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THE INSECT GROUPS

Four main groups of insects feed upon the seeds of trees of the family Leguminosae in arid and semi-arid zones. These are Coleoptera (Beetles), Hemiptera (Plant Bugs), Lepidoptera (Moths and Butterflies) and Hymenoptera (Phytophagus Wasps). Of these four, the Coleoptera are by far the most important, containing within the group the Bruchidae (Seed Beetles). The larval stage of this latter group live for the greater part of their developmental period within a single seed. The larvae of Lepidoptera and Hymenoptera develop on seeds rather more indiscriminately, often attacking several seeds but without completely destroying them. There is very little biological information on these two groups. Also little is known of the plant host relationship of the other main group, the Hemiptera, or the extent of their feeding range and the resultant damage to pods and seeds. The Hemiptera, in an agricultural situation, can cause considerable loss of yield to crops such as cowpeas (Vigna unquicularis). Losses attributable to Hemiptera on cowpeas in the field in Uganda are stated by Nyiira (1978) to be as high as 40%.

The larval stage of Coleoptera, Lepidoptera and Hymenoptera cause damage by boring into the seed from an egg laid on the pod or in the pod tissue. Hemiptera feed externally in the larval and adult stages by inserting sucking mouthparts into pod tissue. Any damage during the formative growth of pods or seeds as the direct result of insect attack must affect the total seed production of the tree.

The pressures exerted by seed feeding insects cause undue stress on the plant's functions. The plant in turn responds by producing a greater amount of seed. A first priority therefore is an early study of the local ecology of the trees, particularly if the species concerned are known to have a history of insect association, so that some form of control strategy can be formulated.


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