Managing tricky vegetable pests: Cucumber beetle

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

Cucumber beetles show up in Minnesota every year. This article provides a brief overview of cucumber beetles along with recommended management strategies you can use after your cucurbits are planted.

Cucumber beetles: an overview

Cucumber beetle, Gerald Holmes, Bugwood

Striped cucumber beetles, Acalymma vittatum, are a common insect pest of cucurbits. While they are almost always present, in some years they are worse than others. Striped cucumber beetle overwinters as an adult, typically in debris or field margins near cucurbit plantings. They tend to become active in early June in the southern part of the state, and become active further north in mid to late June.

As soon as cucumber beetles locate your cucurbit fields, they begin to mate and lay eggs at the base of plants. Larvae emerge underground and feed on plant roots, so they are not easy to detect until after they pupate in the soil and emerge as adults, typically 40-60 days later. 

Cucumber beetle damage

Striped cucumber beetles do damage in two important ways:

  
1. Bacterial wilt. The first adults to show up in June feed on young plants, causing defoliation. More importantly, they often carry a disease called bacterial wilt (Erwinia tracheiphila). Carriers can infect plants when their mouthparts or feces come into contact with open wounds in the plant from feeding, or other mechanical damage. Symptoms of this bacteria (wilting during the day, recovering at night) don't show up for 1-3 weeks after infection. So, it's critical to manage cucumber beetles carefully, even if you're not seeing any symptoms. Young plants are most susceptible to infection. Cucumbers and melons are very susceptible; squash and pumpkins are less so.

2. Feeding damage. Cucumber beetles can cause substantial damage simply from defoliation, especially when plants are small, up to the 3 true-leaf stage. However, they can cause substantial damage throughout the summer, and in particular can create cosmetic damage on developing fruit.

Management strategies

After cucurbits are planted, you are somewhat limited in approaches you can use. These early season preventative controls it may be too late for include using metallic plastic mulches, which disorient cucumber beetles, and using row cover.

Kaolin clay

Kaolin clay, photo Natalie Hoidal.

At this point in the season, a common strategy is to spray your plants with kaolin clay. Kaolin clay is uniquely effective against cucumber beetle because it builds up in their antennae and creates a gummy coating, making it difficult for them to navigate. Kaolin clay will not kill cucumber beetles, so it should not be used as a rescue treatment. Rather, it's meant to repel them from directly feeding on your crop. If you're already seeing high populations in your fields, it can still be a good idea to use kaolin clay, but consider pairing it with an insecticide application.

Physical removal 

Cucumber beetles often congregate. While the thresholds for spraying (see below) are 1-5 beetles per plot, we often observe 100+ beetles on a single plant, and limited populations elsewhere in the field. If you're seeing concentrated populations and have a large enough production area to sacrifice a few plants, consider flaming the beetles in a concentrated area. In situations like this, it may also be practical to use an insect vacuum. Insect vacuums are expensive and hard to come by, but some growers have created inexpensive vacuums out of modified leaf blowers. 

Additionally, if you're seeing plants with bacterial wilt symptoms, remove the plants immediately. They will not survive anyways, and they are a source of bacterial wilt in your field. Not all cucumber beetles automatically have bacterial wilt, rather they need to feed on a plant that is already infected, then move to an uninfected plant to spread it.

Insecticides approved for organic systems

Organic insecticides often don't work well on cucumber beetles. Growers in Michigan have had good results with the highest labelled rate of Pyganic 5.0 EC in acidified (pH 5.5 - 5.7) spray water. 

Beauvaria bassiana, entomopathogenic nematodes, and other biocontrol products have shown some promise in lab settings, but have not demonstrated significant control in field trials.

Insecticides for conventional systems

Many conventional insecticides are effective for managing cucumber beetle. See the Midwest Guide Vegetable Production Guide for a full list. 

Striped Cucumber Beetle Thresholds

The following thresholds have been developed for striped cucumber beetle. Growers are encouraged to spray when populations reach the following levels. Thresholds are lower for melons and cucumbers than for squash and pumpkins due to bacterial wilt susceptibility. It is recommended you scout more thoroughly in cucumber and muskmelons  (check 48 plants throughout field weekly to detect beetles at low numbers), while scouting in more tolerant crops can be more lax (8 plants throughout field weekly). Spray within a day of finding beetles reaching the threshold.

 Thresholds when plants are (3 leaves or less)

Cantaloupe/muskmelons 0.5 beetles / plant

Cucumber 0.5 beetles / plant

Watermelon 1 beetle / plant

Squash 1 beetle/ plant

Pumpkin 1 beetle / plant

Thresholds when plants are larger (4+ leaves)

Cantaloupe/muskmelons 1 beetle / plant

Cucumber 1 beetle / plant

Watermelon 5 beetles / plant

Squash 5 beetles / plant

Pumpkin 5 beetles / plant

Beetles are often highly attracted to cucurbit flowers, and will hang out inside and feed on flowers. This damage doesn't reduce yield.

How can you prevent cucumber beetle damage next year? 

Image: Howard F. Schwartz, Bugwood

• Remove all residues from the field to prevent overwintering. If you're operating a no till or minimal till system, chop/shred residues to facilitate faster decomposition.
• Use varieties that are less attractive. ATTRA has a very comprehensive cucumber beetle management document with various ranking lists of variety attractiveness. 
• Use straw mulch or plastic mulch. Reflective plastic works particularly well. This won't stop them totally, but makes their lives a little harder. Straw makes it harder for them to move around and provided habitat for predators, reflective mulches can repel them.
• Use row cover at the beginning of the season. Remove row cover as crops being to flower.