A very interesting question! In the case of protecting stored grains/seeds etc from insects, it appears that one traditional method of protection is the use of other plants as natural insecticides. e.g. in China, plants such as neem and chilli are used as a natural insecticide for grain storage. In Ghana, tobacco leaves which can not be sold are used by the farmers to protect their grain. However, there is very little information on how these plants are applied to the stored commodity (see this document from the FAO http://www.fao.org/docrep/x2230e/x223...)

The traditional use of plants as insecticides led to interest from scientists who are keen to find alternatives to chemical pesticides. Many chemicals are being withdrawn from use on the basis of environmental concerns and an alternative is desperately needed to improve global food security.

I have been scanning the scientific literature on bean weevil feeding in order to try and come up with an answer to this and I think that the answer must lie in the control of the adults on the plants prior to egg laying. Due to their importance as pests of stored seed/grain, weevils have received some attention and have been subject to a number of experiments ultimately aimed at controlling them post-harvest.

An area of interest in the literature is the use of botanical essential oils to repel or kill insect pests. Essential oils have received attention as in addition to the antimicrobial and antifungal activities, they also have insecticidal activity as both fumigants (spraying of gas) and contact insecticides.

One study I found (Sertkaya, 2013; see image) recorded a 100% adult mortality when bean weevils were exposed to preparations of the essential oils of thyme and oregano. The oils of rosemary, basil and lemon balm also caused insect mortality, but to a lesser extent. It is worth noting that these studies were done in the laboratory and not on plants out in the field but I wonder if it may be worth conducting a little home experiment by rubbing some thyme or rosemary oil onto the leaves of your plants?

Additional information

Acanthoscelides obtectus (bean weevil) life cycle

Adult bean weevils overwinter indoors in stored seeds or outdoors in protected areas. The adults become active when the bean plants are flowering and and they lay their eggs either in holes they have chewed in the pod, or in natural wounds. The larvae hatch in 20-30 days and move through the pod to the seed. The larvae burrow through the seed coat and feed on the interior. The larvae pupate inside the seed and exit by chewing a circular hole in the seed. Newly emerged adults will migrate to new bean plants and after mating, females will again lay their eggs in the pods to complete the cycle. The entire life cycle can be completed in as little as 21 days in favorable conditions, allowing 6-8 generations to be produced each year!

Female weevils that emerge from stored seed behave slightly differently and will lay their eggs close to the seeds. When the larvae hatch out, the will move to the seed and bore through the coat.....In addition to the adults, I would imagine it is at this point - as they are moving to the seed - that they are susceptible to the insecticide (neem, rosemary etc)