Pea Weevils – The Ugly TruthApril 25, 2012
I’m reminded once again that even if you practice crop rotation, there’s no guarantee a garden pests won’t find their way into your garden.
Recently, I began noticing notching on the leaves of some of my peas. At first, I chalked this damage up to birds that like to sit on pea trellises and peck at the edges of pea leaves doing quite a bit of damage. But then I realized the damage was far too uniform to be the result of random bird pecking. And, I began to notice similar notching on the edges of fava beans growing in another bed that birds couldn’t access.
So, I knew it was time to look a the problem a little more closely.
I wasn’t able to find any pests around the peas, so I took my investigation to the favas. Inside the favas, along the stems, I observed tiny ants. They weren’t farming aphids, which they’re known to do on aphid-attracting favas. But, they weren’t cutting leaf edges either. Instead, they were sipping nectar, which the fava excretes from a spot near the base of each leaf. (More on that available here.)
So, the mystery of the notched leaves continued. I did some reading on the internet, which left me confused. Was the damage due to bean weevils or pea weevils? Both are discussed in detail all over the web, but the names are often used interchangably, leaving me with even more questions. (Hopefully, this post will help folks ID pea weevil more easily than I was able.)
So, I took those questions to WSU Entomologist Sharon Collman who teaches an on-going extension course in pests and disease called ‘Bugs & Blights’. After exchanging just a few emails with Sharon, we had the mystery solved and an IPM (Integrated Pest Management) plan in place. When I asked Sharon if I could share this information with gardenhelp.org readers, her response was fantastic: “My goal is to teach people and the more people that pass on information the more people learn.” That’s a statement after my own heart. So here’s how we drilled down on the problem together:
First, Sharon clarified that Bean Weevils and Pea Weevils are actually different insects. Bean Weevils aren’t even true Weevils though they are a close relation, and that’s all I’m going to say about Bean Weevils.
Next, she asked, “Are you thinking of pea weevils that notch the edges of newly germinating peas?”
That question, which identified the damage I was seeing, identified the damage as that of the Pea Weevil.
Sharon went on to explain: “If (this is the damage you’re seeing) I?ve seen them in the daytime though I think they may do the notching at night. They are small and I think a bit striped.”
Unfortunately, despite checking during the day and at night with a headlamp, I haven’t been able to catch one of these suckers in action.
But perhaps they’ve finished their Spring gorge on my peas and favas.
To my relief, Sharon tells me: “…Once the peas reach 6 leaves apparently they won?t cause significant damage to the peas. These do not infest the peas in the pod but rather the larvae feed on root nitrogen fixing bacterial root nodules. Plants of the family Leguminosae (legumes) are the major food of the adult weevils. The larvae feed only on Rhizobium sp. nodules of these plants.”
The favas have built up at least six leaves at this point; the peas are a little further behind in growth and have sustained more damage than the favas. The peas were planted in a more exposed site, earlier in the season than the favas. This may be why they’re in worse shape. Fortunately, both crops continue to grow, and the now-identified damage has begun to taper off.
Sharon’s final bit of insight: “I guess if (the weevils are) really abundant they might cause some stunting but usually in home gardens if the plants make it to 6 leaves, the plants will do fine. If your population is too high you may have to pick off some, or I guess spraying is an option. Some people grow peas in a gutter inside then slide the pea plants into the furrow so that they have a head start on the season and maybe on the weevils which have an annual life cycle….Pea leaf weevil is probably the least of a garden’s worries. It’s a once a year thing so once you get through the spring you are good to go till next year.”
So here’s hoping I get a decent crop of peas and favas this spring despite this infestation. And, for the next couple of years, we’ll be growing no legumes in those areas. Hopefully, this will break the life cycle of this annoying pest.
Any larvae feeding on the root nodules should be removed and disposed of when the crop waste is removed from the soil after harvest. I will not be planting any bush, pole or runner beans in these beds this summer (as I’d originally planned). Any larvae that remain should be without a food source for the remainder of this year, following harvest. Should they still survive — perhaps on annoying wild vetch weed roots — and attempt to emerge next spring, they will find that there are no peas or beans to enjoy in those beds. And hopefully this will help us reach the goal of eliminating the pest by breaking its life cycle. No pesticides required!