Managing Leaf Beetles without Chemicals

Thirty percent of all animals on Earth are beetles, with 34 times more beetle species than bird species.  Within this immense order of insects (Coleoptera) is the family Chrysomelidae, the leaf beetles.  It is a family of much interest to farmers and gardeners, as leaf beetles can be devastating herbivores.  Leaf beetles common in New England gardens include the Colorado potato beetle, cucumber beetle, lily leaf beetle, and the flea beetles that perforate the leaves of broccoli and eggplant.

Cucumber beetles

Striped Cucumber Beetle    There are gardening seasons in which slugs and Japanese beetles are the only really troublesome herbivores, and then there are seasons in which a specialist herbivore appears in plague proportions throughout a region.   In 2010, for example, Maine gardeners were talking about the hoards of striped cucumber beetles feeding on cucumbers and squash plants in their gardens.

I first heard about this 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, cucumber beetles entered the conversation with another gardener as we walked past her 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.  Carbaryl is toxic to birds, fish, tadpoles, salamanders, shrimp, bees, and other non-target insects.
It is also highly toxic to honeybees and native pollinators, including leaf-cutter bees, alkali bees, and bumblebees, and to ladybug beetles and parasitic wasps, two very important beneficial predators in our gardens.

Other non-chemical controls for cucumber beetles, 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 also makes sense, as these plants are favorite larval hoses.  Row covers supported over the cucurbit plants with wire hoops will exclude the beetles.  (See “When and How to Use Floating Row Covers”, below.)

Lily leaf beetles

Lily leaf beetle.

Lily leaf beetle.

Native to Europe, the lily leaf beetle was discovered near Montreal, Canada in 1945.  For decades its damage was limited to the Montreal area, but in the summer of 1992 it was discovered in Cambridge, Massachusetts, apparently entering the U.S. with bulbs shipped from Europe.

Lily leaf beetles are now devouring lilies in New England gardens.  Lilies are their preference, although they will feed on frittillaria, Solomon’s seal, potato, hollyhock, hostas, and flowering tobacco (Nicotiana).  They lay eggs, however, only on true lilies (Lilium); daylilies (Hemerocallis) are not a host.

The adults spend the winter in the soil or plant debris, often some distance from their host plants.  They emerge from late March through June, mate, and the females lay their reddish-orange eggs on the underside of leaves in an irregular line.  Throughout the spring, routinely inspect your lilies for eggs and either crush them or remove and burn the affected leaves.

The adult is a striking creature, about ¼-inch in length with a shiny scarlet body, black head and legs.  They squeak when you squeeze them, a defense mechanism to deter predators.
Don’t let this squeaking deter you from hand-picking them off of your plants, preferably in the early morning while they are sluggish.  Either crush them or throw them into a jar of soapy water.

The larvae resemble slugs with swollen orange, brown, yellowish, or greenish bodies and black heads.  To deter their predators, including gardeners, they secrete and carry their excrement on their backs.  These larvae can be hand-picked if you are not squeamish, or you can wear gloves.  Those that escape your efforts will give rise to a second generation of adults that emerge and feed until fall; they do not mate or lay eggs until the following spring.  Look for these new adults and eliminate as many as possible.

Colorado potato beetle

Colorado potato beetle.

Colorado potato beetle.

We’ve been growing potatoes in Marjorie’s Garden for several years and the Colorado potato beetle has yet to find us.  The reason is a floating row cover over the potato bed from the moment sprouts emerge in spring until harvest.  (See “When and How to Use Floating Row Covers”, below.)

Installed with plenty of slack between bed edges, our potato bed row cover literally floats on top of the growing plants.  And because producing potatoes does not involve pollinating insects, the row cover can remain over the potato bed throughout the summer.

Flea beetles

Flea beetles on tomatillo.

Flea beetles on tomatillo.

Small “shot-holes” in leaves are the work of flea beetles, tiny black or dark brown beetles that use their large rear legs to jump like fleas when disturbed.  There are several different species of flea beetle ranging in size from 1/16- to 1/5-inch in length, each species showing a preference for certain crop families.  Crops most likely to suffer attack include the cabbage family, potatoes, tomatoes, eggplants, beets, corn, grapes, and spinach.

Most flea beetle damage is done in spring.  Overwintering adults begin feeding in May, attacking the leaves of seedlings and young transplants.  They feed for several weeks, the females taking time out to lay eggs in soil cracks at the base of the plants.  The growth of flea beetle-infested plants can be retarded and heavily-infested plants may be killed from a combination of beetle feeding and disease transmission.

The worm-like larvae feed on small roots and root hairs with minimal effect on the host plant (with the exception of the tuber flea beetle’s damage to potatoes), then pupate in the soil.  The adults emerge in time to begin a second generation within the same year.

Populations of flea beetles can be reduced with yellow sticky traps placed within the host crop.  Other non-chemical controls include trap crops, such as radish, planted near the crops to be protected, and floating row covers that exclude beetles flying in from overwintering areas.  Because adult flea beetles overwinter under leaves and soil clods in the garden bed, the effectiveness of row covers is dependent on crop rotation.

When and How to Use Floating Row Covers

 Floating row cover over a potato crop.

Floating row cover over a potato crop.

Floating row covers are a chemical-free way of denying plant-eating insects access to your vegetable crops.  Most garden row covers are a polypropylene fabric of varying weights.  For insect control, we prefer a lightweight row cover (0.45 oz. per sq. yd.) that retains very little heat in the soil while transmitting 95 percent of available light.  Water from rain or irrigation easily passes through the fabric.
Available in rolls of varying length from garden stores and online, standard row cover widths range from 5 1/2 to 8 feet, and wider is always better.  Be sure to buy a width that will accommodate the upward growth of the plants.

For leaf and root crops, row covers can be kept on throughout the growing season.  For crops that require pollination by insects for fruit production, row covers must be removed during flowering.

Some crops, such as tomatoes, peppers, and others with fragile growing tips, do better if the row cover is supported with hoops.  Many gardeners use hoops made from 9-gauge wire cut into 6-foot-long pieces.  The ends of the hoops are pushed into the ground.  In raised beds framed with timbers, small holes can be drilled in the top timber to support the hoops.  Other options for hoops include inexpensive plastic pipe, the ends pushed into the soil or slipped over rebar stakes.

When Floating Row Covers Work (and When They Don’t) 

Using floating row covers successfully requires knowledge of how insects overwinter. For example, some herbivores, like the Colorado potato beetle, spend the winter as adults in the soil near the plants on which they fed as larvae.  If potatoes are planted in the same bed the following spring and the bed is covered with fabric, these adults will emerge to find their favorite food handy beneath a cover that protects them from their natural enemies.  Other herbivores that overwinter near last year’s plant host include the onion maggot, corn rootworm,  and flea beetle (affects many vegetable seedlings).  Clearly, crop rotation must be used along with row covers to foil these herbivores.

Some herbivores, such as slugs, cutworms, millipedes, and sowbugs, overwinter in scattered locations around the garden.  These insects have the greatest potential for causing plant damage under row covers, since they could emerge anywhere in the garden.  Frequent inspection is the key.   If noticeable populations of these herbivores are found, the row cover should be removed to allow beneficial insects access to their prey.

Floating row covers should be used only to prevent establishment of an herbivore capable of serious damage, such as the Colorado potato beetle or the cabbage worm caterpillar, but they should not be used at the expense of building strong populations of beneficial insects.  Cover everything and the beneficials will disappear, a recipe for disaster.

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About Reeser Manley

I was born in Laramie Wyoming but moved to the southeast at an early age. I was educated through my B.S. in Biology in the Columbus, Georgia area, then crossed the Chattahoochee River to earn my M.S. in Botany at Auburn University. For the next ten years, I worked as Horticultural Manager for the George W. Park Seed Co. in Greenwood, S.C. At 40 years of age, I decided to return to graduate school and in 1994, I earned a Ph.D. in Horticultural Science from Washington State University, Pullman, Washington. Fast forward through 10 years at university (7 at UMaine, Orono) and you find me teaching high school science in Eastport, Maine, the edge of the world, and writing a weekly garden column for the Bangor Daily News. My new book, The New England Gardener's Year, a Month-by-Month Guide for Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, and Upstate New York”, will be published later this year by Cadent Publishing. You can learn more about the book by visiting its Facebook page: http://www.facebook.com/pages/The-New-England-Gardeners-Year/187285218055676.)