Greenhouse Pest Management Is An Internal Issue

Year-round greenhouse pest management can be challenging for small-scale hobby farmers. Here's what to look for and how to keep pests outside.

by Moira McGhee
PHOTO: LedyX/Shutterstock

Pests can wreak havoc in a greenhouse. Literally thousands of insect species feed directly on the tissues of living plants.

The warm, humid conditions and abundant food supply inside a greenhouse provide an ideal environment for pest development on a year-round basis.

Natural enemies that keep these pests under control outdoors aren’t always present in a greenhouse. Pest situations can develop rapidly and become chronic without early detection and immediate eradication.

Insect herbivores injure plants through direct feeding. But some also transmit diseases, which is often more serious.

Insect problems can be especially challenging for small-scale growers who may experience multiple pest problems at the same time. Prevent pests from sucking the life out of your plants with comprehensive year-round greenhouse pest management.

Prevalent Pesky Pests

Depending on what you grow, there are numerous pests that could cause major problems in your greenhouse. However, some common insects to watch out for include aphids, fungus gnats, thrips, whiteflies, cutworms, leafminers and mites.

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The high humidity and moist growing media in greenhouses provide the perfect breeding ground for many of these pests. And all of them can be extremely difficult to eradicate in traditional soil and hydroponic operations.

“Greenhouse pests tend to have a broad dietary niche, which means they’re not particularly picky eaters,” says Jeremy Jubenville with the Michigan State University Extension of Kalamazoo County.

Jubenville serves as the floriculture and greenhouse educator for southwest Michigan. He works primarily with commercial flower growers in seven counties but occasionally works with small, diversified farms; indoor vertical farms; and hydroponic operations in the area.

Common Foes

Depending on the time of year and the crop, some of the most common pests in commercial greenhouses in southwest Michigan are thrips, spider mites, fungus gnats and aphids.

“We occasionally see broad mites on certain plant species. And whiteflies, mealybugs and other scale insects can also be a huge problem in certain crops,” he says.

“Our most problematic pests have been found feeding on hundreds of different plant species. That being said, most generalist herbivores will display a preference when given the choice, so we see some patterns.”

Some of the patterns Jubenville sees in his area, include:

  • Spider mites on tropical foliage plants and grasses
  • Mealybugs and other scales frequently found on citrus and other tropical plants
  • Aphids infesting containerized vegetables, particularly peppers, eggplant and leafy greens
  • Serious whitefly infestations on poinsettias
  • Broad mites infesting various impatiens, begonias and a few others
  • Thrip damage on sensitive plants, including verbena

“It’s important to note that this barely scratches the surface,” Jubenville says. “Each group of pests I mentioned is fully capable of infesting almost every crop you can imagine.”

Seasonality of Pests

Small-scale greenhouse growers in mild climates are used to dealing with pest problems year-round. Growers in areas with four seasons, however, may inadvertently overlook insect infestations in the winter months.

Cooler temperatures don’t necessarily provide you with a reprieve from pests. Monitor for problems throughout the year, and stay prepared to combat pest problems no matter the season.

Daniel Banks, founder of Next Generation IPM (Integrated Pest Management) in Denver, Colorado, states that fungus gnats, onion thrips, hemp russet mites, spider mites, rice root aphids and whiteflies are among the most damaging and hardest-to-control pests in his line of work.

“Although thrips, spider mites, foliar aphids and hemp russet mites tend to become more of an issue during spring and summer, all pests in our area can be issues in colder months, if perpetual production is in place and pest infestations are allowed to roll from one crop to the next,” he says.

A Pest Oasis

Jubenville agrees that all the pests they normally see in southwest Michigan’s warmer months still exist in the colder months.

“A greenhouse is essentially a warm, humid oasis in the middle of the cold, dry winter,” he says.

“Most plant pests can thrive in the greenhouse during colder months as long as it’s warm enough and there is food to eat. Commercial greenhouses are equipped to provide a favorable environment for growing plants.

“Unfortunately, those conditions are also favorable to most insect pests as well. Although most pests won’t thrive in a minimally heated greenhouse, they will survive. So, you may not notice a tiny infestation until March or April when things really start to heat up.”

Jubenville also points out that pest pressure varies from year to year and is different with every greenhouse.

“With pests like aphids, spider mites, whiteflies and broad mites, there may be seasons where you manage to avoid an infestation,” he says. “In other years, it’s possible you may find all of them in your crops, which would be unfortunate.”

Comprehensive Pest Management

Different types of insect herbivores damage roots, chew leaves, suck sap, destroy flowers or devour fruit.

No matter what type of crop you raise in your greenhouse, instigating a comprehensive pest management system is vital to protect your profits. Depending on the size of your greenhouse and what you grow, greenhouse pest management techniques vary to some degree.

However, basic integrated pest management practices are similar no matter how big your greenhouse or what you use it for.

“Good cultural practices are the key to any successful pest management program,” Jubenville says.

“Start with clean plant material in clean greenhouses. When I say clean, I mean scrubbed, sanitized and weed-free. Weeds are underrated reservoirs of insect pests and plant diseases. Keeping your greenhouse weed-free is well worth your time. Plus, your greenhouse will look better.”

Banks adds that controlling the growth of weeds and volunteer crops near the outside of your greenhouse is also important. It prevents movement of pests into the interior of your greenhouse.

“It’s typically easier to control insect and mite pests in greenhouses, because the grower has increased control of the environment,” he says.

Integrated Pest Management

“I really like to utilize an integrated pest management approach. This incorporates cultural practices that suppress pests, mechanical barriers to block pests from entering the facility and environmental modifications that reduce pest and disease development.

“These preventative areas are coupled with an active monitoring program to quickly detect the presence of pests.”

Jubenville’s pest management tips for organic growers and those who want to operate without the use of pesticides are the same as those for conventional growers. He explained that successful pest management is a cycle that begins and ends with a clean house and includes:

  • Sanitation Before you start growing the crop, eliminate weeds, scrub or power wash all surfaces to get rid of dirt and algae, then use a sanitizer.
  • Prevention Avoid bringing pests into the greenhouse. Consider all the ways a pest might enter and address it.
  • Monitoring Weekly Scout for pests.
  • Early Intervention Tamp down an infestation as soon as possible. All the pests mentioned have exponential population growth curves, so one female can turn into thousands in a relatively short period of time.
  • Record Keeping Record when, where and how many pests were found in a spreadsheet. Track population growth and use the data for future planning.
  • Sanitation Sanitize after the crop has been harvested.

Pest Management Minus Insecticides

It’s very difficult to manage pests in a greenhouse without the use of some management products.

“This is especially true if the crop has a high aesthetic value with a low threshold for damage, such as ornamental plants and fresh vegetables,” Jubenville says.

Use insecticidal soaps, horticultural oils and insect pathogens if you want to avoid conventional insecticides. Spray thoroughly for good coverage—the first two require direct contact.

“Biological control also works very well for protected indoor agriculture,” Jubenville says. “We have growers in southwest Michigan that are experts at using biological control agents. Several of them haven’t had to spray an insecticide in years.”

Banks likes to use biological controls preventatively in situations where pest pressure isn’t a current issue, but historical data indicates it may become one again.

However, in situations where an active pest infestation needs combated, he typically employs a hybrid approach that utilizes biological controls as well as organic pesticides.

Prevention Is Preferred

Preventing a pest outbreak is always preferable to curing the problem after infestation and damage have already occurred.

As Jubenville previously stressed, having good sanitation practices is vital to prevention. He also suggests not bringing outdoor plants into your greenhouse without treating them. Patio plants and plants you’ve dug up from the ground are likely to have pests on them.

Also, if you grow in your greenhouse year-round, pests have a consistent food source. You must continuously monitor your plants and treat them as needed to prevent a much larger outbreak when outdoor temperatures rise.

In the spring and summer when pest populations outdoors increase, the propensity for them to make it into your greenhouse goes up. Installation of insect screening on intakes helps. So does limited greenhouse access and ensuring your greenhouse doesn’t open directly to the outdoors.

“Also, use a well-drained media and avoid overwatering,” Banks says.

“Avoid overfertilization, which many insect pests and pathogens thrive on. Maintain environmental parameters appropriate to your crop. Maintain good airflow and adequate plant spacing, and avoid excess humidity. Implement a good scouting program and use preventative biological controls.”

Pest management is an integral part of any greenhouse operation. Without sound cultural practices and an integrated greenhouse pest management program, your crops could fall prey to a wide variety of insect herbivores no matter the season.

Stick to year-round monitoring and consistent sanitation practices to help protect your greenhouse production all year long.

Sidebar: Early Detection

Once most insect species are established on a widespread scale, the hope for eradication diminishes exponentially. Early detection combined with a rapid response is vital in combating pests before they have a stranglehold on your greenhouse production.

“A consistent scouting program is essential, especially in organic and pesticide-free systems,” says Jeremy Jubenville with the Michigan State University Extension of Kalamazoo County.

“I suggest using yellow sticky traps and taping plants gently onto a white surface, such as white paperboard, to see what falls out. You should also pick up plants and examine them. Look at the underside of the leaves. The main idea is to get a representative sample. Don’t feel like you need to pick up every plant. Nobody has time for that.”

Daniel Banks, founder of Next Generation IPM,  agrees that utilizing sticky cards for identification of flying insects works well for early detection.

“Using a scouting approach tailored to your greenhouse that incorporates random leaf pulls and plant inspections is also good,” he says. “Always check historical trouble spots, such as an area close to air intakes or with poor drainage.”

This story originally appeared in the November/December 2019 issue of Hobby Farms magazine.


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