Thirty percent of all animals on Earth are beetles with 34 times more beetle species than bird species. Thus the garden in tune with nature is likely to harbor more species of beetles than any other insect. Some, like the cucumber beetle, flea beetle, Colorado potato beetle, and Japanese beetle, are herbivores. Others are decomposers that break down organic matter.
Among the beetles that feed on garden plants, the vast majority go about their daily business hardly noticed. For example, flower longhorn beetles show up in Marjorie’s Garden in early July to feed on the pollen of native rose blossoms. For a few days almost every rose flower is host to at least one of these orange-striped beetles. Then, just as suddenly, the beetles are gone.
A few herbivorous beetle species capture our full attention as they show up in large numbers to feed on garden plants, some transmitting plant diseases as they feed. Striped cucumber beetles are a good example. It appears that this gardening season, like 2010, may go down in Maine gardening history as a peak year for this herbivore.
Cucumber beetles can devastate plantings of cucumber, squash, and other cucurbits. In addition to feeding on the leaves and flowers, these beetles also transmit a bacterial wilt disease that can cause death of the plant.
Striped Cucumber Beetles
The following is excerpted from THE NEW ENGLAND GARDENER’S YEAR by Reeser Manley and Marjorie Peronto (2013, Cadent Publishing, Thomaston, Maine).
In 2010, gardeners in New England were talking about the hoards of striped cucumber beetles feeding on cucumbers and squash plants in their gardens. I first heard about the beetle scourge while visiting a private garden on Mt. Desert Island where deer-fenced plots were planted to vegetables for local food pantries. In the winter squash plot, hand picking the beetles every early morning was necessary to keep them at tolerable levels. The beetles are like me before morning coffee, lethargic and easier to subdue.
A day later, while visiting with another gardener in her Bucksport garden, cucumber beetles entered the conversation as we walked past the pumpkin patch. She was using yellow sticky traps to control the beetles but mentioned that sucking them up with a portable vacuum also worked. Now there’s a market that the Dust Buster folks have overlooked!
And then I received a reader’s email with two attached photos, one of her vegetable garden’s squash plants, the other a close-up of a single squash flower crawling with cucumber beetles. She wanted to know what they were and what to do about them. I immediately replied, giving her all of the non-chemical remedies, including vacuuming, hand-picking, and yellow sticky traps, then wished her luck.
Later that day I received a reply thanking me for my prompt response to her concern. She went on to say, “my husband ran out and bought a package of Sevin-5 (5% carbaryl) and got rid of those pesky bugs”.
Sadly, cucumber beetles were not the only creatures eliminated by this errant act.
Between two and four million pounds of carbaryl (typically in the form of Sevin) are used every year on U.S. lawns and gardens, despite the fact that it has been labeled as a mutagen (causing genetic damage) in humans by the National Institute of Occupational Safety and Health. The U.S. Environmental Protection Agency classifies carbaryl as carcinogenic in humans; it has been linked with cancer among farmers. Toxic to birds, fish, tadpoles, salamanders, shrimp, bees, and other non-target insects, it kills by inhibiting normal nervous system function.
Carbaryl is highly toxic to honeybees and native pollinators, including leaf-cutter bees, alkali bees, and bumblebees. So while my reader may very well have eliminated the cucumber beetles on her squash, she also minimized her squash harvest by killing off the essential pollinators.
Carbaryl also kills ladybug beetles and parasitic wasps, two very important beneficial predators in our gardens. How long will it be before species diversity returns to near normal after an application of carbaryl?
When a herbivore such as the cucumber beetle shows up in the garden, the first step is to identify it. Kudos to my reader for at least wanting to know the identity of her squash-eating bug before finding a remedy; there are still a lot of people who will reach for the Sevin dust or other toxic chemical without knowing whether an insect is friend or foe.
Once you have identified the herbivore as one that will need control, learn all you can about its life cycle. In the case of the striped cucumber beetle, it goes like this. Unmated adults overwinter in wooded areas beneath the litter and under rotting logs, often as much as a mile from the garden. They leave their hibernating quarters in the spring when temperatures reach 55 degrees (F) and initially feed on pollen, petals, and leaves of willow, apple, hawthorn, goldenrod, and asters. As soon as cucurbit plants (cucumber, squash, pumpkin) appear in your garden, the beetles fly to these plants, often in large numbers.
The beetles soon mate and continue feeding throughout the season, laying eggs 8 to 25 days after mating. Each female deposits up to 800 orange-yellow eggs in small clusters or singly in soil cracks at the base of the cucurbit plants.
Eggs hatch within 8 days and the white larvae (1/3-inch long when full-grown) spend about 15 days feeding on the roots and fruit stems in contact with the soil. The pupal period lasts for about a week. The total time from egg hatching to adult for the first generation is about one month. In Maine, only one generation occurs each season, but in southern New England, two generations may occur in one year.
Knowing the life cycle, non-chemical controls other than those already mentioned come to mind. Growing cucurbits on trellises should help reduce larval feeding on fruit stems. Removing goldenrod and aster plants from the immediate vicinity of cucurbit crops makes sense. Row covers supported over the cucurbit plants with wire hoops will exclude the beetles.
But indiscriminately killing off most of the insect life in your garden, including pollinators and beneficial predators, is not an option. Toxic chemicals, including insecticides, herbicides, and fungicides, have no place in the ecologically-functional garden.