Pea and Bean Weevil Mitigation

I'm working a new allotment plot this season and, as I normally do, I planted broad beans. I watched the little seedlings grow, but my glee quickly turned to concern when I noticed a curious problem develop. All the young leaves acquired these little notches along their edges. Below, you can see what I mean, like little peck marks, but they seemed too uniform and neat for pigeon damage...

I felt puzzled. So, as I do, I commenced some research and RHS quickly offered the answer: pea and bean weevils (Sitona lineatus).

The leaf damage is caused by adult weevils. They overwinter in field margins and then fly in spring seeking out legumes, preferably beans and peas, on which to feed. Once fed, they lay their eggs in the soil around these same plants. Their larvae then feed on the plant's nitrogen fixing root nodules (housing rhizobia). To quench your curiosity, you can see clusters of rhizobia filled root nodules growing on the broad bean roots below.

My natural next question was how to control these weevils. I didn't feel satisfied with the RHS's suggestion that control is unnecessary because most plants tolerate this damage. I needed to know more.

This led me to a fascinating review article. It is written for commercial agronomists but, I would argue, is equally as relevant to allotmenteers and home growers. 

It confirmed my hunch. We can do more than helplessly watch our plants struggle along. The key is understanding what the larvae feed on, the rhizobia. 

It turns out, legumes vary the number of rhizobia root nodules they grow depending on how much nitrogen is already available in the surrounding soil. When soil nitrogen levels are high, legumes don't bother growing lots of nodules. They only produce a few. 

If the weevils eat them, it doesn't matter much to the plant because it has access to enough nitrogen from the soil anyway. There is a further benefit too. The larvae have few rhizobia on which to feed, so only a few weevils make it to adulthood reducing the number of weevils next year. 

The moral of this story is now obvious. Fertilise your peas and beans with good quantities of nitrogen rich compost. Top dressing the beds with further compost throughout the season is bound to help too. This will not only allow your current peas and beans to thrive despite the weevil damage, but also naturally reduce the pest pressure in future years.

Problem solved. Compost, glorious compost, I do love you!