Cool-Season Insect Control

Whiteflies  Photo by Robin Siktberg

AS growers ramp up production in late winter and into the early spring, they are often caught off-guard when they discover insects or mites feeding on the crops very early in the growing cycle. It can be very frustrating for growers to have to deal with insect pests this early in the season. Although the amount of insect activity and crop injury is usually minimal this time of the year (particularly when the production temperatures are low), if undetected, the populations of these pests can gradually increase and lead to significant crop injury later in the production cycle as temperatures rise.

The presence of insects and mites in the early spring occurs frequently with perennials and woody ornamentals that have been overwintered, but they commonly occur in other crops under cool growing conditions, as well. In some instances, there are dormant life stages and eggs that overwinter on plant debris, weeds and “pet plants” carried over from the previous growing season. In other cases, new populations are introduced into greenhouses from starting materials obtained from outside sources.

Temperature Affects Insect Development And Activity

Many insect pests and mites are capable of surviving under low temperatures and others have “dormant” immatures and/or eggs which can survive during cold periods. Insects and mites are essentially cold-blooded, and their metabolism and activity are greatly influenced by environmental temperatures. Up to a certain point, as the temperatures increase, so does their rate of development and activity. Conversely, as temperatures decrease, insect and mites develop more slowly and are much less active.

In general, most insects and mites do not become active until temperatures reach approximately 50°F. Although insect and mite activity will occur at cool temperatures, their rate of development and the amount of activity (movement and feeding) is relatively slow compared to what is observed under more normal growing conditions. As the temperatures warm up, insects and mites are more active, feed more and eventually become reproductive. Aphids are the exception and are capable of reproducing at temperatures less than 50°F.

Controlling Pests Under Cool Conditions

When considering if control measures are necessary under cool production temperatures, keep in mind that many insecticides and miticides are only effective when they come into direct contact with the pest. Additionally, systemic pesticides only move up the plant while the plants are actively growing (plant growth is also greatly reduced with low temperatures).

In many instances, control strategies are relatively ineffective under cool growing conditions because the temperatures are too cool to promote uptake of systemic products. Contact pesticides often fail since the pests are not very active, limiting the likelihood of them coming into contact with these products.

Under normal growing temperatures, contact insecticides and miticides are most effective when they are applied during the coolest parts of the day (mornings and evenings), as this is when insects and mites are generally most active and have the greatest chance of coming into direct contact with the pest control products. However, under cool growing conditions, it is best to apply insecticides and miticides during the warmest time of the day when the pests are most active, increasing the likelihood they will come into direct contact with the active ingredients of these products.

Many growers are using biological predators to control several of their insects pests and mites. Unfortunately, these natural enemies are also less active and do not actively seek out their prey as well under cool temperatures. However, as the temperatures rise, their level of activity (hunting and feeding) also increases and closely mimics the amount of activity of the pests you are trying to control. Although they are not as active under cool growing conditions, the presence of biological predators can be helpful for controlling certain pests when the temperatures do increase and the insects and mites you would like to control become more active.

Increasing The Effectiveness of Spray Applications

Since controlling pests can be challenging under cool growing conditions, it is helpful to take steps to maximize the effectiveness of any control strategies that are implemented.

The objective of spray applications of insecticides or miticides is to provide a uniform distribution and deposition of active ingredient throughout the target area which will effectively control the intended insect or mite. Below are several guidelines that will help growers achieve optimal efficacy from these applications.

1. Apply pesticides that are effective at controlling the desired pest(s). It is best to properly identify the type of insects or mites that are present before making applications to control them.

2. Make applications to interrupt the most vulnerable pest life stages. Many products do not control all life stages. Choose products that are effective at controlling the life stages of the pests that are currently on the crops.

3. Ensure consistent, thorough and uniform coverage. Deliver chemicals to plants in such a manner that the spray solution is applied to both the upper and lower leaf surfaces uniformly. Leaves and stems that receive inadequate coverage are not protected as well as where adequate coverage is delivered.

4. Use an appropriate spray volume to improve coverage and the effectiveness of pesticides. Growers should know in advance how much spray volume needs to be applied to any given area before the applications are made. They should also know the output [gallons per minute (gpm)], the application pressure [pounds per square inch (psi)], and the droplet sizes [microns] of the spray equipment being used. This information is helpful for growers to determine how much spray solution needs to be applied and the approximate time needed to make the applications.


One of the most important aspects of crop production is to start the growing season with few or preferably no pests in the greenhouses and to maintain this pest-free status throughout production. Production facilities should be thoroughly cleaned several weeks before new crops are grown. Be sure to remove all leftover plant debris, organic material, weeds, and stray plants, which can serve as a source of insect pests that may infect future crops.

After the greenhouses are cleaned and free of all weeds and debris, a great deal of potential problems can be kept out of the production area by inspecting incoming plant materials. All new plant materials, such as unrooted cuttings, plugs, liners and newly transplanted containers, should be free of any insects or mites prior to entering your production facilities.

These preventative strategies combined with the guidelines discussed above will go a long way toward minimizing crop injury from insect pests and mites during the cool late winter and early spring production cycles. Additionally, having fewer pest problems early in the growing season will help reduce the potential of these pests from causing crop injury during the warmer growing periods ahead.  GG

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