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Gardening Wars: When Plants Blight Back Part 2: Plant-eating Pests and their Predators

Proper identification of insect colonies can be tough for hobby gardeners. Fortunately, a magnifying glass paired with this simple guide to identifying common pests in the growroom can steer you in the right direction. The following is a crash course on the top four most prolific pests found in hydroponic systems. Spider Mites One of the most infamous pests in hydroponics is the two-spotted spider mite. Ranging in color from brown, to red, to yellow and green, these mites are typically found on the undersides of leaves on many different plants. It might be necessary to use a magnifying glass at a 15-20-times zoom to even be able to see two-spotted spider mites since they are extremely small. They also come in several different variations—some of them don’t even have two spots. Spider mites prefer hot, dry months but they can persist through the winter months as well. For the hydro growers out there, spider mites can be present year-round and pose some pretty serious threats to the health of plants. There have been reported cases of spider mites so bad they have killed an entire crop overnight, through a combination of their voracious appetites and their uncontrollable rate of reproduction. Watch out for fine webbing on the branches, leaves or flowers of plants, as this is often an indication that a moderate-to-severe colony has moved in. The presence of small, chlorotic spots on leave surfaces is one of the first signs of an infestation. These spots are actually bite marks from mites feeding on the sap of plants. Graying or yellowing of foliage usually always accompanies a spider mite infestation as well. If you are confident a spider mite colony has invaded your growroom, there are several biocontrols that are effective against them. The most popular defense against spider mites is one of several different kinds of predatory mites commercially available in the United States. But even if a spider mite infestation is treated before a severe amount of devastation has taken place, the damage incurred is typically irreversible, and there will be some level of detrimental effects on the quantity and quality of a harvest. Whiteflies Another common pest found in hydroponics is the whitefly. Whiteflies are tiny, white-winged insects that usually gather on the undersides of leaves. They are active during the daytime and can be seen with the naked eye, making them slightly easier to identify than other insects. The presence of whitefly eggs is another sign of an infestation. The eggs are usually laid in circular patterns on the undersides of leaves and are round and white in color. One of the telltale signs of a whitefly infestation is the honeydew they secrete on the foliage of plants as a result of feeding on the sap by piercing into the phloem of plants. It is a sticky substance left over on the surface of leaves. Honeydew carries the risk of producing fungal diseases and subsequently weakening the plant. But the most serious threats posed by the presence of whiteflies are the harsh diseases they commonly carry, which they transfer to the plant during feeding. If an infestation gets to this point, it may be too late to recover the crop, depending on how fast the disease spreads. A serious whitefly infestation may be difficult to control, as whiteflies are known for developing a tolerance to chemical pesticides relatively quickly. The best way to control an infestation is to use integrated pest management, where several different control methods are implemented. Washing off the whiteflies and their eggs is a good place to start, followed by the use of a biocontrol—predatory mites and parasitic wasps are two effective options. Fungus Gnats One of the biggest nuisances in the hobby gardener’s growroom, the fungus gnat, tends to have more indirect consequences than the direct damage brought on by spider mites. Adult fungus gnats appear as small, delicate black flies with long antennae. Their most distinctive physical trait is the presence of a Y-shaped vein on their wings. Adult fungus gnats don’t typically carry many risks, besides flying around in a grower’s face and being extremely annoying, but they have been known to be a vector for plant pathogens such as pythium. The larva of fungus gnats, on the other hand, typically carry the most harmful side effects to plants. When present as a part of a large population, fungus gnat larvae will feed on the root zone of plants, stunting growth and diminishing overall plant health. Fungus gnats typically lay their eggs in the grow media, and when the larvae emerge, they appear as wormy-like, whitish in color, up to 5-mm long, with a black head. They start off in the top layer of the grow media, and work their way down to the root zone, feeding on organic matter and plant roots. If you see signs of fungus gnats, make sure you’re not overwatering your plants. Gnats thrive in moist conditions. Look out for slime trails on the surface of your grow media, which looks similar to the tracks of common snails and slugs. This could indicate a fungus gnat problem. Although preventive measures tend to work the best to avoid problems with fungus gnats, there are ways to eliminate an infestation. Predatory mites are commonly used to prey on fungus gnat pupae and larvae in the uppermost layers of the soil where these pests most commonly exist. Nematodes are also an effective remedy when applied as a soil drench against fungus gnat populations. Thrips If you notice an abundance of scar-like lesions on the foliage or stems of your plants, you may be encountering a thrip infestation. These critters have an elongated body, typically about 1-mm long, and can range in color from clear white to varying shades of brown, and sometimes even brighter colors such as orange and yellow. Although they are not known for their flying abilities, thrips have a fragile set of wings that extend the length of their bodies, causing them to resemble miniature feathers in their entirety. Many growers who encounter a thrip population in their growroom will only experience minor cosmetic damage. In some cases, the biggest threat is the risk of introducing a plant disease to your crop. One of the most common plant diseases carried by thrips is Tospovirus. Plants experiencing this disease will exhibit an inward curling of leaves, bronzing of foliage and dark spots on the leaves. As the disease progresses, flowers of infected plants may appear deformed, and an increase in the presence of dark streaks will occur. There are several different species of thrips, so it is important to properly identify which one is plaguing your plants. Varying pesticides affect each species differently. Researching the specific thrip species’ natural enemy is a good way to determine which predator should be administered. Generally speaking, predatory thrips and mites or parasitic wasps are effective against a colony of plant-eating thrips. If the population is small enough, removing the thrips by gently shaking the plants in an area far away from the growroom is a good way to prevent the population from growing. Proper identification of insect colonies can be tough for hobby gardeners. A magnifying glass may be necessary to decipher one species from another, and subsequently treating the plant for that specific insect. It is always best to get a second opinion from an expert when the symptoms are questionable, or ubiquitous across multiple types of common pests. For insignificant pest populations, it may be best to start by simply washing or shaking off the insects in a protected area, as some forms of pest management can also have an adverse effect on plant growth. For a serious infestation, it is important to act quickly as the damage can be exponential from one day to the next.