Insect damage such as Mexican bean beetle Epilachna varivestis
I think these types of feeding window opening could be caused by a leaf eating beetle as suggested or a lepidopteran moth and you will need to catch the insects in the act and identify them. Many of lepidopteran are nocturnal and are best seen at dawn. Some of the beetles have a dropping behavior and can be collected from the base of the plants or on the soil. If you have larvae stages keep them in a ventilated jar with fresh foliage to get adult stages for identification. Proper identification will be useful for identifying potential management strategies.
If you have neem trees in your vicinity they are a good biorational biological control for the insect pests of cowpea.
The extraction of the principle is by alcohol either the fruit with seed or the leaves are the active agent
The mode of action is to play havoc with insect juevenile hormone which is not found in mammalians like us.
In colonial days I believe the British planted neem which comes naturally where ever they had power.
If the rotation of control is needed the Bacillus thuringiensis occurs in strains which attack either lepidopteran or beetle types so ID would be very important.
Another I notice on those cowpeas is a tip chlorosis which can be associated with alkaline soil conditions leading the deficiencies of Fe, Mn and/or Zinc.
You may want to get a soil pH reading to confirm the condition and address the issue which compromises the plant immunity defensive system.
Getting the nutrition right can often greatly increase the ability of the plants to deal with weeds, pests and diseases.
It could be damage by leaf beetles like Disonycha glabrata or Epilachna varivestis. Also slugs can damage leafs of cow pea and beans. Try to check at night if you see slugs.
Such damage symptoms in cowpea leaves, in my cowpea research was due to locust attacks.
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