What is pesticide resistance?

Pesticides (insecticides, miticides, bactericides and fungicides) are still widely used in greenhouse production systems to alleviate problems with insect and mite pests, and diseases. However, continual reliance on pesticides can promote the development of pesticide resistance. Pesticide resistance is a very important factor that greenhouse producers must take into consideration when dealing with pests (insects, mites and diseases) in greenhouse production systems.

This article is the first in a series of six articles we plan to develop in 2019 and 2020 focusing on pesticide resistance and resistance management for insect and mite pests, and diseases. The first article of the series describes pesticide resistance as it relates to arthropod (insect and mite) pests and plant pathogens, highlighting the differences and similarities.

1. Pesticide resistance: insect and mite pests

Resistance is the genetic ability of some individuals in an arthropod (insect or mite) pest population to survive an application or applications of pesticides (insecticides or miticides). In other words, the pesticide(s) no longer effectively kills a “high” number or percent (>90%) of individuals in the insect and/or mite pest population. Resistance develops at the population level and is an inherited trait. Surviving insect or mite pests can pass traits (genetically) to their offspring (young) or next generation, thus enriching the gene pool with resistant genes. In addition, resistance indicates a change in the genetic composition of an insect or mite pest population in response to selection by a pesticide (insecticide or miticide) over time. “Selection pressure” for resistance increases as application frequency increases, especially when insecticides and miticides with the same mode of action are applied in succession. Moreover, the frequency in which resistant genes occur in a pest population determines the rate that resistance can develop.

So, every time a pesticide (insecticide or miticide) is applied, this places “selection pressure” on insect and/or mite pest populations, which consequently alters the frequency of resistant and susceptible individuals in the population. Insect and mite pest populations can develop resistance to pesticides using different resistance mechanisms. The two most common mechanisms are based on metabolic and physiological resistance. Metabolic resistance involves degradation of the active ingredient by an insect or mite pest. For instance, when a pesticide enters the body, specific enzymes detoxify or convert the active ingredient into a non-toxic form. The active ingredient is then excreted out with other waste products. Physiological resistance occurs when an insect or mite pest modifies or alters the target site of the pesticide, which decreases sensitivity to the active ingredient at the physical point of attachment (lock and key scenario). One form of resistance that can occur among resistant insect and mite pest populations is referred to as “cross resistance.” Cross resistance is based on a single resistance mechanism conferring resistance to pesticides in the same chemical class and/or having similar modes of action. Cross resistance is common among aphid and spider mite populations.

The biological factors responsible for promoting resistance in insect and mite pest populations are listed below:

• Short generation time
• Multiple generations per season or cropping cycle
• High female reproductive capacity
• Broad range of host plants fed upon

2. Resistance management: Insect and mite pests

The primary way to alleviate insect and mite pest populations from developing resistance is to rotate pesticides with different modes of action. The mode of action is how a given pesticide (in this case, insecticide or miticide) affects the metabolic and physiological processes of an insect or mite pest. There are two mode of action types affiliated with insecticides and miticides: narrow and broad spectrum. Narrow spectrum or site-specific mode of action pesticides are active on specific target sites in the central nervous system or enzymes associated with metabolism. Broad spectrum mode of action pesticides are active on a variety of target sites or possess multiple modes of action. The modes of action of insecticides and miticides used in greenhouse production systems are available on the Insecticide Resistance Action Committee (IRAC) website (irac-online.org). In addition, the mode of action of a product is listed on the package as a number (e.g., 5, 6 or 28) or combination number and letter designation (e.g., 4A, 7A or 21A).

3. Pesticide resistance: plant pathogens

Dr. Cloyd’s description of why arthropod pests can become resistant to a product are about the same as the reasons for plant pathogens becoming resistant. Bacteria can have a generation time of merely 20 minutes. Fungal generation times range dramatically with some fungi-making spores (reproducing) in a few days to weeks. In addition, some fungi have both asexual and sexual reproduction — like some insects. In this case of sexual reproduction, the genes are essentially recombined, making the possibility of resistance much more likely.

Some plant pathogens also have a very broad host range, which makes it easier for resistance to develop: Botrytis, some bacteria like Erwinia, some powdery mildews and Rhizoctonia and Pythium causing root rot. Others are narrow, like some leaf spots and Fusarium wilt.

The spread of disease is largely passive for plant pathogens, unlike insects, which can move from site to site. Wind (or fans), handling by workers and water are the most common way some plant pathogens get around our growing areas. Examples of plant pathogens moved by water are Pythium and Phytopthora. Those easily moved by wind or fans are Botrytis, downy mildew, rust and powdery mildew. This method of dispersal means that somebody else’s resistance can become yours.

4. Resistance management: plant pathogens

For plant pathogens the keys to resistance management involve a series of issues. First and foremost, the cause of symptoms must be determined. Using the wrong product for a disease results in no control, which can be confused with resistance. Use of the correct product at the correct rate and interval are the best ways to make sure you do not provide pressure on the fungus or bacterium, resulting in the development of resistance. It is well-known that using lower than a lethal dose of a product targets the weakest or most sensitive individuals and builds the proportion of the population with a high level of resistance until it is the only thing present.

Application of a product too often in a program exposes the fungal or bacterial pathogen to a single mode of action, resulting in resistance development the same as insects and mites.

This is especially true if you are using a narrow mode of action product, which does the same thing to plant pathogens as arthropod pests. Tank-mixing or alternating products with the same mode of action does the same thing. It is always necessary to know the FRAC number to avoid this mistake by alternating numbers in successive applications. Using a product with multiple modes of action, which can come from a single active ingredient or a pre-mix of two or more ai’s, is a good way to avoid resistance development.

The next article in the series will address the history of pesticides in regard to the development of resistance to older and newer pesticides.

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. rcloyd@ksu.edu or 785-532-4750

Dr. A.R. Chase is president of Chase Agricultural Consulting. She has more than 35 years’ experience in research, diagnostics and practical consulting in plant pathology. She has been retired from the University of Florida – Mid Florida Research and Education Center in Apopka since 1994, but remains on staff as a professor emeritus. archase@chaseresearch.net or 928-649-0400