cucumber beetles

Asked June 29, 2020, 11:49 AM EDT

We have a heavy infestation of cucumber beetles and flea beetles in both veggie and flower garden this year, worse than past years. Tried beneficial nematodes (three separate applications this year) to no avail. We have always gardened without using any pesticides and would be very reluctant to use any thing that would endanger bees or other beneficials. We also have dozens of black swallowtail caterpillars currently inhabiting our garden and do not want to put them at risk. It seems that every "safer" method of dealing with these pests presents some level of danger to beneficials. What would you recommend as a safe but effective course of treatment? Are there predatory insects that can be obtained and introduced? Can you please "rank" potential treatments according to two factors: most effective/least harmful?

Oakland County Michigan

3 Responses

For smaller plantings, some growers will use row covers to protect the seedlings until the 3 true leaf stage and to exclude the beetles. When the plants begin to flower it is necessary to remove the covers in order for pollination to happen.

If you haven't tried it, Neem oil is a common natural insecticide that can be purchased as a ready-made spray in many garden centers. Neem oil kills cucumber beetles upon contact. According to the National Sustainable Agriculture Information Service, when pure neem oil is used, it is also effective for treating cucumber beetle larvae and eggs. Neem oil used appropriately won't harm bees, butterflies and ladybugs. Pollinators are becoming rare. ... Since neem oil only targets bugs who chew on leaves, neem oil insecticides are safe to use around butterflies, ladybugs, and most other beneficial insects.

There is research on-going to look at other organic means to control this beetle. While the link below will give you some other options, I'm not sure any of them will help you with your efforts to protect beneficials.

We also suggest rotating your crops if you are not doing that already. Another suggestion would be to delay planting until the last week of June so that the adults look elsewhere to do their damage. Other research has shown that using a black plastic mulch can reduce the survival of this beetle by 50%.

For contolling the flea beetles, Spinosad (see is probably the best option. While not specifically labelled for flea beetles it is labelled for vegetable gardens. Spinosad is certified organic.

Thank you for your interest in saving our beneficial insects. Good luck eradicating your cucumber beetles.

We can use some more guidance in how to use the Neem oil. Our bottle of Garden Safe Neem Oil extract concentrate (meant to be diluted 2-4 tablespoons in a gallon of water, and applied with a sprayer) says to spray on foliage, underside as well. "Kills on contact": does this imply that the Neem oil has to be sprayed directly on the beetle? or that the beetle will pick it up from the foliage? The container also states that it is harmful to bees if there is direct contact, so not to spray when bees are active (so, I assume, spray later in the evening). Additionally: what is the shelf life of Neem oil? The bottle we have was bought prior to last year: is it too old?
Related question: we made 3 applications, in a heavy concentration, of predatory nematodes, purchased from one of the supplier companies. We made these applications 2-3 weeks apart, starting after last frost, wetting the ground both before and after application, concentrating on the vegetable and flower garden areas, along with adjacent lawn areas. We haven't noticed a benefit from this application, so of course we're wondering if this was correctly used, or if others have found this to be an effective remedy for the beetle problem. Some feedback on the use of predatory nematodes, please?

According to the OSU Extension publication, How to Reduce Bee Poisoning From Pesticides, neem oil, an insecticide made from a neem tree, does not have a precautionary statement regarding bees on its label. Therefore, no bee caution is necessary. The neem must be ingested in order for it to be toxic to bees. Try not to spray on flowers, and please be sure to follow instructions on the label.

Neem oil has a minimum of a one year shelf life unopened. As for the mixed solution, you only want to mix up the amount necessary for the job at hand and it is recommended to use that batch within approximately 8 hours of mixing due to the neem oil breaking down once mixed. I suggest a new bottle for most effective outcome.

Neem oil is practically non-toxic to birds, mammals, bees and plants. ... It is important to remember that insects must eat the treated plant to be killed. Therefore, bees and other pollinators are not likely to be harmed.

Timing is critical when applying beneficial nematodes. They should be applied to the soil of infested areas to control the pupal stage (early) of the cucumber beetle. If adult feeding damage is identified, apply kaolin clay to plant foliage. The film left behind disorients insects and prevents feeding.

Beneficial nematodes are perishable and don't have a long shelf life so might be difficult to find locally. Your best bet is to check with your local well-stocked garden center. Depending on the time of year, they may have the nematodes in stock. Otherwise, they can order for you.

A word of caution about using nematodes for managing cucumber beetles: There is limited scientific research in this area. There have been studies done, but typically these are limited to controlled environments (like testing in a laboratory or a greenhouse setting, but not out in the field or garden exposed to the natural environment.)

Take some time to read the packaging: There are many different types of beneficial nematodes available to purchase and not all nematodes work against all pests. Some of the types that have been at least lab tested against cucumber beetle are: Steinernema sp. and Heterorhabditis sp.

Keep in mind that these beneficial nematodes only affect the soil-dwelling portion of the cucumber beetles life. The nematodes won't affect the adults that cause the above ground foliage damage-only the immature larva in the soil. So timing of application of the nematodes is important.

And the last caution: the damaging-causing adult beetles are very mobile. So just because you are diligent in managing them in your own garden-it doesn't mean they won't just fly in from neighboring yards!