In order to be able to answer this question clearly we need to know the nature of damage, i.e. for example whether damage was on the leaves, buds, growing apex, roots or any other part of the plant in question. We may also need to know the type of crop you are referring to because different plant species respond differently to insect damage. I hope you will clarify your question for better response and discussion.

One possible explanation is that cowpea has undeterminate growth (modular growth); as a result during vegetative growth it can recover by growing new branches

Sorry Ndiso, part of my earlier response required indication of of the type of crop. I noted later that you were referring to cow pea. My understanding of cow pea is that removal or damage to some eaves will have no significant effect on grain yield. This is why farmers pick some leaves for vegetable consumption without significant reduction in grain yield. The possible explanation for this observation is that cow pea pods do photosynthesize and depend very little on current assimilate for pod filling. In addition, pod filling in cow pea also depends on stored assimilate in the stems and branches. When current assimilates are low, the plant mobilizes these assimilates for effective pod filling. We also need to note that although cow pea is indeterminate in growth habit, vegetative growth is highly suppressed during reproductive growth, which means that there will not be any new fully grown leaves after reproductive development. From this explanation it appears that the cow pea plant depends very little on the leaves during reproductive growth and hence, vegetative damage by insect pests will have only minimum effects on grain filling and hence, productivity. In this crop, insect pests attacking the reproductive structures will have large effect on grain yield. Such pests should be controlled rigorously.