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MANAGING TROUBLESOME INSECTS IN THE VEGETABLE GARDEN
by George Bushell
The vast majority of insects in a vegetable garden do no harm. Only a few are actually harmful. And their impact can be minimized easily by practising a little prevention, tolerance and control.;
Start by ensuring that your plants are healthy and growing vigorously - use plenty of compost, ensure that drainage is adequate and always practice crop rotation. Increase bio-diversity by planting only small quantities of the same vegetable in any one place. Remove vegetable refuse from the garden and compost it so as to reduce the overwintering success of some insects.
Encourage natural controls toads, birds, and predatory and parasitic insects. Members of the umbelliferae family, such as dill, are especially good at attracting parasitic wasps. Become familiar with the large number of predatory insects that eat egg, larval and even adult forms of pests (see references at end of article). Never kill an insect unless you know it is, or could become, a pest. You might mistakenly kill a beneficial ground, rove or tiger beetle, an assassin, pirate, big-eyed or lady bug, a tachinid, hover or robber fly, a praying mantis or a spider.
When direct control of a specific pest is needed, handpick or trap, use barriers or try homemade natural repellant solutions. Some people claim that certain plants, such as marigolds, repel insects, but I have not found them effective. Time your plantings to miss the first, and usually most destructive, generation of a pest if it is particularly bothersome in your area.
For severe infestations of moth or butterfly larvae, Bt can be used as a last resort by directing small amounts of it right at the pests; don't spray clouds of it over affected plants. Never use broad spectrum insecticides even if from a natural source (e.g. rotenone).
And finally, practice tolerance. Vegetables and fruit from the home garden need not look perfect discard some produce, cut away insect damage, and enjoy the flavor and nutrition of produce grown naturally in your own garden.
In spite of our best efforts, however, there are times when control must be directed toward a specific pest if we are to harvest enough produce to make our gardening worthwhile and rewarding. Some of the most troublesome are discussed below.
This pest was brought from Europe and has few natural enemies in North America. The adult white butterflies with 2 or 3 black spots on each wing start appearing in late spring or early summer. They lay small eggs, singly, on all members of the brassica family cabbages, cauliflower, broccoli, etc. With a number of generations each year, the larvae can be destructive throughout most of the growing season, eating and soiling leaves and heads. If you are continuously diligent, and only have a few plants, the larvae can be hand picked. A fabric cover, such as Reemay, can prevent the butterflies from depositing eggs on your plants in the first place. This is one of the few pests I recommend using Bt on as long as you carefully apply the mixture to the centre of each plant, where the cabbage larvae almost always feed. The chance of harming other butterfly species is very small since cabbageworms are about the only caterpillars you will find in the center of a brassica plant. The larvae of the cabbage looper moth can be controlled in a similar manner.
Corn Earworm/European Corn Borer
The larvae of these two pests do the most noticeable damage to corn ears, although the borer can affect all parts of the corn plant, sometimes burrowing into the stalk and causing it to break over in a wind. The corn earworm moth cannot overwinter in Canada but flies northward from the USA looking for host plants in summer; and as implied by the name, the European Corn Borer was imported from Europe some time ago (the adult is also a moth). Earworm larvae enter the cob by way of the silk and do most of their damage at the tip of the ear. Borer damage to the ear, when it occurs, is more randomly located as the larvae can enter the cob at any point. Earworm infestations can be lessened by selecting cultivars with tight fitting husks. Many varieties are also resistant to stalk damage and lodging (search your seed catalogues for the appropriate varieties). A little mineral oil on the silks just as they start to dry up after pollination is complete can lessen earworm damage. Use succession planting only the planting that coincides with the first and most destructive generation of larvae will likely be affected. And most important, practice a little tolerance for home use, one can easily remove larvae and cut out any affected part.
I have observed this large worm, up to 4 inches long with an impressive horn, only once on my tomato plants during 18 years of gardening in the Ottawa area. Growing up in southwestern Ontario, I remember seeing them more often. In Canada, they occur mostly in extreme southern Ontario and Quebec, and can be easily hand-picked when you detect foliage and/or tomatoes that have been eaten. The larvae are often parasitized by trichogramma wasps. You may want to save a worm in a jar with some tomato leaves and observe the transformation to our largest and most impressive moth, the sphinx, with a wingspan measuring up to five inches.
Colorado Potato Beetle
In this case, the potato, or host, was imported to North America, and not the pest. Apparently the Colorado Potato Beetle lived in obscurity in the foothills of the Rockies until it was introduced to the potato in the 1880s. It spread eastward and was soon found in association with potatoes wherever they were grown except in the southern USA. This pest will eat other members of the solanaceae family (eggplants, peppers, tomatoes) if its favorite, the potato, is unavailable. However, in the home garden, it is relatively easy to control. The adults emerge from the soil in late spring or early summer, usually as the first potato leaves begin to push through the soil. At this time, collect as many adults as possible to prevent them from laying eggs. The adults themselves do little damage to the foliage; it is their larvae that are destructive. While collecting adults, inspect the underside of the leaves for bright orange clusters of eggs, and destroy them (simply rub them against the leaf). And if a few larvae do manage to hatch, it is a simple matter to hand pick them, as well.
These tiny white grubs can, on occasion, damage the roots of cabbages, broccoli, cauliflower and Brussells sprouts to the extent that the plants wilt and eventually die. The adult flies, 1/4 inch long and similar to a housefly in appearance, emerge in spring (late May and early June in Ottawa) and lay their eggs at the base of young transplants. There is often a later generation, but it rarely does much damage. A water resistant paper disc fitted snuggly around the base of susceptible plants will prevent egg laying. The same fly lays eggs on emerging radishes, turnips and rutabagas. Stagger your radish plantings throughout the season and you will likely only experience damage to an early and a late crop or cover with Reemay (very effective). Delay planting rutabagas to early summer (after the middle of June in Ottawa) so as to miss the first and most destructive generation of grubs. Large rutabaga roots will only suffer a little surface damage in August or September when the second generation of grubs appears.
A small fly is also responsible for the tiny white grubs that can destroy onion plants or ruin their bulbs in southern Canada and the northern USA. Again, the flies emerge in spring (late May in Ottawa) and lay eggs near young onion plants. White onions appear more susceptible than yellow or red types. However, Reemay placed over the onion bed when plants or bulbs are set out and left in place until early June has been totally effective at preventing damage in my allotment garden (neighbours with uncovered plants often have their onions severely affected). Later plantings usually suffer little damage, but an early start for onions produces larger bulbs.
Carrot Rust Fly and Carrot Weevil
The larvae of these two pests can damage the crown and/or roots of carrots, parsnips, and celery. According to Agriculture Canada, the carrot weevil is most common in muck soils of southern Ontario and Quebec. The Carrot Rust Fly is most destructive in southern B.C. where three generations per season are possible. I have rarely experienced any damage from these pests as I do not plant carrots until after the middle of June, thus missing the first and most destructive insect generation. Should you notice their tell-tale brown colored tunnels on your roots, a floating row cover is very effective if installed at planting time.
This pest was also introduced from Europe. The larva of the tiny adult fly, hatched from eggs deposited on host plants in early spring, tunnels between the upper and lower layers of the leaf surface producing yellow, brown or white patches. Damage is most significant when it occurs on the edible leaves of plants such as spinach, chard and beets. Cover your seed beds immediately after planting with a floating row cover if you have problems in your garden. Remove all infected leaves so that no larvae develop into adult flies. Later plantings (from early June onward in Ottawa), are rarely affected to any degree by this pest.
Cutworms are the larvae of night flying moths. These moths deposit their eggs in the soil. Most species remain buried in the ground during the day and only surface at night to eat the leaves of young plants. Normally, there are enough weeds in the garden to keep them satisfied. However, when we enthusiastically till our gardens in the spring, we often remove all of the vegetation but not the cutworms. Is it any wonder that the larvae then attack the only food source around young tomato and cabbage transplants, or newly emerging beans? To protect transplants, place two or three toothpicks or other small sticks around plant stems to prevent the cutworm from chewing through the stalks. If you find a fallen plant, sift the soil with your fingers around the base of the plant until you find the worm if you dont, it will probably fell another transplant the following night. And leave a few weeds in your garden when transplanting. Cutworms usually prefer these young succulent morsels to your "hardened" transplants.
Striped or spotted cucumber beetles can be a serious pest in eastern North America. The yellow and black adults emerge in early summer (June in the Ottawa area) and eat the leaves and blossoms of cucumbers and melons, particularly. Squash are less affected in my garden. The adults can spread cucumber mosaic and bacterial wilt. They lay eggs at the base of their host plants the larvae can weaken the plants later in the summer. A few plants can be covered by a floating row cover to prevent access but you will have to remove these covers after the first generation of adults has disappeared to allow cross pollination. You can try to trap adults by placing yellow painted discs or boards covered with a sticky substance (e.g. tanglefoot, vaseline) among the vines.
This pest is a tiny black, brown or bronze-colored beetle that jumps like a flea when disturbed. It chews tiny holes in its host plants, but usually only causes damage to young brassicas, turnips, rutabagas and eggplant. I have only experienced serious problems with eggplants they have been turned yellow and sickly on some occasions. However, a floating row cover for 3 or 4 weeks after transplanting has proven completely effective for me in fact, the heat loving eggplant benefits in other ways from these covers.
This beetle, which is a serious pest in the eastern USA, the Niagara Peninsula, and other isolated parts of extreme southern Ontario and Quebec, was accidentally imported to New Jersey in 1916. The metallic-looking beetle, similar in life cycle to our June beetle (white grub), eats a large variety of foliage. They are most active in Canada on warm July days. Hand picking and floating row covers are most effective where they are a problem.
This import, with few natural enemies, has become a real pest in some localities of eastern North America. It is most active for a few hours after dusk, resting and hiding during the daytime. Its preference for small, dark, daytime hiding places allows one to trap considerable quantities place sections of old pipe, bamboo, crumpled newspapers, etc., around the garden and remove the earwigs daily (a pail of hot water is very effective at destroying them). You can also trap them during the night when they are active place yogurt or cottage cheese containers filled with water in the ground. Earwigs will climb in and drown. A homemade repellent sprinkled on particularly susceptible plants, or rows of plants, can provide some protection until vegetation is large enough to be no longer desirable experiment with garlic, pepper powder, mustard, etc. mixed with water, and see what works.
Slugs and Snails
These night feeders are especially troublesome in humid, moist climates particularly in coastal British Columbia. Again, try trapping them if they become a pest. Lure them under boards and pots placed around the garden, then collect and destroy daily. Trap in shallow containers placed in the soil and filled with a water/malt/sugar mixture or with beer if you can bring yourself to give good beer to your slugs. Hunt at night with a flashlight, sprinkle a little wood ash on the soil, or place old copper strips on the ground to deter them.
An organic garden, managed naturally, usually suffers little damage from insect pests, in spite of the long list of adversaries listed above. Nevertheless, a little knowledge of insect life cycles and habits will allow you to deal with any problems that do arise in a manner that does not harm the vast array of beneficial flora and fauna that also live in our gardens.
The following references have colorful pictures of pests, while the last two also contain color photos of beneficials.
Insect Identification Sheets. Communications Branch, Agriculture Canada, Ottawa, ON, 1973.
Rodaless Color Handbook of Garden Insects. Rodale Press, Emmaus, PA, 1979.
The Organic Gardeners Handbook of Natural Insect & Disease Control. Rodale Press, Emmaus, PA, 1992.
Copyright © 1993.George Bushell
Reprinted with permission. All rights reserved.
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