Bean leaf beetle
Cerotoma trifurcata
Round shot holes punched through bean leaves, and a small black triangle behind the head that identifies the beetle whatever color it happens to be.
How to know it
A beetle 5 mm long, varying from yellow-tan to red to almost green, usually with four black squares on the wing cases — but the one constant is a small black triangle immediately behind the head. That triangle is the identification when nothing else is. It drops and plays dead when disturbed.
What you'll see
- Neat round holes punched through the leaves, mostly between the veins, rather than chewed from the edge.
- Seedlings with their cotyledons and first leaves riddled, which can kill them outright.
- Shallow scars grazed on the surface of the pods.
- Pods clipped off at the stalk and dropped, on a heavily infested plant.
- Beetles dropping off the plant as you approach.
Where it comes from
Adults overwinter under leaf litter in woodland edges and hedgerows and move into the earliest beans they can find in spring — which is why the first sowing always takes the worst of it. Two generations a year in most of the range. The larvae feed on the roots and nitrogen nodules underground, which does more harm than is usually credited.
Warmest and worst in the midwest and south. The first bean planting in the neighborhood is a magnet, and gardens next to woodland or overwintering cover carry more.
Dealing with it
- Cover seedlings with fine mesh until they are well established and flowering; the seedling stage is where the real loss is.
- Hand-pick in the early morning, holding a tray under the plant since they drop.
- Spray insecticidal soap or neem on a heavy seedling infestation.
- Pull and bin plants that are being clipped of pods rather than nursing them.
- Accept shot holes on a mature, vigorous plant — the leaf area lost is smaller than it looks.
Keeping it out
- Sow a little later so the crop is not the earliest bean available to the overwintered beetles.
- Rotate beans away from last year ground and clear all bean debris in autumn.
- Clear leaf litter from the garden boundary where the adults overwinter.
- Grow a trap row of an early bean and pull it once the beetles gather on it.
- Encourage ground beetles, spiders and soldier beetles by mulching and leaving cover.
What it goes for
Snap, dry and lima beans, soybeans, cowpeas and peas.
Who hunts it for you
- Ground beetleCarabidae Fast, glossy night hunters that patrol the soil surface for slugs, cutworms, and root-fly larvae. Give them daytime cover — thick perennial edges, stepping stones, a log — and skip the tillage that wrecks their burrows. Grow to recruit it: Lavender,Mint,Thyme
- Soldier beetleCantharidae Soft-winged beetles seen mating on flower heads in high summer; adults and larvae both take aphids and soft grubs. Open composite flowers are their landing pads and hunting grounds. Grow to recruit it: Echinacea,Rudbeckia,Sunflower,Yarrow,Zinnia
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com