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Heat-Loving Pests

Marissa Schuh - Horticulture IPM Extension Educator, Reviewed by Natalie Hoidal

While the prolonged heat is testing irrigation systems and human endurance, some insects are thriving.  Here’s a rundown of three pests whose populations boom during periods of hot, dry weather.  The pests themselves are small in size and not common troublemakers on many farms, but hot weather allows them to quickly build their populations to damaging levels.  

For all these pests, a cold front or some rain will tamp down their populations, but read on with ways to deal with them as this heat wave continues.


Aphid Basics

Minnesota is home to many species of aphids, a few of which can cause problems in vegetables. We most often see aphids on brassicas and cucurbits.   

Female aphids spend the summer giving live birth to genetic clones of themselves. Photo: MedievalRich, CC BY-SA 3.0, via Wikimedia Commons.

Aphids damage plants in a few ways.  

  • They cause direct damage to plants as they use their straw-like mouth parts to suck juices from the plant.  This can weaken the plant and cause leaf distortion.

  • Aphids secrete a sticky substance called honeydew, which can get on the plant’s leaves and fruit, and grow mold.  This can be hard to remove from things like pumpkins. 

  • Finally, aphids are capable of transmitting viral diseases when they feed on an virus-infected plant then move on to feed on an uninfected plant.  

Extreme aphid feeding can damage leaves, and honeydew can cause sticky molds to collect on leaves and fruit. Photo: Marissa Schuh, University of Minnesota Extension.

Managing Aphids 

Aphids are primarily controlled by predatory insects (ladybugs, minute pirate bugs, syrphid fly larvae, lacewing larvae, and parasitoid wasps).  This means that aphids are often not an issue in systems where broad-spectrum insecticides are avoided.  If aphid populations are in concentrated spots in the field, spot spraying can be a good way of controlling aphids and preserving natural enemies.  This can even be with a hose, as high pressure water blasts aphids off leaves.

If using an insecticide, consider using softer, aphid-specific chemistries early in the season.  Cross reference this list with the Midwest Vegetable Production Guide for products labelled for the crops you are growing.  Preserving natural enemies now means they can patrol your fields for all season long.

A ladybeetle (or ladybug) larvae feeding on an aphid. Photo: Winston Beck, Iowa State University,

For more information, see the Midwest Vegetable Production Guide. For organic growers, New York State Organic IPM guides do a good job laying out options.


Thrips Basics

There are many species of thrips, but the ones we typically see causing issues are small (1/16 inch), yellow, oblong, and quick-moving.  In Minnesota, we most often see thrips in onions, and occasionally cole crops.

Their mouthparts puncture leaves roughly, giving fed-upon leaves a scarred brown or silver appearance.  In onions, damage appears as white or silver areas where leaves meet at the base of the plant.

Thrips pressure is based on both weather and farm location.  

  • Thrips population boom in hot, dry weather.  Heat allows them to reproduce quickly (landing them in this article), and rain knocks thrips of plants, slowing their growth down or killing them.  

  • Farm location and time of season also plays a role.  Thrips feed on whatever is living and juicy in the general landscape, and dry weather and harvest/mowing of neighboring areas can cause thrips to migrate.  This means that thrips populations can spike on vegetable farms when nearby wheat is harvested or when hay is mowed.

Thrips (yellow, long flecks) leaving behind silvery white areas of feeding damage. Whitney Cranshaw, Colorado State University,

Thrips Management

Again, natural enemies are important in managing thrips.  Thrips are also very susceptible to water, be it in the form of rain or overhead irrigation.  Water knocks them off the leaves (or in the case of onions, collects within the bases of all the leaves and drowns the thrips).

Because of thrips's biology, they are resistant to many insecticides.  For more information, see the Midwest Vegetable Production Guide.  Thrips management can be very involved, if you are having consistent thrips issues, please reach out to talk over options.

A minute pirate bug feeding on a thrips. Photo: Whitney Cranshaw, Colorado State University, 

Spider Mites

Spider Mite Basics

The final creature in our heat-loving triad is two-spotted spider mites. Mites are more closely related to spiders than insects, having eight legs and sometimes spinning silk netting around the  areas where they are feeding. They are incredibly small (1/16 inch), and hard to see with the naked eye. Spider mites generally feed on the underside of leaves, puncturing cells and feeding.  This damage can appear as small brown or yellow areas on the leaf, but if feeding is severe the whole leaf can become brown and crispy. 

In Minnesota, we see spider mites watermelons and cucumbers.

Spider mite feeding and webbing on a pepper plant. Photo: Rasbak, CC BY-SA 3.0 <>, via Wikimedia Commons

Managing Spider Mites 

Like thrips, spider mites often flare up after wide-spectrum insecticides are used and have resistance to many pesticides in their populations. To beat this drum again, natural enemies are critical in control, and need to be preserved, especially as it is still early in the season.

For more information on management, see the Midwest Vegetable Production Guide.

Questions about these pests? 

If you have questions about what is going on on your farm or want to talk over management options in your specific cropping system, please reach out.

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