Mexican bean beetles are common pests of garden plants, particularly beans and other legumes. These pests can cause significant damage to plants by feeding on leaves, stems, and pods, leading to reduced yield and stunted growth. They are most active during the warmer months and lay their eggs on the undersides of leaves, which hatch into larvae that feed voraciously on plant tissue.
Symptoms
- Leaf Damage: The most obvious symptom of a Mexican bean beetle infestation is leaf damage. The beetles feed on the undersides of leaves, leaving behind holes or chewed areas that can cause the leaves to turn yellow or brown. In severe infestations, only the leaf veins may be left intact, resulting in a lace-like appearance.
- Stunted Plant Growth: The feeding of Mexican bean beetles can also stunt the growth of plants. This is due to the significant damage they cause to the leaves and stems of the plant, which can interfere with the plant’s ability to photosynthesize and produce energy.
- Reduced Yield: Infestations of Mexican bean beetles can also result in reduced yields of beans. The damage to the pods and stems can cause the plant to produce fewer beans or cause the beans to become misshapen.
- Presence of Beetles and Larvae: In addition to leaf damage, another symptom of a Mexican bean beetle infestation is the presence of adult beetles and their larvae. The adults are oval-shaped and about 3/8 inch long, with a yellow or bronze color and 16 black spots on their wing covers. The larvae are spiny, plump, and range in color from yellow to orange. They are typically found on the undersides of leaves, where they feed and cause damage.
What is a Mexican Bean Beetle
- Adults: Mexican bean beetles are oval-shaped insects, about 3/8 inch long, and are yellow or bronze in color. Their wing covers are adorned with 16 black spots, giving them a distinctive appearance. They have short antennae and legs, and are often seen crawling or flying around the affected plants.
- Larvae: The larvae of Mexican bean beetles are spiny, plump, and range in color from yellow to orange. They have six legs, and can grow up to ¼ inch long. They are usually found feeding on the undersides of leaves, where they cause the most damage to the plant.
- Eggs: Mexican bean beetle eggs are yellow and oval-shaped, and are typically laid on the undersides of leaves in clusters of 30-40 eggs. They hatch into larvae after about four days, which then begin feeding on the leaves.
- Pupae: Mexican bean beetle pupae are brown, about ¼ inch long, and are found on the undersides of leaves or in the soil near the affected plants. They emerge as adult beetles after about five days.
Treating a Mexican Bean Beetle Infestation
It may be necessary to use multiple methods to effectively treat Mexican bean beetle infestations in the garden. For example, using row covers in combination with hand- picking and pesticide applications can be more effective than relying on a single method. The best approach will depend on the severity of the infestation, the size of the garden, and the resources available.
Cultural Control
- Crop rotation: Rotating crops from year to year can reduce the build-up of Mexican bean beetles in the soil and reduce their populations. This method works by interrupting the life cycle of the beetles, as they are specific to beans and are unable to feed on other plants.
- Plant resistant varieties: Some varieties of beans are less susceptible to Mexican bean beetle damage, and planting these can help reduce infestations. This can include varieties with tough leaves or a fast growth rate, which makes them less appealing to the beetles.
- Remove crop debris: Clearing away dead plants and leaves from previous seasons can reduce the habitat for Mexican bean beetles and limit their populations. This helps to reduce the number of overwintering sites available for the beetles.
- Early planting: Planting beans early in the season can help them mature before the Mexican bean beetles become most active, reducing the damage they can cause. Early-maturing beans may have time to grow and produce pods before the beetles arrive, limiting the damage they can do.
Physical Control
- Hand picking: Regularly removing adult beetles and larvae by hand can help reduce infestations. This method is best done in the morning or evening, when the beetles are less active and easier to find.
- Row covers: Using row covers can prevent Mexican bean beetles from accessing the plants, effectively excluding them from the area. The covers should be securely fastened and extend several inches into the soil to prevent beetles from entering from the sides.
- Sticky barriers: Placing sticky barriers around the base of plants can trap Mexican bean beetles and prevent them from reaching the leaves. This can be done using sticky bands or sticky cards, which can be made by applying a sticky substance to a piece of cardboard or paper.
Chemical Control
- Pesticides: Spraying plants with insecticidal soap or pyrethrin-based pesticides can kill Mexican bean beetles and their larvae. These products work by disrupting the normal functioning of the beetle’s nervous system, causing them to die. It is important to follow all label instructions and to consider the impact on beneficial insects and the environment before using pesticides.
- Neem oil: Using neem oil as a preventative or to treat infestations can help reduce populations of Mexican bean beetles. This works by interfering with the beetles’ feeding and reproductive processes, making it more difficult for them to survive and reproduce.
Biological Control
- Parasitic wasps: Introducing parasitic wasps, such as Pediobius foveolatus, can help reduce populations of Mexican bean beetles by laying their eggs in the larvae. The wasp larvae then feed on the beetle larvae, killing them. This method is most effective when the beetles are present in low numbers.
- Predator insects: Encouraging the presence of predator insects, such as ladybugs, can help reduce populations of Mexican bean beetles by feeding on the adults and larvae. This method works best when the beetles are present in low numbers and the predator insects are able to find and feed on the beetles effectively.