Cucumber Beetles
By Bob Williams (April 2020)
At the March HSCSS meeting, Jolie Krupnik’s name was called first during the attendance prize drawing. She chose the main prize: a plant from the genus Ibervilla, which is a member of the Cucurbitaceae family. Just by the family name, you know this plant is related to cucumbers and squash. The discussion turned to cucumber beetles and how they are damaging to cucurbits. I decided to investigate and see if this is true and what can be done.
There are two types of cucumber beetles. The adult spotted cucumber beetle is yellow, with 12 black spots on its back. The striped cucumber beetle also has a yellow body, but has three black stripes along its back. Both are the same size, roughly a quarter of an inch long. These bugs are cute little things, but not that good for plants.
The larvae are white grubs with brownish heads. The larva of the spotted cucumber beetle, known as the corn rootworm, is a severe problem for corn and other agricultural crops. Adult cucumber beetles overwinter in the garden, in compost piles or in trash heaps, then emerge in spring. They can also overwinter next to building foundations, where it is warmer.
The adults feed on weeds and other plants until their preferred food source – cucurbits such as cucumbers, squashes and melons – are available. Once the beetles locate cucurbits, they feed on the plants, and the females lay their eggs in the surrounding soil. After the eggs hatch, the rootworm larvae feed on below-ground cucurbit roots and stems until they hatch. Then the adult insects emerge from the pupae, and the cycle starts all over again. The entire life cycle of the insect is about eight weeks.
There are two ways that cucumber beetles can damage plants. First are the larvae. They have a very hearty appetite and can devour the roots in a very short time. The adult beetles feed on the leaves of plants. This is where the real damage occurs. When the beetles eat the leaves and stems, the consumption does not really damage the plants. Bacteria wilt causes the damage.
Bacteria are secreted in the beetle’s stomach. As the insects feed, they spread the bacteria to a plant’s vascular system, which causes the leaves to wilt. If not contained, the wilt will eventually spread and kill the entire plant. Also, plants infected with bacterial wilt attract more cucumber beetles, which will eat the infected leaves and continue spreading the bacteria throughout the garden.
Cucumber beetles are also a primary carrier of cucumber mosaic virus. They are very destructive pests, wreaking both direct damage and serving as carriers for a variety of bacterial and viral diseases. I guess we could call this a BEETLEJUICE problem.
From the above description, it looks like we’re facing an uphill battle to control these bugs. There are some things we can do, however. First, some insecticides in the ground will kill the larvae. Placing beneficial nematodes will also control the larvae. This is the easy part.
Next is how to control the beetle itself. There is a considerable amount of information about control, mainly for commercial growers. One article says to put your plants out later in the season. Since the life cycle is eight weeks or so, putting your plants out in mid-summer is a possibility. This may not be something one would want to try with a show-quality plant.
There is something else that can be put on your plants to deter this menace: kaolin clay. Kaolin clay is a fine, white clay that is used to make porcelain. Marketed as Surround WP, it is dusted on plants. It does not kill cucumber beetles, but drives them to your neighbors’ plants.
There are two sure-fire ways to keep the bugs away. First is to keep plants indoors in a sunny room or in your greenhouse. As long as the beetles don’t have a way to open your door, they won’t be a problem. For those who want to take on a construction project, building a structure that will enclose your plants within a fine screen enclosure will fill the bill. Your plants will enjoy the great outdoors without living in fear of bacteria and viruses.
Sources:
The Spruce – Identifying and Controlling Cucumber Beetles and Their Larva – https://www.thespruce.com/identifyand-control-cucumber-beetles-2539841
Plant Natural Research Center – https://www.planetnatural.com/pest-problem-solver/gardenpests/cucumber-beetle-control/
Epic Gardening – https://www.epicgardening.com/cucumber-beetle/
Gardening Know How – Tips On Using Kaolin Clay In The Garden –
https://www.gardeningknowhow.com/plantproblems/pests/pesticides/kaolin-clay-insect-control.htm