Handling Insect Pests in Your Greenhouse

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Yellow sticky traps like this one can help get rid of insect pests like whitefly.
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Plants in containers can become rootbound, with compacted, crowded rootballs that lack air and soil. Repotting plants regularly prevents this from happening.
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A neem tree is one of many solutions that can help to drive away unwanted insects in your greenhouse.
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Roger Marshall covers all aspects of owning and maximizing the potential of a greenhouse in “The Greenhouse Gardener’s Manual.”

Wanting to get the most out of your greenhouse? In The Greenhouse Gardener’s Manual (Timber Press, 2014), author Roger Marshall provides information on how to choose, heat, irrigate and maintain your greenhouse. This excerpt, which details the best methods for controlling unwanted insect pests, is from Chapter 10, “Controlling Greenhouse Pests and Diseases.”

Buy this book from the MOTHER EARTH NEWS store:The Greenhouse Gardener’s Manual.

Talk to any greenhouse grower and they will tell you that the two worst pest problems are aphids and whitefly. You might also find sow bugs, mealybugs, scale insects, and red spider mites. Larger greenhouse pests include slugs and snails that hitch a ride under the rim of large pots or in the drain holes of larger pots. Although screened windows can keep out butterflies and moths, you may also find a caterpillar or two feeding on your plants. Lastly, rodents can get into the greenhouse. Mice are a common problem and I’ve had groundhogs, skunks, and even a fisher that ate all the goldfish in my greenhouse pond one night when I forgot to close the door.

The best way to control insect pests is to be careful what you bring into your greenhouse. An isolation chamber can be a convenient way to keep new plants separate from the main population of greenhouse plants until you are satisfied that they are pest-free.

Whether to use insecticides is a personal decision. I prefer not to spray pesticides of any kind inside the greenhouse because they can kill off beneficial insects along with problematic ones and I find the effects of the product are multiplied by being in an enclosed structure. I have noticed that the odor of a pesticide in the greenhouse can last several days. Also, continual use of the same insecticide can eventually give rise to insecticide-resistant bugs.

My preference is to take the approach of Integrated Pest Management (ipm). This means I always begin with the least toxic type of prevention before moving on to more drastic measures. I start with barriers and screens to keep out pests, rotate crops, eliminate host plants in the vicinity of the greenhouse, isolate incoming plants, keep the greenhouse clean, handpick pests, and finally, bring in natural enemies such as ladybugs to feed on harmful insects. All these controls can easily be integrated into your greenhouse practice before you need to resort to chemical pesticides.

Preventing Problems in Your Greenhouse

Most beginning gardeners take many years to figure out the two most important tools are not a trowel and a spade, but observation and feel. When it comes to keeping plants healthy your best bet is to inspect your plants, no matter if you only have one plant or thousands. You need to touch them and more importantly, touch the soil, especially the soil around container plants.

When looking at your plants, try to see how they are enjoying where you put them. Are they growing? Are the leaves green or are they yellow or worse still are they brown? Are the leaves normal or have they curled? Are the plants standing tall or are they wilted? Are there any insects or insect eggs on them? Have the plants bolted?

Learn what your plants should look like when healthy so you can identify problems quickly. It also doesn’t hurt to knock a plant out of its pot and check the roots. If the roots are going around the bottom of the pot, the plant may be rootbound and it’s time to transplant. Container plants should be checked this way at least once a year.

Greenhouse Barriers and Traps

You’ll never be able to keep every insect out of your greenhouse. They fly in when you open the door, they crawl through minute cracks and openings, and they can hitch a ride in on your boots, tools, or new plants. However, you can keep the vast majority of insects at bay with the use of screens placed across vents, windows, and other openings. The problem with screens is that they cut down on the amount of light that gets into the greenhouse. Ideally, you will use screens whenever the windows and vents are open and remove them to allow in maximum light when the greenhouse is closed up.

Another way to help keep insects out is to have an air lock between the outdoors and the inside of your greenhouse. If you are growing crops that are prone to particular pests, such as cabbage white caterpillars on brassicas, you can also use floating row covers placed over the plants to protect them.

Sticky traps are another way to control pests in the greenhouse. Some common traps include yellow sticky traps for whitefly, and red balls for coddling moth and fruit flies. The insects are attracted to the color of the trap and get stuck on the adhesive covering. It’s a good way to monitor and identify pests that may be lurking around your plants. Put them under the benches, close to intake vents, and near windows and doors.

Isolation Chambers for Your Greenhouse

You have a greenhouse full of plants and buy a new plant which turns out to be buggy. Pretty soon your entire greenhouse is full of bugs. To prevent this from happening, you can isolate any new plant or cuttings until you are satisfied that they are bug free. In most cases this means the plant is in isolation for about two weeks.

An isolation chamber can be quite simple. One option is a small plastic greenhouse or cold frame kept in the air lock or just outside the greenhouse in summer. You can also use a large aquarium tank with a tight-fitting lid, although these can be quite heavy and are not very portable. The isolation chamber should stay at the ambient temperature for your greenhouse and will also allow the plant to become acclimated before it is moved into the general plant population.

Natural Insect Repellents and Controls

There are very few plants that do not attract insects but there are some that repel them. Rue (Ruta graveolens), for example, repels bees and some beetles. Spearmint repels aphids, and strong-smelling herbs like rosemary, thyme, and wormwood are all distasteful to the olfactory senses of many moths and flies. By interplanting these insect repellents in your greenhouse, you can help to drive out pests. For example, a pot of mint placed next to any of the brassicas will help keep cabbage white butterflies away. Similarly, a pot of rosemary will keep bean beetles and carrot flies away.

Another option is to plant a neem tree (Azadirachta indica) or two. Neem trees are the source of a natural insecticide that can be used in the greenhouse, but growing your own plant in your greenhouse is another safe way to deter insects. To make your own insecticide, simply chop neem leaves in a blender and add water. Add a dash of liquid soap to help the solution stick to leaves. Leave the solution for twenty-four hours and strain off the liquid. Spray it onto your plants. Neem will break down in the greenhouse sunshine in a few days.

An alternative that is the complete opposite of repellent plants are those plants that attract certain insects. These indicator plants can be used to demonstrate when insects are present. For example, whiteflies are attracted to lettuce and tomatoes. By checking these plants regularly for insects, you can ascertain whether your controls are effective.

Organisms and insects that eat pests are known as biological controls or beneficial predators. You can order these from suppliers online. One well-known predator is the ladybug. These little black-spotted red beetles are voracious eaters of aphids and if they are contained in the greenhouse will eat every aphid in sight. They can be purchased by mail order but you may have to buy far more than you need and may find that they leave the greenhouse just as soon as they have devoured every aphid. (The ladybugs in my greenhouse did a terrific job, but I have no idea what happened to the two thousand beetles I released in the greenhouse. They were almost totally gone within six weeks—but so were the aphids.)

Another aphid predator is the green lacewing, although if they run out of aphids they are just as likely to eat each other. Damsel bugs and assassin bugs will also feed on aphids.

The larva of the midge Aphidoletes aphidimyza bite into aphids and inject a toxin. They usually prey at night and all you might see are sawdust-like aphid carcasses on leaves. One drawback to Aphidoletes aphidimyza is that it likes warmth and works best in summer.

Several parasitic wasps and predatory mites are also efficient biological controls. Aphytis melinus can control scale and Encarsia Formosa seems to do the best job on whitefly. Trichogramma wasps attack caterpillars. The predatory mites Amblyseius californicus and Phytoseiulus longipes can control spider mites.

Other Solutions

As with outdoor garden beds, it is a good idea to practice crop rotation to help reduce the incidence of soilborne diseases in your in-ground greenhouse beds. For example, if you are growing winter greens, you might change to bedding plants in spring and tomatoes in summer, then clean the beds before planting autumn crops.

You can also consider leaving your greenhouse empty for the summer when all your plants can easily survive outside and thoroughly cleaning it before moving your plants back in (carefully inspect plants before bringing them back into the greenhouse). A more radical solution to remove severe infestations is to let the entire greenhouse freeze for a few days in winter. That will effectively kill off any insects, but if you cannot let your greenhouse freeze, let the temperature drop to just above freezing so that insects become sluggish, and infected plants can be easily removed without the insects flying off the plant before you can get them out of the greenhouse.

Using Insecticides in Your Greenhouse

If you do choose to use insecticides, start with the least-toxic products first. The products listed here have low toxicity, but they can still damage beneficial or harmless insects along with the pests, so use them sparingly. If you are applying these products, follow the directions on the package and be sure to observe any safety instructions, such as wearing a mask, respirator, gloves, or any other recommended clothing.

Insecticidal soap sprays kill pests by dehydration. You can make your own soap spray by adding 2 spoonfuls of liquid soap (not detergent) to a quart (1 liter) of water.

Horticultural oils are refined petroleum or vegetable oils that work by smothering pests.

Pyrethrum is derived from chrysanthemums. It is a broad-spectrum insecticide that affects the insects’ nervous system.

Rotenone, which occurs naturally in the seeds and stems of several plants, is a broad-spectrum insecticide and piscicide. Do not use it near any fish you have in a greenhouse pond or aquaponic system.

Using Fungicides in Your Greenhouse

If conditions are right you can easily get fungal molds in the greenhouse. In some cases mold can affect an entire fruit crop and all your hard work caring for your plants may come to nothing. The easiest way to prevent molds from occurring is to ventilate your greenhouse well. If you do not get cross-ventilation when the greenhouse windows are open, make sure you have a fan blowing across or above your plants to ensure air is well circulated. If that fails, you should remove infected plants, and you may have to spray with a fungicide depending on the severity of the infection. The main fungicides for use in the greenhouse are sulfur, copper, and Bordeaux mixture (a combination of copper sulfate and lime), but in many cases these work best as preventative measures rather than curative ones.

Cleaning Up Pests in Your Greenhouse

Some cleaning strategies can help to minimize pests and diseases:

Remove old plant matter such as dead leaves to eliminate food sources for insects that live on decaying plants.

• Give your plants plenty of room and good air circulation.
• Mop up pools of standing water helps to eliminate algae, moss, and insect larva.
• Use sterile potting soils, and sterilize compost before using it.
• Clean gardening tools, seed flats, and other equipment.
• Remove or isolate infected plants.

This excerpt has been reprinted with permission from The Greenhouse Gardener’s Manual, by Roger Marshall and published by Timber Press, 2014.