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Managing tricky vegetable pests in 2020: Cucumber beetle

Author: Natalie Hoidal, UMN Extension Educator

2020 has been a great year for cucumber beetles. Their populations are thriving across the state, and at many farms, they are reaching are reaching treatment threshold levels. This article provides a very brief overview of cucumber beetles along with recommended management strategies. 

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, some years, like 2020, seem to be 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 are becoming active now further north. 

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

Most farmers have already planted all of their cucurbits, so a few strategies are too late to try. These early season preventative controls include using metallic plastic mulches, which disorient cucumber beetles, and using row cover. 

Kaolin clay
Kaolin clay, photo NH
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
A combination of pyrethrin insecticides and neem can effectively manage cucumber beetle, though you will likely need to apply more than once. Pyrethrins are contact insecticides that will kill most insects, including beneficials, so take care to apply either early or later in the day. Neem has a few modes of action, but mainly it has an antifeeding effect. There is one OMRI approved product on the market, Azera, that contains both of these chemicals. Azera is the only recommended OMRI approved product in the Midwest Vegetable Production Guide for Commercial Growers. Pyrethrin and neem products can also be purchased and used separately. 

Spinosad is also an option. Keep in mind that if spinosad is your go-to for other insects like flea beetles, potato beetles, etc., you should take care to prevent resistance and over-use. Beauvaria bassiana 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 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. 

Cantaloupe 1 beetle / plant
Cuumber 1 beetle / plant
Watermelon 1 beetle / plant
Squash 5 beetles / plant
Pumpkin 5 beetles / plant

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 ope
    rating 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
  • Use row cover at the beginning of the season

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