Garden & Greenhouse


Identifying and Treating Greenhouse & Indoor Garden Pest Insects

Posted January 18th, 2017 by Garden & Greenhouse in , ,

Few things are as devastating to an indoor garden or greenhouse as a pest insect infestation. The isolation from nature, which gives an indoor garden or greenhouse heightened control over many variables, is actually a disadvantage when it comes to pest insects. In nature, predatory insects keep the population of most pest insects in check. Some plants even use pheromones when being attacked by a pest insect to signal predatory insects to come to their aid.

In an enclosed environment, like a greenhouse or indoor garden, pest insects are not kept in check by their natural predators and can quickly reproduce, overtaking an otherwise healthy garden. When a pest insect does show up in the garden, correct identification and immediate treatment are a grower’s best chances to eradicate the problem before it gets out of hand. Even when the pest insect itself isn’t visible, there may be other indicators that a pest insect is present. Horticulturists who know what to look for will be able to identify problems much earlier and begin treatment sooner.

Daily Monitoring

Monitoring an indoor garden or greenhouse daily is one of the best ways a gardener can prevent many different problems. Daily monitoring allows the horticulturist to check on hardware such as lights, fans, filters, and also to closely examine the plants for deficiencies or insect damage. Gardeners who monitor the garden daily and log data on a regular basis become “in-tune” with their gardens and are more likely to be successful horticulturists. A grower should start his or her daily monitoring by making sure all ballasts, fans, bulbs, air conditioning units, and CO2 enrichment equipment are functioning properly.

A digital humidity and temperature monitor, which keeps track of the highest and lowest temperature and humidity of the last 24 hours, is a really handy tool to have. These atmospheric highs and lows can then be logged into a garden journal. After a grower has examined all of the mechanical aspects of the garden, he or she should examine the plants themselves. Looking over the entire plant from the bottom upward is a good place to start. Next, randomly select four or five leaves on each plant and examine them closely. Anything out of the ordinary should be addressed immediately. The more common pest insects each have their own tell-tale signs.

Spider Mites

The first sign of a spider mite problem usually shows up in the form of yellow speckling on the surface of the leaves. The speckling from spider mite damage will resemble yellow spray paint misted on the leaves. Closer examination of the bottom of the leaves will reveal clusters of mites and their eggs. In more extreme infestations, webbing may be found in-between or on the tips of branches and leaves. This webbing looks very much like a spider web and is how these nasty bugs received their fitting name. Diligence and early detection are keys to eliminating a spider mite problem.

Miticides (insecticides created specifically for mites) are generally the best option for treatment. Follow the manufacturer’s instructions for initial treatment and re-treatments. With most miticides, a grower will need to treat his or her garden every 3-4 days for at least one month. The reason for this aggressive type of treatment is that most miticides will kill adult mites, but not the mite eggs. Multiple treatments will ensure that any eggs hatched between treatments will not be able to establish a new population of spider mites. Although some miticides claim to destroy the eggs, spider mite eggs are extremely resilient to treatments and seem to be able to hatch no matter what.

Fungus Gnats

The first sign of a fungus gnat problem is usually the small mosquito-looking black or gray insects that fly around aimlessly. They are most prevalent right after a watering or when the soil is disturbed. The larvae of fungus gnats look like tiny, light colored worms that wiggle around in the top layer of soil. They can sometimes be seen “dancing” in standing water after a feeding.

The best defenses against a fungus gnat problem are soil drench insecticides (which are used to treat the eggs and larvae found in the soil) combined with sticky traps to catch the adults. Another simple technique to reduce the likelihood of fungus gnats is to water planting containers from the bottom instead of the top. This will allow the upper layer of soil to dry out. The insect’s ability to reproduce will be reduced since fungus gnats require moisture in the top few inches of soil in order to lay eggs.


The first sign of a thrip problem is usually “shiny streaks” which show up on the surface of the leaves. The shiny trails are actually the areas of the leaf where the thrip larvae have been feeding. Gardeners may also notice tiny black specs on the leaf surface which is the larvae’s fecal matter. To the naked eye, thrip larvae resemble fast moving grains of rice. The larvae can be many different colors, but are usually yellowish-green.

As with fungus gnats, a multifaceted approach is required to control thrips because they live a portion of their life cycles on the plant and the other portion is lived in the soil or medium. The combination of sticky traps and an insecticide specific to soft-bodied insects will take care of the adults and larvae above ground. For the eggs and larvae in the soil or medium, the grower should implement a soil drench insecticide or predatory nematode solution.


In most cases, the gardener will see the aphids themselves before they notice any damage to the plants. Aphids are small, pear-shaped insects that are usually found in dense clusters on the underside of leaves and/or on tender new growth tips. There are many different varieties of aphids and they come in all sizes and colors (green, brown, yellow, pink, or black).

Any insecticide specific to soft-bodied insects will work well for treating an aphid infestation. Insecticidal soap and neem oil are both effective at treating aphids. As with spider mites, diligence is important for complete eradication.


The first thing a gardener will notice about a whitefly problem is the tiny white moths which fly around when the foliage is disturbed. Another tell-tale sign of whiteflies is the shiny sheen they leave behind on the leaves. In most cases, whiteflies are more of an annoyance than a detrimental problem. However, if left untreated, a large population of whiteflies will leave a lot of residue on the leaves of the plants. This abundance of residue could eventually develop into a black pathogenic mold. Yellow colored sticky traps work great for catching the adult whiteflies. In addition to the sticky traps, an insecticide specific to soft-bodied insects should be used to treat the eggs and larvae.


The first sign of mealybugs is usually cotton-like, fluffy masses found in crotches or joints. These “cotton balls” are actually clusters of the slow moving mealybugs. Even though these bugs are slow to reproduce, they should be treated immediately to reduce any possible adverse effects.

Insecticides specific to soft-bodied insects work great for mealybugs; as long as a gardener is diligent. It is recommended to continue treatment for a week or two after the last signs of mealybugs have been seen. Unfortunately, these crawlers have a tendency to reappear weeks, if not months, after a gardener thinks he or she has eradicated the problem. Physically removing the cotton-like clusters with a cloth or by hand (prior to foliar treatments) can also do wonders in reducing the likelihood of re-infestation.

Beneficial Insects

One of the best ways an indoor gardener or greenhouse hobbyist can mimic nature’s defense against pest insects is to introduce beneficial insects into the garden. Beneficial insects are the natural predators of the pest insects. As with other treatments, positive identification of pest insects is imperative for success. In other words, the type of pest insect must be known so the appropriate beneficial insect can be introduced into the environment. The five most popular beneficial insects used by greenhouse hobbyists and indoor gardeners are ladybugs, predatory nematodes, green lacewing larvae, predatory mites, and pirate bugs.

Ladybugs will literally eat any insect, larvae, or egg it can fit in its tiny jaws. The relatively low cost of ladybugs makes them the most cost effective beneficial insect solution for a grower. Another advantage of ladybugs is that both the adult and larvae stages are active predators.

Predatory nematodes are tiny worm-like creatures that can be watered directly into the medium and will feed on the eggs and larvae of soil born insects. These effective little creatures are great for treating thrips and fungus gnats.

Green lacewing larvae are true opportunists and will feed on any insect, larvae, or egg they can inject with their paralyzing venom. Green lacewing larvae feed on a variety of pest insects, including thrips, aphids, mealybugs, and whiteflies.

Predator mites seek out and feed exclusively on spider mites. There are a few different species of predator mites available to the indoor gardener or greenhouse hobbyist.

Pirate bugs feed on a variety of bugs, including thrips, aphids and even spider mites. They also feed on all the stages of a pest insect’s life cycle (eggs, larvae, and adults) which makes them a popular choice for battling thrips and spider mites.

Pest insects are the achilles heel of indoor gardeners and greenhouse horticulturists. A protected indoor growing environment, which is perfect for optimizing many of the variables responsible for plant growth, is also a perfect place for pest insects to monopolize. Due to the amount of damage they can do in a very short period of time, pest insects need to be addressed immediately. Horticulturists who monitor their gardens daily for the indicators of a pest insect’s presence will be much more likely to recover from an attack without sacrificing a large portion of the garden’s yield.

Catching a pest insect problem early on can also mean a less aggressive treatment regimen will be required. In fact, if caught early enough, many indoor horticulturists and greenhouse hobbyists can solve a pest insect problem with the help of beneficial insects. Regardless of the treatment he or she chooses to use, a gardener must possess the knowledge to properly identify a pest insect. If identified correctly and early enough, a gardener will be able to rectify the problem with the least amount of disruption to the garden’s overall performance.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at

Read more articles:

Hydroponic Systems Can Have Pest Control Problems Too

Natural Mite and Pest Control Options for Your Orchids

Natural Pest Control for Greenhouses and Indoor Gardens

Natural Pest Control for the Garden

Pest Control Using Predatory Insects

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