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Major arthropod pests of greenhouse crops include aphids, fungus gnats, leafminers, mealybugs, shore flies, spider mites, thrips and whiteflies. These pests have various plant feeding behaviors. Phloem feeders include aphids, mealybugs, soft scales and whiteflies. Xylem feeders include true bugs, spittlebugs and leafhoppers. Leafminers tunnel through plant tissues. Spider mites are chlorophyll feeders. Thrips are mesophyll and epidermal fluid feeders.
Pest biology, life cycle
To deal with these insect and mite pests effectively, it is important to understand their biology and life cycle. There are a number of reasons these pests can be difficult to suppress with pest control materials and biological control agents. These include:
- Cryptic feeding habits (pests reside in areas not accessible to pest control materials and/or biological control agents)
- Variability in life stage susceptibility (eggs and pupae vs. larvae and adults)
- Reproductive capacity (number of offspring produced per female)
- Dispersal ability (movement within and among greenhouse crops)
- Reproduction system (breeding system that promotes resistance)
- Generation turnover (number of generations per year or per cropping cycle)
- Wide host range (feed on many different plant hosts)
Poor control factors
There are a number of factors responsible for poor control of insect and/or mite pest populations. These include:
- Spray timing - Applying pest control materials when susceptible life stages such as larvae and adults are either absent or not predominant.
- Spray coverage - Not applying the spray solution to the leaf undersides where most of the life stages are located.
- pH of spray solution - Depending on the pest control material, if the solution pH is above 7, degradation of the active ingredient may occur.
- Frequency of applications - Not applying pest control materials frequently enough to substantially impact pest populations.
- Migration of insects or mites into greenhouses from outside - Flying insects enter greenhouses from weeds or outside crops.
- Resistance to pesticides - Pest control material no longer kills a sufficient number of individuals in a pest population to prevent damage.
Resistance problems
Resistance is the genetic ability of some individuals in insect or mite pest populations to survive an application or applications of insecticides or miticides. The pest control materials no longer effectively kill a sufficient number of individuals in the pest population.
Resistance develops at the population level and is an inherited trait. The surviving pests can pass traits genetically to their offspring or next generation thereby enriching the gene pool with resistant genes.
The amount of “selection pressure” or frequency of applying pest control materials is the main factor that influences the ability of a pest population to develop resistance to insecticides or miticides. This increases the frequency/proportion of resistant individuals.
Promoting resistance
There are several biological factors that are responsible for promoting resistance in insect and mite populations. These include:
- Short generation time (development from egg to adult)
- High reproductive rate (number of offspring produced per female)
- Haplo-diploid (arrhenotoky) breeding system. Only the male has one set of chromosomes so that any new genetic features arising from mutations will be immediately expressed (fixed). This increases the potential for resistance development.
Resistance can be more problematic in greenhouses because environmental conditions such as temperature, light and relative humidity are conducive for development and reproduction. The constant exposure to insecticides/miticides results in intense selection for resistance. The greenhouse’s environment limits immigration of susceptible individuals resulting in a high proportion of resistant individuals in the pest population.
Insect pests such as western flower thrips that migrate into greenhouses from nearby agricultural fields (e.g., soybean, corn and vegetable) may be exposed to insecticides with the same mode of action. Every time a pest control material is applied “selection pressure” is being placed on an insect and/or mite population. Also, plant material from outside suppliers may be treated with the pest control material or one with a similar mode of action.
Resistance is increasingly becoming a worldwide problem. Expanding international trade of plant material not only spreads arthropod pests, but may also spread resistance genes associated with each pest.
Preventing resistance
The primary and best way to mitigate insect and mite populations from developing resistance is to rotate insecticides and miticides with different modes of action. The mode of action is how a pesticide such as an insecticide or miticide affects the metabolic and physiological processes of an insect or mite pest. Pest control materials should be rotated by common names or active ingredients not trade names.
In general, rotate products with different modes of action:
- Every one to two weeks depending on the time of year.
- Within a single generation apply insecticides or miticides with similar modes of action.
Many pest control materials have site-specific modes of activity which means they are active on a specific target site in the insect or mite pest nervous system. As such, these pesticides are more prone to insect and mite pest populations developing resistance.
The information on pesticide labels can assist growers in practicing resistance management. For example, group 5 mode-of-action pesticide labels instruct users not to make more than two consecutive applications of these insecticides. If additional treatments are required after two consecutive applications of group 5 insecticides, users are instructed to rotate to another class of effective insecticides for at least one application.
Avoiding resistance
Crops should be scouted regularly to appropriately time pest control material applications in order to target the most susceptible life stages (e.g., larvae and adults) of insects and mite pests. Implement proper cultural (watering and fertilization) and sanitation (weed and algae removal) practices. Screen greenhouse openings to prevent insect pests from migrating into greenhouses.
Implement the use of biological control agents and/or natural enemies. Use synergists, which are compounds or molecules that are inactive/non-toxic as a pest control material. However, when these compounds are mixed or applied in combination with insecticides, they either enhance or synergize by increasing the toxicity (activity) of certain insecticides. Use pest control materials with broad modes of activity such as insect growth regulators, insecticidal soaps, horticultural oils, selective feeding blockers and beneficial bacteria and fungi.
Raymond Cloyd is professor, extension specialist in Ornamental Entomology/Integrated Pest Management, Kansas State University, (785) 532-4750; rcloyd@ksu.edu.
This article is adapted from the presentation “Bug-Zilla: problem pests and why they are so difficult to control” at the 2010 Northeast Greenhouse Conference & Expo (www.negreenhouse.org).
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