are greater than they indicate. Damaged seed is usually unmarketable as a result of contamination with frass and webbing.

Investigations showed that, although larvae may be carried into stores when the crop is harvested, L. ptychora cannot become a pest of stored cowpea seeds because it is unable to develop on dried seeds.

The possibilities of chemical, cultural and biological control are discussed, the latter not at present being promising. Field trials with γ BHC, dieldrin, carbaryl, isobenzan and phosphamidon were carried out; only the last-named, which is a systemic insecticide, showed promise of affording satisfactory control. The difficulties of successfully attacking L. ptychora with insecticides are discussed, and it is concluded that chemical measures will need the support of modified cultural practices, particularly those directed towards destruction of stand-over and volunteer cowpea plants, if a satisfactory level of control is to be achieved.