Gardeners who grow cucurbits (cucumbers, squash, melons, etc.) often struggle with cucumber beetles, small elongated beetles that attack their leaves, stems, roots, fruits, and, especially, their flowers. There are actually two common species in Eastern North America, the striped cucumber beetle (Acalymma vittatum), yellow with black stripes, and the spotted cucumber beetle (Diabrotica undecimpunctata), yellow with black spots. West of the Rockies, the Western striped cucumber beetle (Acalymma trivittatum) replaces the Eastern species, but only an entomologist would notice the difference.
Both are attracted to cucurbitacins, bitter biochemical compounds found in cucurbits. They are normally used to discourage predators through their bitter taste, but these beetles have adapted to cucurbitacins and have even learned to use them to find their host plants, as they can smell cucurbitacins from far away.
Cucumber beetles not only feed on cultivated cucurbits, but also wild ones, notably the wild cucumber (Echinocystis lobata). They will also feed on other plants for short periods, especially early and late in the season, but they absolutely require cucurbits to complete their reproductive cycle.
Both adults and larvae are harmful. The adults eat the leaves, stems, flowers and fruits (especially overripe fruit), while the larvae attack roots, stems and fruits that touch the soil. Adults are most visible on flowers and sometimes you can find dozens of them devouring a single bloom.
In addition, to damaging plants by eating them, cucumber beetles can carry diseases from one plant to another, notably bacterial wilt, fatal to cucurbits. It causes the plant to wilt as if it needed watering, but no amount of water will help it recover.
Life Cycle of a Cucumber Beetle
Both species overwinter in the soil as adults, usually where their hosts were planted the previous year, therefore, in vegetable gardens, but sometimes on the edges of nearby woods. They emerge from the ground when once soil temperatures are greater than 50˚F (10˚C). Since gardeners don’t plant cucurbits outdoors that early in the season, they’ll feed on other plants temporarily, often without significant damage, although they are harmful enough to seedling corn plants to have earned a second name: corn root beetle. When cucurbits do become available, that is, after they are sown or planted out, the beetles then change hosts and fly to their favorite prey, attracted by the scent of cucurbitacins.
A few weeks after they emerge, the beetles begin to mate and then lay eggs in cracks in the soil, usually around cucurbits in the case of the striped cucumber beetle, although the spotted cucumber beetle seems to prefer grasses and other plants, notably corn. The small wormlike larvae feed on roots and stems, and sometimes overripe fruits that touch the ground. However, unless the beetle population is exceptionally high, they rarely cause significant damage. After a short pupation, they metamorphose into adults and the cycle begins again. There are usually two generations per summer.
How to Control Cucumber Beetles
Most gardeners only react to cucumber beetles when they see them, thus when the infestation is already well underway, and that is less than ideal. Notably the insects may already have transmitted bacterial wilt to their host plants.
About the best you can do when they are present is to handpick them (do so early in the morning, because beetles hide among foliage during the day), dropping them into a bucket of soapy water. You can also also pick them up with a hand vacuum cleaner. Or spray the plants with an insecticide. Neem oil is probably the most effective biological insecticide (it is no longer available in Canada), but you can also try insecticidal soap and pyrethrines, or a mixture of both (such as End-All). Some people also claim great success with garlic sprays.
Preventing cucumber beetles is much easier then suppressing them.
The most effective method I know of is to cover the plants or seedlings in spring with a floating row cover. For this product to be effective, it is important to carry out a crop rotation, sowing or transplanting your cucurbits in a spot where they were not grown the previous year. Otherwise when adult beetles emerge from the ground in spring, they’ll find themselves prisoners under the floating cover… with an abundance of seedlings to eat!
When your cucumbers, melons, squash, etc. begin to bloom, you’ll have to remove the cover to provide to allow pollinating insects access to the flowers. (Pollination is required for cucurbits to produce fruit). At this point, though, the adult beetles in the sector will either be dead or will have gone elsewhere to feed. Once the row cover is removed, some adult beetles from gardens nearby may find your plants, but the population will usually be very low and they are unlikely to cause significant damage.
Another method of prevention is to grow a trap crop, possibly a cucumber with especially bitter fruits (they contain more cucurbitacins than other varieties). Start a few trap plants indoors, then in early spring, one or two weeks before you intend to plant or sow your cucurbits, put them outdoors to attract adult beetles. When they show up, destroy the pests by dropping them in soapy water.
Studies also show that cucumber beetle infestations are less severe in the gardens where the soil is covered with a thick mulch. This is partly because adults seem to prefer exposed soil in which to lay their eggs, but also the presence of beneficial predators is greater under a mulch (ground beetles, especially, are voracious beetle predators) and that may also explain in part the effectiveness of mulch in reducing beetle numbers.
Intercropping (polyculture), that is to say to mixing cucurbits with other vegetables rather than planting them in a row or in a monoculture, can also help to reduce the beetle population.
The use of yellow sticky traps placed near cucurbits can also help. Adults are attracted to the color yellow and land on the traps where they remain stuck. This treatment is most effective early in the season, catching adults when they first emerge from the soil.
There are also pheromone lures designed specifically to catch cucumber beetles, but they are not usually available in local garden centers. You can order them from certain mail order suppliers of organic pesticides such Nic Natural Insect Control or Peaceful Valley. Usually the lures are designed to be combined with yellow sticky traps. Like sticky traps used alone, they will be most effective when you install them early in the season.
Beetle Resistant Varieties
A final possibility is to grow varieties of cucurbits that are naturally resistant to cucumber beetles. These plants contain few curcubitacins and beetles have a harder time finding them.
Here are some suggestions:
Buttercup squash: ‘Ambercup’
Cucumber: most burpless varieties are beetle resistant, including the following: ‘Beit Alpha’, ‘Big Burpless’, ‘Burpless Beauty’, ‘Burpless Bush’, ‘Burpless Tasty Green’, ‘English Telegraph’, ‘Garden Sweet Burpless Hybrid’, ‘Green Knight’, ‘Marketmore 80’, ‘Muncher’, ‘Orient Express’, ‘Palace King’, ‘Perseus’, ‘Suyo Long’, ‘Sweet Burpless’, ‘Sweet Slice’, ‘Sweet Success’, ‘Sweeter Yet’, ‘Summer Dance’, ‘Tanja’, ‘Tasty Green’, ‘Tendergreen’ (‘Tendergreen Burpless’).
Melon: ‘Classic’, ‘Galia’, ‘Passport’, ‘Pulsar’, ‘Rising Star’, ‘Super Star’
Pumpkin: ‘Big Max’ ‘Baby Boo’
Summer squash: ‘Cocozelle’, ‘Caserta
Zucchini: ‘President’, ‘Black Jack’, ‘Green Eclipse’, ‘Seneca Zucchini’, ‘Senator’, ‘Super Select’, ‘Dark Green Zucchini’, ‘Embassy Dark Green Zucchini’.