Don’t let pests prevail

Features - Pest & Disease

Acquiring a basic knowledge of the different categories of insecticides, and how they work, will better equip you to handle pest infestations.

February 24, 2016

In many cases, applying insecticides in the late morning or early afternoon is most effective, as pest populations are more active during those times.
Photo courtesy of Raymond Cloyd

Insecticides are an integral component of most pest management programs associated with greenhouse-grown horticultural crops. Insecticides are relatively inexpensive, easy to apply and effective (in most cases).

Greenhouse producers apply insecticides to maintain insect pest populations below damaging levels or suppress existing insect pest populations so as to avoid any losses associated with marketability or salability due to a reduction in aesthetic value. The types of insecticides that may be used on greenhouse-grown horticultural crops include contact, stomach poison, translaminar and systemic.

Contact insecticides kill insect pests (e.g. aphids, caterpillars, thrips and whiteflies) by direct contact or when an insect pest walks or crawls over a treated surface. The insect pest walks across that treated surface and then insecticide residues enter the body and move to the site of action.

The activity of stomach poison insecticides is affiliated with an insect pest feeding on treated surfaces such as leaves and ingesting the insecticide residues, which are then absorbed through the stomach lining. Insects stop feeding in 24 to 48 hours with death usually occurring within two to four days.

Translaminar insecticides work by penetrating leaf tissues and forming a reservoir of active ingredient within the leaf, which provides residual activity against foliar-feeding insect pests.

Systemic insecticides applied to the growing medium, as either a drench or granule, involve the active ingredient being taken-up by the root system, which is then translocated or distributed throughout the plant. Systemic insecticides are primarily used to prevent outbreaks of phloem-feeding insect pests such as aphids and whiteflies.

The two major categories of insecticides that target insect pests are: broad-spectrum and narrow-spectrum/selective insecticides. Broad-spectrum insecticides are active on a variety of different insect pests, which is helpful when a diversity of horticultural crops is grown in a greenhouse. Narrow-spectrum or selective insecticides are only active on certain types of insects. For example, the product Dipel, which contains the active ingredient, Bacillus thuringiensis subsp. kurstaki, only has activity on caterpillars. Another example is the product Gnatrol, which contains Bacillus thuringiensis subsp. israelensis as the active ingredient, and is only active on fungus gnat larvae.

The effectiveness of an insecticide depends on several factors related to application including: coverage, timing and frequency. Thorough, uniform coverage of all aboveground plant parts, including leaves and stems, is important in suppressing insect pest populations. Both upper and lower leaf surfaces must receive a sufficient volume of spray solution. Be sure to determine the location of pests, and then direct spray applications to those plant parts to achieve maximum coverage; subsequently increasing insecticide effectiveness. Leaf undersides must be sufficiently covered, as this is where the life stages (eggs, larvae/nymphs, pupae and adults) of most insect pests are located.

The use of water-sensitive paper or spray cards may be used to assess spray coverage by quantifying spray droplet distribution and deposition. The strips of water-sensitive paper turn blue when exposed to water droplets. Spray cards can be randomly distributed among a crop and securely attached to plants. This practice should be conducted routinely to determine droplet size and density. Moreover, using spray cards will help to evaluate the performance of the spray equipment and applicator efficiency.

Proper timing of insecticide applications will also maximize effectiveness. Insecticide applications conducted when insect pest populations are extensive results in taking longer to lower the numbers below damaging levels. Therefore, more frequent applications will be required, especially when dealing with multiple age structures or overlapping generations. In addition, insect pests may 1) have developed into life stages, such as eggs or pupae that are tolerant of insecticide applications; 2) already be causing substantial plant damage; or 3) be in locations on plants such as unopened terminal buds or flowers that are difficult to reach with sprays. Always time insecticide applications when insect pest numbers are low. Insecticides should be applied in the early morning or late afternoon, which is generally when most insect pests are active, although activity depends on ambient air temperatures. Applying insecticides when insect pests are less active will result in minimal efficacy (based on mortality).

Most insecticides only kill the young (larvae or nymphs) and adult life stages of insect pests with no direct effect on eggs or pupae. Therefore, repeat applications are warranted in order to kill stages that were initially not susceptible to previous applications such as larvae or nymphs that were in the egg stage, and adults that were in the pupal stage. When dealing with overlapping generations and different age structures simultaneously, repeat applications will be required. In some cases, depending on the target insect pest, two to three applications may be needed when insect pest populations are abundant.

Frequency of application also depends on the season. For instance, during cooler temperatures, the insect life cycle (egg to adult) and the length of development between generations may be extended compared to warmer temperatures, which may result in fewer insecticide applications; however, one common problem is that intervals between spray applications are too long (e.g. >10 days), which leads to insufficient suppression of insect pest populations.

In order to maximize insecticide activity against targeted insect pests, be sure to time applications accordingly, cover all plant parts with the spray solution and conduct frequent enough applications to suppress insect pest populations.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.