Understanding Greenhouse Pesticides

What is a pesticide and how do you use pesticides in a greenhouse?

Pesticides are any substance that kills, repels, or mitigates a pest. Pesticide active ingredients can be synthetic chemical products, natural chemical products, essential oils, beneficial bacteria, and fungi, or plant growth regulators. The US Environmental Protection Agency (EPA) determines labeling requirements for safe handling of pesticides and acceptable limits (tolerances) for residues left on the products they are applied to.

Greenhouse IPM Tips for Preventing Pests

  • Look for regular opportunities to monitor plants closely like when pruning or transplanting
  • Keep relative humidity below 55%
  • Good airflow from fans prevents fungus and helps plant development
  • Clean clothes, gloves, hairnets, and shoe covers should be worn in the growing area
  • Have an effective integrated pest management program for your greenhouse

Pesticide Product Categories

Federally Registered Pesticides include herbicides, insecticides, rodenticides, antimicrobial products, and biopesticides that are not determined to be a minimum risk to human health. These pesticides undergo formal EPA review, risk assessment, and registration.
List 25(b) Federally Exempt Minimum Risk Pesticides are pesticides that are not required to be federally registered and have been demonstrated safe for their intended uses. Not all 25(b) pesticides are also exempt from EPA tolerance guidelines. 
Exempt Pesticides have no tolerance limit when used on consumable crops. If a 25(b) pesticide is not also tolerance exempt, it is advisable to test plants for residue levels even if not required by the state. It is important to remember that 25(b) pesticides are safe for intended use, but because of the federal legal status of the plant, no product is intended for use with the plant.
Organic Pesticides are those which have been evaluated by the National Organic Standards Board (NOSB). Most natural pesticides meet NOSB standards and most synthetic products are prohibited.

Biofungicides, Biopesticides, and Predaceous Insects

A biopesticide is a naturally occurring pest control substance such as microorganisms, plant-produced natural chemicals, and plants that have been genetically modified for resistance (PIPs). Predatory insects and beneficial soil bacteria work best as a preventative treatment. For treating active issues, natural pesticides and beneficial bacteria usually need to be reapplied frequently, especially when there is new growth that needs to be protected. 
Natural biochemical pesticides are non-toxic substances that interfere with the pest’s lifecycle. They can be scents that disturb mating or attract insects into a trap. Some essential oils like lemongrass, clove, cinnamon, peppermint, and thyme are effective against bacteria and fungi as well. 
Microbial biopesticides are like probiotics for soil and they work very similarly to the good bacteria that defend the human gut from pathogens. Microorganisms that are beneficial to plants can be added to soil to mitigate pathogen establishment. They can defend the plant’s roots from bacteria, viruses, and fungi. The basic concept is that the good microorganisms out-compete the bad ones for space and resources.
Plant-incorporated-protectants (PIPs) are genetically modified organisms, like Bt Corn, that have the pesticidal protein from Bacillus inserted into their own DNA. This allows the plant to produce the same compound that the beneficial bacteria produces.

Pesticide Selection

Because greenhouses present different conditions than outdoor crops, not all pesticides are appropriate for use in the greenhouse. Like herbicides, pesticides should be labeled for use in the greenhouse environment. It is important to choose the right pesticide and the right application method. A particular pesticide should have a label indication for the type of crop it is to be used on.
When selecting a pesticide, you must consider the type of greenhouse pest, the life stage of the pest, the crop age and degree of damage, the risk to other crops in the greenhouse, and the crop value. Applying a greenhouse pesticide can preserve the yield and quality of the crop but it can also present hazards and cause damage if the product is not appropriate for the crop. Greenhouse pesticides come in a variety of ingredients and forms. Many are made for spray application, but powders, granules, and baits are available as well.

Tips for Effective Pesticide Application

  • Be sure to apply pesticides at the right time in the insect’s life cycle
  • Pests can develop resistance to pesticides, so they need to be rotated
  • Rotation of pesticides should include different classes of pesticides and pesticides with different modes of action
  • Always follow the label and instructions for use
  • Store and handle pesticides safely

Mixing Pesticides: pH and Growth Regulators

If a pesticide seems to be ineffective, the case may be that it was not used at the correct strength or frequency, that the pest is resistant to the particular product, the product may have degraded from exposure to heat or light, or it may be improperly mixed. Many of these factors are eliminated with good chemical hygiene like proper storage and ordering products as-needed instead of stockpiling. Improper mixing can be inadvertent for even the most cautious users.

Many pesticide users are unaware that the pH and alkalinity of their water can lower the effectiveness of some types of pesticides, and even deactivate them completely. Even water that has been tested and/or treated to be suitable for irrigation can be unsuitable for use with pesticides and growth regulators. Depending on the type of pesticide, it may work better or worse with differing levels of pH. Few pesticides perform best with neutral to basic water, slightly acidic water is more commonly ideal.

Example: Organophosphates, synthetic pyrethroids, carbamates, chlorinated hydrocarbons, and the growth regulator ethephon among others break down (hydrolyze) in water with a pH over 7. For each increment of pH over 7, they will break down 10x faster. Hydrolysis can happen fast, even instantly in water with a high pH, and also it can happen over time. After mixing, the solution should be used immediately to prevent it from becoming inactive.

Read the labeling on a pesticide carefully to check for guidance on the pH of water that it should be mixed with. If you need to correct the pH of your water, a buffering agent can be used. Most buffering agents will change color as the pH changes. Water should be tested with a pH meter before mixing with pesticides since the pH of water can change depending on season and rainfall. Some buffering agents do not work well with particular pesticides, so check with the manufacturer if that is a concern.

Spray Pesticide Application

Proper greenhouse pesticide and herbicide application are important to ensure that the product is effective and to avoid overuse that can lead to groundwater contamination and product waste. Most commonly, products are applied with a sprayer. Hydraulic sprayers use a pump to mix the product with water that carries it to the spraying target under pressure. Hydraulic sprayers may be booms or handheld “gun” systems. Low-volume sprayers use a water or oil carrier that is injected into an airstream.

he air stream in the pump is created by a fan, blower, or air compressor. Low-volume sprayers create a smaller droplet size than hydraulic sprayers so less product can be used to achieve good coverage. However, these smaller droplets do evaporate more quickly. Most pesticide labels give mixing instructions per 100-gallon dilution. This will differ from the concentration applied per acre or per 10,000 square feet. The ratio has to be adjusted for the additional water introduced through the sprayer. Another precaution to be mindful of is that some sprayers need to be calibrated to ensure proper function. Be sure to calibrate if needed before spraying.

Pesticide Impact Beyond Consumers

Pesticide use is a growing concern for the environment as it is for consumers. Improperly managed agriculture can lead to water depletion and damage to the surrounding environment were significant. Aquatic wildlife, land wildlife, and water sources, and other plant life are threatened by harm from pesticides and herbicides. Risks of pesticide exposure are important to workers as well. Growers must do their part to be sure that they are using safe procedures, mixing and using products correctly, and applying them properly. Safe pesticide and herbicide use is just as important as effective pesticide and herbicide use.

Recordkeeping for Pesticide Use

Keeping records of regular greenhouse pest monitoring and greenhouse pest control measures is an important step in integrated pest management. Records allow trends to be tracked and the effectiveness of control measures can be evaluated. At the end of each growing season, the greenhouse management should look over records to identify trends and evaluate the effectiveness of the treatments, and the effectiveness of their monitoring program as a whole. In addition to personal use, pest control records are also a requirement for compliance with FDA, EPA, and USDA codes for use of restricted pesticides. The USDA publishes a free guidance for what information must be kept in records.

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