Bean leaf beetles
- The bean leaf beetle (Cerotoma trifurcata) is a pest of snap beans (also called string beans or green beans).
- Adult beetles feed on the undersides of leaves, creating round, 1/8 inch diameter holes.
- They can also feed directly on the pod.
- Adults are active mid-May to early June and mid-July through September.
- Bean leaf beetles are generally more common in southern Minnesota than in the northern part of the state.
How to identify bean leaf beetles
- Adults are about ¼ inch long, oval-shaped insects, with heads visible from above.
- Most bean leaf beetles in Minnesota are yellowish-green with four black spots and black markings along the outside margins of the wings.
- Some are red and some lack spots.
- All bean leaf beetles have a black triangle at the top of their wing covers.
Life cycle of bean leaf beetles
- Adult bean leaf beetles spend the winter in the soil under leaves (especially in wooded areas), in clumps of grass or inside dried curled leaves in leaf litter.
- They emerge from mid‑May to early June. They feed first, then mate.
- Females lay clusters of about 12 orange eggs in the soil around the base of the beans.
- In addition to snap beans (also called green beans or string beans), they also attack soybeans, clover, dry edible beans and several leguminous weeds.
- Eggs hatch into larvae one to three weeks later, depending on temperature.
- White larvae feed on the bean roots, causing only minor damage to plants.
- They feed for about two to three weeks then transform into pupae in the soil.
- Adults emerge from mid‑July through August.
- There is usually only one generation per year in Minnesota.
- In the southern part of the state, two generations can occur.
- The first generation appears in July and the second appears in late August and September.
- High numbers of bean leaf beetles can be attributed to mild winters or adequate snow cover that insulates and protects adult populations in the winter.
Damage caused by bean leaf beetles
- Adult bean leaf beetles prefer to eat tender young plant tissue.
- High populations of adults can defoliate the first true leaves and kill young seedlings.
- Extensive feeding can weaken the plant and reduce the yield.
When pods form later in the season, adults will also feed on their outer surface. This feeding only affects the appearance of the pods.
Bean leaf beetles are known to carry and spread some plant diseases. Fortunately, this is not an issue in home gardens as most snap bean varieties are not affected by these diseases.
How to protect your plants
Delay the planting time in spring
You can minimize the risk of bean leaf damage in spring by delaying the planting of snap beans. Snap beans take about 60 days to grow.
- In southern Minnesota, plant in early to mid‑June to minimize the damage.
- You can plant them as late as mid-July in southern Minnesota and at the end of June in northern Minnesota.
Examine your plants
If you have had bean leaf beetle infestations in the past, it is important to monitor your garden for them.
- The best time to check is in the afternoon between 12 and 4 p.m.
- Check your plants early in the season when they can suffer most damage due to beetle feeding.
- Check for beetles as well as signs of feeding damage.
If you find moderate or severe injury (about 25 percent defoliation, or 6-10 holes/leaf) on 10 percent or more of your plants, you should protect your snap beans, especially after the first set of true leaves is present.
As snap beans grow larger and develop more leaves, they become more tolerant of defoliation.
Remove beetles off plants
- Remove bean leaf beetles in your garden to reduce their numbers.
- Bean leaf beetles often drop to the ground when plants are disturbed.
- Position the pail underneath the plant to catch them as they fall.
- This method may not be practical in larger gardens.
If necessary, spray your snap beans with a pesticide to protect them from bean leaf beetles. Be sure they are numerous enough to justify treatment. It is less necessary to treat bean leaf beetles later in the summer.
Examples of effective pesticides:
- Pyrethrins (effective less than one day)
- Neem (effective for several days)
- Spinosad (effective for about one week)
- Malathion (effective for about one week)
- Carbaryl (effective for about one to two weeks)
- Pyrethroids, including permethrin, lambda cyhalothrin and cyfluthrin (effective for two to three weeks)
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018