Thrips, Aphids and Spider Mites…Oh My!

Greenhouse growers nationwide are well versed in thrips, aphids and spider mites. These three pests are the most persistent greenhouse pests across the country. Thrips, Aphids and Spider Mites

“Mealybugs and scale, whiteflies and leafminers are also important,” said Frank A. Hale, Ph.D., professor of Horticultural Crops Entomology, Entomology and Plant Pathology at the University of Tennessee Extension. “Many of the pests found in greenhouses are cosmopolitan and thus found in greenhouses around the world.”

“The big three pest pressures we have here – thrips, mites and aphids – vary geographically by species, but they are ubiquitous greenhouse pests,” said Heather Anthony, Plant Science, instructional support technician at State University of New York at Cobleskill.

An insect’s resistance to insecticide influences how challenging a pest is to control. “The sweet potato whitefly, Bemisia tabaci (Gennadius), has different strains or biotypes. These biotypes, called B biotype and Q biotype, have differing levels of resistance to various insecticides,” Hale said.

Aside from knowing the species specific to your area, it’s essential to consider insects that attack field grown crops and have the potential of migrating indoors. “It is important to know what crops are planted around the greenhouses. Some of the insects that affect outdoor crops could come into the greenhouse,” said Luis Cañas, Ph.D., associate professor, Insect Ecology in Controlled Environments at The Ohio State University.

Thrips, Aphids and Spider Mites

Ornamentals vs. edibles

Ornamental and edible crops often share the same pests. How consumers value the crop helps determine which pests are most troublesome. In ornamental crops, aesthetics is a key selling factor. Customers aren’t in the market for flowers with noticeable leaf damage.

“The plants need to look great, therefore, it is important to prevent even small pest populations,” Cañas said. “Thrips tend to be an important pest in flower crops.”

Edible crops may be able to tolerate a small degree of aesthetic damage, but the vegetables must be free from insect damage. Lettuce, leafy greens or vegetable plants for use in home gardens are an exception. Customers will expect the high quality aesthetics associated with flower crops.

“On edibles, usually aphids are at the top of the list of major problems,” he said.

Read more: Use of silicon for healthier greenhouse plants

Controlling greenhouse pests

Following an integrated pest management (IPM) approach reduces pest pressure. IPM is a set of principles that focuses on long-term prevention of pests or the damage they cause rather than reacting to an insect infestation.

The first step: start clean.

“Make sure new plant shipments are inspected for insects and diseases,” Cañas said.

Ideally, newly arriving plants should be placed in a different greenhouse or in an area of the greenhouse away from other plants to reduce the opportunity for the spread of insects or disease. Starting clean includes separating holdover plants from newly established plants.

“I have been to a greenhouse where hanging baskets held through the winter contained thrips. Those thrips migrated to the (new) bedding plants on the benches beneath and spread impatiens necrotic spot virus to most of the new spring crop. All the flats had to be thrown out,” Hale said.

The next step is sanitation. “Keeping leaf litter out of plant pots, off benches and floors is worth its weight in gold,” Anthony said. “Removing leaf litter not only removes latent pests, (but) it also takes away places where pests like to hang out.”

An IPM approach includes plant fertility. Healthy plants deter pests from getting out of control. Healthy plants are able to bounce back from an early onset of pests. “The industry refers to these small pest outbreaks as hot spots, and they are very easy to see and treat in healthy crops,” she said.

Maintaining the proper ambient conditions in the growing environment supports healthy plant growth. “Light, ventilation, relative humidity and temperature all factor into this,” she said.

Pest scouting routines detect pests before an infestation occurs. “Scouting for pests daily and weekly so that outbreaks are found as early as possible and can be controlled through the least invasive methods, such as pruning or washing off infested plant tissue, spraying horticultural soaps or oils, or applying biological control agents [which] equal beneficial predatory insects,” Anthony said.

Hale recommends yellow sticky traps for monitoring pest populations. The traps are especially helpful for observing thrips or leafminer adults when the population levels are still low.

When insecticides are needed, choose carefully. “It is critical to establish good rotation programs using products from different mode of action groups,” Cañas said.

Biological controls are another preventive tool. Understanding their proper use and how compatible the biological control agent is with other current practices, including insecticides, is critical.

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Emerging pests

Fortunately for greenhouse producers, emerging pest pressures for 2016 are mostly outdoors. “There have not been reports of new invasive species attacking plants in greenhouses,” Cañas said.

Invasive species like the brown marmorated stink bug, found in eastern states, attacks many crops outside, including vegetables. “We don’t know yet if it will affect vegetable crops in a greenhouse, so we have to be on the lookout,” he said.

Hale cautions growers to be aware of the European pepper moth, Duponchelia fovealis (Zeller). “This is a new pest from Europe that greenhouse growers should be on the lookout [for]. This caterpillar can feed on all plant parts. Especially troubling is its direct fruit damage and stem feeding, which can cause the plant to wither or the stem to collapse,” he said.

Although growers can breathe a sigh of relief that there are no new pests to focus on, thrips and aphids continue to be a major problem. Last year in Ohio there were more problems with aphids than in previous years.

“A potential reason is that growers [previously] used insecticides known as neonicotinoids to control aphids,” said Cañas. “These insecticides were very effective at controlling aphids because they had systemic properties, meaning they moved inside the plant and could reach the aphids when they fed on those plants.”

Because neonicotinoids are designed to control insects, there has been concern about their impact on pollinators. As a result many growers have stopped using neonicotinoids and this has coincided with more aphid problems.

Read more: Managing soil to reduce insect pests

Biological controls

Biological control agents (BCAs) have been proven to be effective in reducing pests. However, Anthony said, using BCAs requires knowledge of the pests in your greenhouse regarding their life cycle and crop preference.

You also need to understand how specific BCAs can be used to control a specific pest and how BCAs are affected by pesticides, which includes “organic pesticides.” All these factors determine which BCAs can be compatible for use in a biocontrol program in your greenhouse.

“You have to adhere to very strict sanitation, insect scouting and growing protocols, and even the pesticide applications of things like insect growth regulators, which are often very specific to a particular pest or pests, in order to have a successful biocontrol program,” Anthony said.

Biological controls are for preventative use. They cannot control heavy outbreaks of pests.

“If an insect infestation is found, in my personal opinion, it is too late to start a biological control program,” Cañas said.

To start a biological control program you need to plan ahead, establish contact with the potential suppliers to find out what they can offer. Then, establish a plan of action that calls for periodical releases with the idea of preventing any pest from getting established,” Cañas said.

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