The Greenhouse as an Ecosystem
(Page 8 of 9)
November/December 1984
By Colleen Armstrong
Seed Companies for Greenhouse Culture
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Johnny's Selected Seeds, Albion, ME 04910
Stokes Seeds, Inc., Box 548, Buffalo, NY 14240
Tsang and Ma International, P.O. Box 294, Belmont, CA 94002
(Oriental vegetable seeds)
George W. Park Seed Co., Inc., P. 0. Box 31, Greenwood, SC 29647
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[4] WATCH FOR INSECTS AND DISEASE
The first step in disease and insect control is watchful prevention. When problems go unnoticed, the situation quickly becomes uncontrollable. Pruning off old or dead leaves, removing diseased plants, and keeping the greenhouse clean can minimize such disasters. Give your used pots, flats, and plant containers a brisk brushing in soapy water . . . rinse them in a 50/50 chlorine bleach-water solution . . . and let them dry in the sun.
Isolate new plants for two weeks before bringing them into the greenhouse, and scrutinize their soil, leaves, stems, and flowers for tiny insects. A 10X field glass magnifies such minuscule creatures as spider mites, thrips, and immature whiteflies.
Hands, clothing, and shoes are common carriers of disease, so be sure to wash your hands after working with diseased plants. Never smoke inside the greenhouse, since tobacco mosaic virus can be spread by smokers onto tomatoes and peppers. If you do smoke, disinfect your hands before working with plants.
Select disease-resistant varieties. Often the name of a variety includes abbreviations for diseases that the plant has been bred to resist or tolerate. For example, Roma VF is a tomato that is resistant to both verticillium and fusarium wilt.
Periodically, inspect your indoor garden so that you can spot problems before they get out of hand. Well-balanced soils, proper nutrition (no excesses), watering, and ventilation reduce stress in plants and, in return, allow them to ward off pests and diseases.
At New Alchemy, we depend on cultural, mechanical, biological, and nontoxic chemical means for controlling such common insect pests as aphids,. whiteflies, spider mites, thrips, mealybugs, and scale . . . and our four-step procedure should work for you, too.
STEP ONE: Since many insects assume different forms over the course of their development, learn about the insect's life cycle and become familiar with its various stages. (For guides to the identification of common greenhouse pests, consult 2 and 3 in the reading list.)
STEP TWO: Conduct an insect check. Even though the plantfeeding insects are not always pests, it's prudent to keep an eye out for them. Be on the offensive. Once a week, wander through the greenhouse and seek out potential foes. Flip the leaves over to check the undersides. Don't deceive yourself; once you've discovered an intruder, keep tabs on it, because you can count on more coming. People often ask, "How many are too many?" A general rule of thumb for vegetables is that if more than one-third of the leaf surface (underside or topside) becomes infested with herbivorous insects, additional treatment is necessary. For ornamentals, infestations over one-eighth (or in some cases even less) of the leaf area indicate a need for control.
STEP THREE: Have a control program available. Diversify strategies so that the tactics are those least disruptive to natural controls and least hazardous to human health and the environment. Never depend on only one control measure. Combine cultural, mechanical, physical, and biological controls.
Cultural practices that can diminish insect problems include sanitation, weed control, and using the least amount of nitrogen compatible with adequate plant growth, because nitrogen is a proteinbuilding material for the progeny of many insect pests.
Selecting varieties that may be unappealing to pests is another cultural control. For example, tomato plants with especially hairy leaves discourage the tomato leaf miner.
Mechanical and physical controls include hand-picking (or the "squish" technique), quarantining new plants, or trapping insects with sticky yellow cards. An attractive trap plant is nasturtium, which attracts aphids, whiteflies, spider mites, and thrips. '
Biological control is an approach that employs predatory and parasitic insects or other organisms to control insect pests. Natural enemies may be indigenous to your area, or you may purchase them from an insectary.
STEP FOUR: Evaluate the control program to determine the success of treatment actions. The following are suggestions for an int egrated pest management program for common greenhouse pests:
Aphids are small, pear-shaped insects. The green peach aphid ( Myzus persicae) is the most common. However, many other aphids can enter the greenhouse. Adults are approximately 1/16" to 1/5" long. Wingless nymphs vary in color, but they have the same shape as adults. Adult aphids can be winged or wingless. Green peach aphids are pale green and well-camouflaged on foliage. Favored host plants are peppers, cabbage-family members, lettuce, dill, snapdragons, and pansies
. Cultural control: Do not overfertilize. Prune off heavily infested leaves. Hose off aphids with cool water.
Biological control: Native predators include ladybird beetles, green lacewings (Chrysopa carnea), syrphid flies, and predaceous midge larvae called Aphidoletes aphidimyza. Native braconid wasps will parasitize aphids in the warn season. Ladybird beetles (Hippodamia convergens) can be purchased through commercial insectaries.
Chemical control: Insecticidal or household soap will kill aphids but will also kill different stages of beneficial insects. Use sparingly.
Greenhouse whitefly(Trialeurodes vaporariorum): Adults are white, flylike, and 1/20" to 1/12" long. Nymphs are pale green, translucent, and have flat scales with waxy filaments. A 10X field glass is necessary to see the immature stages. Favored host plants are tomato, European cucumber, tobacco, melon, calendula, some geraniums, fuchsia, lantana, petunia, arid nasturtium.
Cultural control: Hairy plants have the best protection.
Mechanical control: Sticky yellow traps, which you can make or buy, set out by a plant's tip growth will lure adults and snag them on the gooey substance. Use the traps early in the season, and remove them once Encarsia formosa (see below) has been introduced into the greenhouse.
Biological control: A specific parasitic wasp, Encarsia formosa, 1/32" long, is available from many insectaries and is a successful biological control agent when whitefly populations are low and greenhouse temperatures are warm (75°F). Encarsia is not effective during the winter months.
Chemical control: You may spot-spray tip-growth areas with insecticidal or household soap, but this is harmful to Encarsia.
Spider mite(Tetranychus urticae): Adult spider mites are very small (1/32"), oval-shaped, and yellowish, with two black spots on their backs. A symptom of spider-mite damage is bronzish mottling on the leaves and thin webbing between the petiole and stem. Favored host plants include European cucumbers, peppers, grapes, and many ornamentals.
Cultural control: Misting the plant and increasing the relative humidity inside the greenhouse will discourage spider mites. These pests thrive under hot, dry conditions and can frequently be found in one hot spot in the greenhouse.
Biological control: A predatory mite, Phytoseiulus persimilis is commercially available. Introduce the predatory mites at the first sign of spider mites. The predators desiccate at temperatures of about 80°F. If your greenhouse temperature exceeds this limit, try Amblyseiulus californicus, which, unfortunately, does not search for spider mites as well as P. persimilis does. In either case, an annual stocking is usually necessary.
Chemical control: A 50/50 water-alcohol solution may reduce spider mites, but do a test spray before treating the entire crop.
Thrips(Thrips tabaci or Heliothrips haemorrhoidalis): These tiny, slender, rodlike insects are 1/25" long and have a hard exoskeleton. Adults are brown, and nymphs are creamy white; both hide under leaf veins, flower buds, and petals. Favored host plants are European cucumbers, melons, peppers, marigolds, gladioli, African violets, and many ornamentals.
Cultural control: Maintain good sanitation. Prepupal and pupal stages rest in leaf litter on soil beds. Eliminate any highly infested leaves, or remove the entire plant
. Mechanical control: Mist leaves and dust with diatomaceous earth if the thrip population is low. Scrape the top 1/2" of the soil away from the infested plants.
Biological control: The predatory mite Amblyseiulus californicus will prey on thrips at a slow rate. Both green lacewings and Orius insidious, the minute pirate bug, will also help control this pest, and both are indigenous to North America.
Mealybugs: There are many different kinds of these white, soft, segmented, oval insects. Between 1/5" and 1/3" long, they are usually covered with a whitish, powdery, waxlike material. Females and nymphs are wingless and are found most frequently on tropical plants. Favored host plants are banana, bamboo, ginger, citrus, and many ornamental houseplants, especially orchids.
Cultural control: Dislodge mealybugs by hosing them down with water. Quarantine new plants and discard any infected plants.
Biological control: A predatory beetle, Cryptolaemus montrougieri, is available at commercial insectaries. This Australian lady beetle is 1/4" long, with a black body and orange head. The larvae look like mealybugs. They prefer warm temperatures (75°F). A commercially available wasp, Leptomastix dactylopii, will parasitize the citrus mealybug.
Scale: There are several species of both soft scale and armored scale. Scale insects, like mealybugs, are plant suckers that distort and yellow afflicted leaves. Females are found on the undersides of leaves and stems. A female and her young live under a protected, waxy covering. Favored host plants are tropical fruit or ornamentals, especially philodendrons, palms, and orchids. (See Backyard jungle on page 174 for more information on scale insects.)
Cultural control: Dislodge scale with a forceful spray of water. Peel off any loose sheath tissue that scale could hide behind. Prune off damaged leaves.
Biological control: Green lacewings and Australian beetles (Cryptolaemus montrougieri) will feed on scale. A parasitic wasp, Metaphycus helvolus, provides variable control of several soft-scale species. All three biological-control agents are commercially available.
Chemical control: Insecticidal or household soap can be applied if beneficial insects are not being used.
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