The bean leaf beetle, or more specifically, Cerotoma trifurcata can be a major pest of soybeans and green beans as well as cucumbers, squash, and pumpkins. Cerotoma trifurcata is not to be confused with the Mexican bean beetle, which is a completely different species!
Bean leaf beetles can be various colors (yellowish-green, red, orange, or brown), but all of them have a distinct black triangular mark at the top of their folded wings. Red ones may be mistaken for a huge, elongated ladybug or the Mexican bean beetle.
There are few things worse than losing the young seedlings that you spent time caring for. What makes these beetles even more of a threat to your plants is that they can be vectors for diseases.
Adults skeletonize foliage and leave damage on bean pods, then lay yellow-orange eggs in the soil. The larvae feed lightly on bean roots but don’t cause extensive damage there. Really, the worst thing to watch for is the adults, as they cause most of the damage, spread plant diseases, and lay more eggs!
There are three main categories for dealing with pests in the garden: cultural, biological, and insecticidal. Cultural control involves physical interventions and barriers. Biological control involves using the natural environment as a defense. Insecticidal control involves chemical intervention.
Depending on the level of infestation, you may use one of these methods or a combination. It’s recommended that insecticidal intervention is only used as a last resort.
Bean Leaf Beetle Overview
|Common Name(s)||Bean Leaf Beetle|
|Scientific Name(s)||Cerotoma Trifurcata|
|Origin||North America, parts of Canada and the United States|
|Plants Affected||Cucumbers, cucurbits, pumpkin, squash, legumes, soybeans, and green beans.|
|Common Remedies||Cultural controls such as physically removing beetles from the plants, crop rotation, drop cloth, and keeping your beds free of plant debris. Biological controls such as attracting predatory insects and, as a last ditch effort, pyrethrins, spinosad, and neem oil|
What Are Bean Leaf Beetles?
Bean leaf beetles, or Cerotoma Trifurcata, are small beetles the size of ladybugs with a similar appearance. They can come in a variety of colors, including yellowish-green, red, orange, and brown.
They are also sometimes confused with Mexican bean beetles, which have a similar appearance and affinity for legumes. Bean leaf beetles, however, have a black band near the outer margin of the wing covers. They also have a black triangle near the head.
These beetles prefer soybean plants and green beans but have also been known to affect cucurbits such as cucumbers, pumpkins, and squash. Overwintering bean leaf beetles emerge in the spring and can cause serious damage to young seedlings. They will skeletonize leaves and damage bean pods.
This type of damage is difficult for tender young seedlings to recover from. Established, more mature plants have a better chance of surviving. In order to control bean leaf beetles, it’s best to understand their life cycle, host plants, and preferred habitat.
Life Cycle of Bean Leaf Beetles
Overwintering beetles emerge in the spring when the soil temperature begins to rise. They choose their overwintering site in plant debris near legumes, like soybean plants. When the adult beetles emerge, they immediately begin looking for a food source and a place to lay their eggs.
Adult beetles live for approximately 40 days, and during that time, they can lay 125-250 eggs. The bean leaf beetle larvae live in the ground and feed on roots until they pupate and emerge as second-generation beetles.
In the adult phase, the bean leaf beetle feeding turns towards the foliage and bean pods of the plants. Depending on the length of your season, the bean leaf beetles may cycle through two generations, laying another round of eggs, resulting in more bean leaf beetle larvae and more adult beetles.
As the temperature grows cooler in the fall, the adults begin to search for plant debris as an overwintering site. The cycle then starts all over again. If you’re wondering how beetles survive freezing temperatures in the ground with no food source, the answer is that not all of them survive.
However, during mild winters, a greater number of overwintering beetles survive, giving a boost to their population and increasing beetle densities. The overwintering adults that survive will emerge again in the spring.
Identifying Bean Leaf Beetle Damage & Their Habitat
As mentioned above, bean leaf beetles can be confused with the Mexican bean beetle, cucumber beetle, or sometimes the common ladybug. Bean leaf beetles can come in various colors, such as yellowish-green, red, orange, or brown, but they always have a black triangle at the base of their brown head when their wing covers are closed.
Yellowish-green coloring seems to be the most common. If visual identification is tricky, inspecting the damage they cause can also be another clue as to whether or not you’re dealing with bean leaf beetles.
Bean leaf beetles feed on a variety of host plants in the legume family. However, they seem to favor soybean seedlings. Bean leaf beetle infestations begin when the soil temperatures begin to rise in the late spring and early summer. The adults emerge about the same time you would plant tender young bean seedlings in your garden.
During the seedling stage, if you observe your emerging soybeans quickly being eaten down to the ground before they can become established, then this may be a sign of bean leaf beetle activity.
Bean leaf beetle infestations continue throughout the season as the first adults lay their eggs in the soil near the bean plants. These larvae feed on the roots and root nodules, impairing the plant’s ability to uptake nitrogen and resulting in yellowing foliage. They do this until they emerge as adults and begin to consume soybean foliage.
If your plants make it past the seedling stage, the damage will appear as beetle populations skeletonize the foliage. In more mature bean plants, the beetles cause damage to developing pods that appear as black dots along the pod. This pod damage results in reduced seed quality.
Bean Leaf Beetles As Disease Vectors
What’s perhaps even worse than the physical damage to plants caused by bean leaf beetles is the fact that they can spread disease to your plants. When bean leaf beetles feed on an infected plant, the virus/bacteria remains in their system. It is then passed along when they feed on the next healthy plant.
Most plant viruses are transmitted this way, with 80% of virus transmission being dependent on insects. This is another reason to attempt to control a bean leaf beetle infestation as soon as you become aware of it.
A few of the viruses that can be transmitted by bean leaf beetles include bean pod mottle virus, southern bean mosaic virus, green stem syndrome, cowpea mosaic virus, and soybean mosaic virus.
Controlling Bean Leaf Beetles
Since bean leaf beetles can appear in the early reproductive soybean stages, it’s important to keep an eye out for them and employ means of control before the damage to your plants is irreversible. These methods fall into three categories: cultural control, biological control, and insecticidal removal.
Since bean leaf beetles overwinter in the area surrounding your bean plants, the best way to disrupt their life cycle is by limiting their overwintering sites. Clean up plant debris in your beds to prevent their ideal overwintering conditions. This can help limit the number of beetles that survive the winter and emerge in the spring.
You can also employ crop rotation as a means to confuse these pests. They will emerge as spring temperatures rise in the location where your beans were planted the year before. But this time, there will be no beans planted there for them to feast on. You may also delay the planting of your beans so that it does not coincide with the emergence of these beetles.
To prevent first-generation beetles from finding your new planting site, you may want to utilize a drop cloth or floating row covers over your plants beginning in early July to provide a physical barrier to keep the beetles away.
Last but not least, if you spot beetles already on your plants but the infestation is still small, you may want to try knocking them off of your plants into a bucket of soapy water to kill them!
Biological Removal Of Bean Leaf Beetles
Another way to stay ahead of bean beetles is to diversify your landscape. By providing many different sources of forage, you can attract beneficial insects to your garden. These beneficial insects will act as natural pest control.
Predatory wasps, specifically parasitic wasps, will inject their eggs into the young bean beetles as well as the larvae of bean beetles. As the wasp’s larvae grow, they emerge from the bean beetles and usually kill them in the process.
It may sound like something out of an episode of The Last of Us, but that’s just mother nature at work! Predatory wasps are attracted to plants with umbrella-shaped flowers like yarrow, fennel, dill, and flowering alliums (like chives).
Insecticidal Removal Of Bean Leaf Beetles
As a last resort, and if your bean leaf beetle populations have reached numbers too great to control with other methods, you may consider the insecticidal removal of these beetles.
Insecticidal seed treatments sometimes involve coating the seed in a pesticide that can protect the seedling for a varying period of time after planting. The seed coat of treated seeds is usually covered in brightly dyed dust, mist, or clay.
Foliar insecticides can be used to treat plants that have made it past the seedling stage. Some organic insecticides include pyrethrins, spinosad, and neem oil. With these, it’s best to treat plants very late in the day when most pollinators are less likely to be active on your plants. Most of these organic methods are unlikely to harm pollinators once they’ve fully dried.
Always remember to follow the application instructions on the label when applying insecticides in the garden, especially on and around edible plants.
Frequently Asked Questions
Q: Are bean leaf beetles harmful?
A: High populations of adult bean leaf beetles can defoliate young seedlings and kill them before they get established.
Q: Where do bean leaf beetles come from?
A: Adult bean leaf beetles overwinter in leaf litter and debris. They emerge in the spring to lay eggs for the next generation.
Q: What plants do bean leaf beetles eat?
A: They have a preference for soybean plants and snap beans (green beans). They are also known to eat cucurbits like cucumber, pumpkin, and squash.
Q: What is a natural remedy for bean leaf beetles?
A: Parasitic wasps are a top predator of bean leaf beetles. Plants with umbrella-shaped flowers attract these wasps, such as yarrow, fennel dill, and flowering alliums (like chives).
Q: How do I keep beetles off my green bean plants?
A: Floating row covers and cloches are good physical barriers to keeping bean leaf beetles off your green bean plants. You can also knock them off of your plants into a bucket of soapy water.
Q: What attracts bean beetles?
A: Well, beans! They are attracted to their preferred food source of soybeans and green beans, mostly.
Q: How long do bean beetles live?
A: Adult females live for about 40 days, during which they can lay up to 125-250 eggs.