5 of the Most Destructive AG Pests and How to Eradicate Them

The Top Most Unwanted Pests in Horticulture and How to Evict Them From Your Operation 

Face it, we’ve all been there. Crops are looking great, the humidity is perfect, and everything is running smoothly. Then you see it- the quick movement of something small. Yellow spots on leaves, and a plant that seems a bit too small for its place in the growing cycle. Upon closer inspection, you see insects and possible other pests. Although it is true that prevention is the best method of pest and disease control for an Integrated Pest Management plan, pests will still infiltrate even the most "armored" of greenhouse or CEA operation. There are five main threats to the greenhouse- aphids, thrips, whitefly, fungus gnats, and mealybugs. How you respond to the pests, and how quickly will determine the overall impact of these pests on your crops and profits. 

Aphids

Aphids are small, soft bodied insects that peirce plant tissues and drink the sap from them. This can create a dew-like substance on plants that is sticky. Some growers call this honeydew. Aphids can be found on the undersides of  young leaves, stems, and other plant materials in clusters. They prefer the younger, more tender plant material to feed on and can be found there in large clusters. Because aphids are non-flying and very small, they can be easy to miss on a broad inspection of plants. Aphids can quickly overtake a growing area in no time at all. 

Signs of Aphids on Plants 

In small numbers, aphids do not cause much harm to a plant and have very little signs. However, in large enough numbers, aphids can really cause a damper on plants causing a range of symptoms or signs in plants. The signs of a major outbreak of aphids on plants are weakened or disfigured plants, small white flakes on plants and plant material, a sticky dew-like substance on leaves, black sooty mold, and in some cases plant viruses. When a grower sees any of these signs, a close inspection of plant material and the undersides of leaves is a must to see if aphids are the culprit or something else entirely. 

Control of Aphids

The first step in the control of aphids is good housekeeping. It is essential to clear weeds and other plant material from the growing area, as well as clean and sanitize surfaces. When aphids are found in the growing area, the plants need to be contained in that growing area to avoid the spread of aphids. Then a grower has some options. They can go the chemical route or the organic route. Chemical insecticides that have an oil component or are a broad spectrum are effective against aphids. However, depending on the insecticide, it may harm beneficial insects as well. For an organic approach, organic oils or soaps are effective in the control of aphids and other soft body insects. In addition to organic soaps and oils, a grower can also release predatory insects such as ladybugs or ladybirds to control aphids. 

Whitefly 

Whitefly can be a common sight in a greenhouse or garden during warmer months. This small, flying, white insect can be found on a host range of more than 250 ornamental and vegetable plants. Whitefly are similar to aphids in the sense that they also peirce and drink the phloem from plants.

Signs of Whitefly 

Whitefly are more noticeable than aphids, as they fly through plant foliage and slightly above the plant materials. When populations of whitefly get too high, plants start to lose vigor and exhibit stunting, yellowing leaves, and premature dropping of leaves. Whitefly will also coat plants with a sticky dew like aphids causing the appearance of plants to decline. This sticky material will also promote the growth of sooty mold. 

Control of Whitefly

SInce whitefly are flying pests, the use of bright sticky paper throughout the foliage of plants can help in the control of these pests. In addition chemical sprays and organic oil sprays can be used in combination with these sticky traps and preventative techniques. The use of beneficial insects, such as a parasitic wasp in the greenhouse can also control whitefly. 

Thrips 

The most common thrip in greenhouses is the Western flower thrip, Frankliniella occidentalis, and are among the most serious of pests in greenhouses. These tiny insects have spread throughout the industry through plugs, cuttings, and small plants. Due to their small size, they are able to hitch rides in flower buds and other small areas that make detection very difficult until it is too late. Thrips have a broad host range and will feed on flower crops like carnations, chrysanthemums, geraniums, and more. 

Signs of Thrips 

Thrips feed on plants by piercing the tissues and feeding on the phloem of plants. This collapses the plant cells and causes deformities in flowers, leaves and shoots. On closer inspections, silvery flecked scars can be seen on the expanded leaves of plants. Other than the direct physical damage of the plants, thrips can also act as vectors of topoviruses such as Impatiens necrotic spot virus and tomato wilt virus into noninfected plants.  Young plants are especially susceptible to thrips and plant virus spread. 

Control of Thrips 

Thrips are stealthy and can catch rides in small plant areas like buds and new leaves rolls. With their ability to hide so well, infestations can occur quickly even under a watchful eye. The best approach is prevention. Growers should have quarantine areas for new plant material such as cuttings and plugs in order to check for thrips and other pests. If a thrip infestation does occur, immediate removal of infected plants should be done and a complete lockdown of the area needs to be done. Non-chemical ways to combat thrips are to use cold pressured water to wash away thrips, good housekeeping by removing sickly or old plant material, or the use of predatory spiders or beneficial insects. In some cases chemical sprays such as acephate, bifenthrin, imidacloprid, or spinosad can be applied. Quick and effective reaction is needed in the control of thrips. 

Fungus Gnats 

Fungus gnats, also known as Bradysia app., can be an issue in the greenhouse in multiple parts of their life cycles. Gnat larvae can be identified by their white and legless bodies that are roughly 1 ⁄ 4 '' when mature with a shiny black head. When they are an adult, they resemble a mosquito in body shape and are ⅛” in length with long legs, clear wings, and long antenna. These insects can be found lying on media in pots or running across leaves and greenery. 

Fungus gnats are commonly mistaken for another greenhouse pest known as the shore fly, however, shore flies are more robust and have short antennae. 

Signs of Fungus Gnats

Fungus gnats are attracted to wet, damp areas where fungi are best able to thrive. This is because fungus gnats eat the fungi as a major part of their diets. Normally this would be beneficial to plants, but when there is no fungus to be eaten, these gnats will then start to feed on plants. These gnats do not have a preference for plant type and will feed on any plants found in a growing area. The larval stage of fungus gnats cause the most damage as they feed on plant roots. These wounds created by the larvae enable pathogens to enter the plant and kill the plant.  A grower should be on the lookout for sudden wilting in plants and yellowing of leaves, in addition to weak growth and overall vigor of the plant. This is due to injury to the roots. 

Control of Fungus Gnats

The control of fungus gnats starts with good housekeeping. This means clearing out old plant material, removing standing water, and controlling the amounts of water applied to plants to help reduce moist soil over long periods of time. If these methods do not work, the use of yellow sticky bug traps or beneficial nematodes are excellent organic methods of gnat control. Growers can also use carnivorous plants such as sundews in some instances. Most insecticides instantly kill fungus gnats, including pyrethroid-based solutions. Due to the topical application of these, repeated doses will be needed to match the life cycle of fungus gnats.

Mealybugs 

There are several most common mealybugs found in greenhouses across the US. These are the citrus mealybug, longtailed mealybug, the obscure mealybug, and the root-feeding mealybugs. Below is a table outlining the differences between these three species of mealybug. 

There have been other mealybugs that have been introduced into greenhouses, however, the above mentioned are the most common. 

Signs of Mealybugs 

Mealybugs infiltrate greenhouses through infested plant material. It is very important to quarantine new plants and plant material from the general growing areas to reduce the risk of spreading pests and disease. There are tell-tale signs of a mealybug infestation that a grower should keep their eyes open for. Just like the other above-mentioned pests on the top 5 list, mealybugs peirce plant material and drink the sap or phloem from plants. Both the nymph and adult stages of mealybugs do this. This will cause stunting of plants, leaf yellowing, and overall distortion of plant parts like leaves and stems. Mealybugs will secrete a dew like substance on plants which serves as a great growing area for black sooty mold. The presence of which should be a key indicator of a pest issue. This black mold inhibits a plants ability to photosynthesize properly and leads to poor growth. 

Control of Mealybugs

Depending on the stage the mealybugs are detected, control of them may be difficult. This is because of their waxy coating. Traditional pesticides are generally ineffective in the control of adult mealybugs and can only be applied to control nymph stages as they lack the wax coat. In general, the control of small populations of mealybugs or in cases where only a few plants are infested, plants should be removed and destroyed. In higher numbers for operations where plants stay for a longer time, the use of beneficial insects can be employed such as parasitoids. Early detection of mealybugs is key in their control. However, if an outbreak occurs control can still be employed as long as there is patience and a strategic IPM plan is in place. It is also recommended that all plant growing areas between growing seasons be power washed to remove any nymph stage pests.

Morr Pest Control 

Morr has been providing excellent pest control methods backed by the industry for years. For more information about how Morr can help create a rock solid IPM plan, and make product recommendations, be sure to reach out to one of Morr’s knowledgeable staff members today! All staff have industry knowledge and real experience in the commercial industry. This ensures that recommendations can be made that are best for each individual operation as there are no cookie-cutter operations. For access to some amazing chemical or organic solutions to your pests be sure to visit Morr today!

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