Bugs and Greenhouses: Keeping the Bugs Out

Authored by: James W. Brown

The insects and mites that could become a problem in your greenhouse live on plants. If you bring plants into your greenhouse from a friend’s greenhouse or even your own yard, the greenhouse pests could travel along with them. It is difficult to impossible to make sure there are no hitchhikers on plants that you bring in. Here's some advice on how to get rid of mites on plants.

Even if you buy plants at a nursery or a garden center, you can not be assured that the plants are perfectly clean. Some vendors will be better than others. If you have been getting plants from a good producer and have not had problems in the past, it would be a good idea to stick with that supplier even if the prices are higher.

Another step you can take is to change the sequence of your gardening habits. Insects and mites can ride along on you. Go to your greenhouse before rather than after you work in your outside garden. The same holds true if you visit a friend’s garden or greenhouse. If you’ve been out and about where there have been plants, you do not want to go into your greenhouse before changing your clothing and brushing your hair. Even if you are out for a walk in the woods, you should clean yourself up and change your clothing before going into your greenhouse. Insects are notorious hitchikers.

A dog may be man’s best friend, but it is not your greenhouse’s best friend. There is probably no problem in carrying a lap dog out to the greenhouse directly from the house, but a dog that takes a detour through your garden and a field or two on its way to the greenhouse, or a dog that lives outside should not be allowed into the greenhouse. It’s likely to bring insects or mites into the greenhouse with it, possibly adding to the greenhouse pests.

Screening the air intakes of the greenhouse will help keep insects and mites out. A very fine screen like a thrips screen will keep almost any insect or mite out. However, it will also restrict airflow through the greenhouse. For this reason, it is necessary to build a box to increase the surface area through which the air can be drawn by the greenhouse equipment. The air intake surface area for a Thrips screen should be five times the unobstructed air intake area. This allows for the collection of a little dust on the screen, but the screen should be cleaned periodically to prevent the build-up of dust and flying plant seeds which obstruct the air flow. Installing a door in the screen box is helpful, enabling you to go inside and hose down the screen from the inside to clean off accumulated dust and seeds.

A double entry way for the greenhouse should be used if insect screening is used on the air intakes. This prevents the wind from blowing into the greenhouse when you enter, and for greenhouse pests to gain entry. When using a double entry, make sure one door is closed before the other one is opened. Some people even look around to see if anything came in with them before proceeding into the greenhouse. This can become a consistent habit, but can become a challenge when a group of three or four people is going into the greenhouse simultaneously. Usually, some done will be polite enough to hold both doors open for others. Unfortunately, this allows the insects to gain entry into the greenhouse along with the people.

Plant beds, potted plants and weeds outside the greenhouse should be eliminated. Any plant material near the greenhouse - especially near the greenhouse door - can serve as a reservoir of pests waiting for an opportunity to enter.

Common Insects and Mites in the Greenhouse

Some insects and mites are more likely to set up residence within your greenhouse than others. Most insects and mites don’t bother us. It is just the few that do that give the whole group a bad name in many people’s opinion.

Before action is taken when an insect has been spotted in the greenhouse, some kind of identification should be made. The first thing that needs to be determined is whether the insect or mite is feeding on the plant. Insects like caterpillars chew holes in leaves. This is fairly obvious feeding activity. Insects like Thrips cut open a number of cells in a patch on a leaf and then drink the cell sap. They will often leave spots of excrement in the feeding patches. If you know what to look for, this type of damage is fairly easy to spot. Other insects and mites on plants suck the juices out of them. Although this can not be easily detected once the insect or mite moves along, the insect or mite spends a fair amount of time boring in to the plant tissue and does not move on quickly. They are usually moving slowly enough that they can be observed. Some of us may need to use a magnifying glass to get a good look at these rather small insects or mites on plants.


Caterpillars are the larval stage of butterflies or moths. They hatch out in the greenhouse from eggs laid by their mothers, who flew or crawled into the greenhouse. If you keep the adults out of the greenhouse, there will be no eggs to produce the caterpillars. Moths and butterflies are fairly easy to keep out of the greenhouse with a little care, a double entry way and a fly swatter.

The Tomato Pinworm is one caterpillar that can become fairly well established in a greenhouse having several tomato plants. Unlike most caterpillars, this larva lives between the upper and lower leaf surfaces. This habit makes it very difficult to eliminate with conventional chemicals or biological stomach poisons. A firm squeeze of the leaf occupant will effectively stop the feeding and development of the caterpillar. In a small greenhouse, this will be effective if efforts are made to keep adults out and if the few caterpillars that develop from the eggs of adults that do get into the greenhouse are hunted down and squeezed. Some growers prefer to wear rubber gloves when doing this job.


Aphids are sometimes referred to as plant lice because they suck the juices of the plant. They are fairly small and easily transported into the greenhouse on incoming plants or the clothing of people who have been around infected plants. Although most aphids are wingless, there is a winged stage that develops when the population gets crowded or in the fall, when the aphids spread. This is a much more mobile form of the aphid.

There are many different kinds and colors of aphids. What you may get in your greenhouse is determined to some extent by what kind of crops are being grown outside in your area and what kind of aphids feed on them. A major crop that is harvested and almost completely removed from the field in a short time period can leave many aphids without lunch. They will move around as much as they can while looking for a food source. Your chance of getting aphids into your greenhouse after an event like this in your area is great unless you take precautions like having a double entry way and having the air intakes screened as discussed earlier. A closer inspection of yourself and others entering the greenhouse is also in order when hungry aphids are looking for a living green spot in your neighborhood. Aphid populations can build very rapidly in a favorable environment, and a greenhouse environment is favorable to many aphids. Except for when they are preparing for over-wintering, aphids bear living young. To make matters worse, female aphids can do this on a repetitive basis without the participation of a male aphid. The offspring are female and can contribute to the population surge by producing their own young within a few days.

Once aphids are in the greenhouse, the use of insecticides will be necessary. Many insecticides are labeled for aphids. Some organic preparations are suggested for aphids. Several beneficials are also effective against aphids.


Whiteflies love a greenhouse environment. Although they can fly, they are not strong fliers and don’t do well if there is much of a breeze let alone a wind. The greenhouse environment protects them against sudden strong air movements. When they fly, whiteflies form a zigzag flight pattern that runs for a short distance that is generally a maximum of three to five feet. They are well known by most commercial and hobby greenhouse growers.

The traditional Greenhouse Whitefly has been joined by additional types of whiteflies. The names include Sweet Potato Whitefly, Silver Winged Whitefly and Tobacco Whitefly. No attempt will be made here to distinguish between the whiteflies here. When, however, one of the other whiteflies comes in, it can drive the traditional Greenhouse Whitefly out. For a while, there can be a mixture of the whiteflies in the greenhouse. Eventually, however, the other whitefly wins out and the Greenhouse Whitefly disappears.

The Sweet Potato Whitefly, the Silver Winged Whitefly and the Tobacco Whitefly are all a little more difficult to control with chemicals and they require a different predator than that which will control the Greenhouse Whitefly.

It would be a good idea to get help identifying the kind of whitefly you have in the greenhouse when you detect a population starting. Check this each year, because the type of whitefly infesting the greenhouse can change from year to year in the same area.

Whitefly adults usually congregate at the top of plants. They lay their eggs on the bottom side of the upper plant leaves. The eggs hatch and go through three larval stages while on the bottom of the leaves. The last two larval stages settle down in one location and suck the juices from the leaf cells. This is followed by a pupal stage from which the new adult whitefly emerges. During the time this temperature dependent, 21 plus day process occurs, the plant continues to grow. Later stages of the life cycle are found lower on vertically growing plants.

Because of the life cycle of the whitefly, it is difficult to control. You may get all the adults but more may hatch out the following day. Even if there are no adult whiteflies on plants brought into the greenhouse, whitefly eggs or immatures may ride in on them. Yellow sticky cards are often used at the top of the plants as whitefly population monitors. They can also be used to help in the early detection of whitefly presence in the greenhouse.

Spider Mites

Spider Mites are very small mites that can be seen if you know what to look for and where to look. Even adult mites do not fly – they do not have wings. They are in the family of spiders and are not technically considered insects. Many insecticides, therefore, will not be effective. Miticides are used to kill Spider Mites.

Spider Mites prefer some plants over others. Cucumber and squash plants, for example, will be inhabited by Spider Mites before tomato plants located in the same greenhouse. However, if the cucumber or squash plants are removed, the Spider Mites will certainly move to the tomato plants rather than starving to death.

Feeding consists of the sucking of plant juices by the immature and mature Spider Mites. Often, Spider Mites will start on the underside of the leaves at the top of the plant. They prefer a dryer, warmer environment, and the top of the plant is usually dryer and warmer lower portions of the plant. Heavy feeding by large Spider Mite populations will give the leaf a speckled, yellowed out appearance. Closer examination, with a magnifying glass if needed, will reveal the Spider Mites.

Webbing that looks very much like spider webbing will be evident on severely infested plants. When the webbing appears, the Spider Mite population is very well established in the crop and greenhouse.

Spider mites can be present in outside gardens and crops. It is very easy to carry them from these locations into the greenhouse if precautions are not taken. Field crops like soybeans are very susceptible to Spider Mites.

Leaf Miners

Leaf Miners are a challenging pest. The adult looks like a small fly. Unless you have a background in Entomology, you probably will not identify the adults as a problem even if you see a few of them on your sticky yellow monitoring cards.

The adult Leaf Miner lays an egg on the leaf of the plant. The larvae tunnels and eats leaf tissue from between the upper and lower surfaces of the leaf. Once the larva is inside the leaf, it is protected against anything that would be sprayed on it. There are beneficial parasites that will control and eliminate Leaf Minors when properly introduced.

Leaf Miners usually pupate in the soil. If the larva drops onto a hard surface like a concrete floor, it will be killed and will not complete its life cycle. Hydroponic greenhouses having concrete floors are less likely to have Leaf Miner problems than greenhouses with other flooring surfaces.


Gnats are usually more a nuisance than a real damaging problem in the greenhouse. Adults lay their eggs in the algae on the growing media or other lighted surface in the greenhouse. The larvae mainly live on dead organic matter. Once in a while, the larvae will nibble on a few young roots.

The adults make a nuisance of themselves by flying up into your face when they are disturbed and detract from your enjoyment of your plants by their presence on your plants.

Beneficial nematodes can be used to control Fungus Gnats. Some chemicals and natural products can also be used to control them. Because there are so many places in the greenhouses that gnats can breed, it is difficult to cover them all with a sprayed material.


Slugs work the night shift. They’re neither insects or mites. They chew on the leaf tissue and leave holes in the leaves. They do this at night and then hide during the day, when it is very difficult to spot them. Many people think they have a mystery problem when they have slug damage. There are a few clues that can be identified. The droppings look very much like those of caterpillars.The slug trails, however, are the dead giveaway clue. Slugs will leave a narrow silvery trail where they have crawled. This sometimes can be seen on the plant stem and leaf where their feeding damage can be seen.

Find the slug’s hiding place in the greenhouse and eliminate it. Slugs will hide under loose dead leaves or under a piece of wood or other items lying on the ground or the floor of the greenhouse. They can sometimes get under plant pots and hide. Hunt them down and escort them out of the greenhouse.

Controlling and Eliminating Pests

Preventing and eliminating pests in the greenhouse is an ongoing effort. Several preventative and control strategies need to be considered. Obviusly, if the pest is kept out of the greenhouse, we don’t have to get rid of it. The barriers, procedures and precautions mentioned earlier in this article need to be implemented to the fullest extent practical and possible. The potential for problems with pests will be reduced by these practices, but not eliminated. If it were that simple, you probably would not be reading this now.

Most pests have a life stage during which they are more vulnerable to control measures. The more you know about the pest and how it grows and reproduces, the better choice you will make when it comes to a control strategy.

At least once a week, take time to inspect the plants in the greenhouse for insect and disease presence or damage. Yellow sticky cards and other monitoring materials should be used as an aid in the early detection of insects in the greenhouse. Some creatures are attracted to the monitoring materials and some are not. You need to use both the monitoring materials and the visual examination of the plants to make an early diagnosis of a problem. The earlier a problem is detected, the earlier the treatment can be started and the smaller the problem will probably become. Beneficials – insects or mites which are natural predators or parasites of problem pests – are available for many of the insect and mite infestations that can occur in the greenhouse. Many commercial growers use beneficials on a regular basis. For a small or hobby grower, the cost of using beneficials can be quite high, relatively speaking, because the cost of using beneficials is mainly in the shipping cost. Although you will need a fraction of the number of beneficials that a commercial grower will need, the cost is going to be almost as much because the shipping charges are as much for a few as for five to 10 times as many beneficials. The effective use of beneficials usually involves three or more separate introductions of beneficials, which translates to three or more separate shipments. Most beneficials cannot be stored away for a week or so after they arrive. They need to be put out into the greenhouse where they do their work and get their lunch, in order to survive.

Conventional chemicals can be used against many of the insect and mite problems in the greenhouse. The number of these products currently on the market, as compared to a few years ago, has decreased. Some have been removed from the market due to chemical regulations and at the same time, fewer new products are being introduced. Many growers, however, choose not to use such products, preferring to grow in an environment without these substances. They may also want to eat produce they have grown that has not been treated with conventional pesticides.


arious other products can be used to control insects and mites. Some products are available that are approved for “organic” production. They offer at least some level of effectiveness. Sometimes, their mode of operation is as a discouragement rather than a killing action against the pest.

Don’t Get Bugged

The bottom line is that while the conscientious greenhouse grower will take every precaution practicable in preventing a pest infestation in his greenhouse, pests will still present a problem from time to time. The best solution is for the grower to arm himself with knowledge so he is ready to swiftly and effectively deal with pests when they appear.