The use of pesticides (insecticides, miticides, bactericides and fungicides) is the primary means of managing insect and mite pests and diseases in greenhouse production systems because they are easy and convenient to apply and effective (in terms of mortality of insect and/or mite pests). However, the continual reliance on pesticides promotes the development of pesticide resistance in insect/mite pest populations.
This article, the second in a series of six, compares and contrasts old pesticides and newer pesticides, and discusses how they influence the potential for insect and mite pests, as well as plant pathogens, to develop resistance.
Insecticides and miticides
Application of insecticides and miticides, in general, has been the primary means of managing insect and mite pest populations in greenhouse production systems for many years. In general, the insect and mite pests have not really changed, only the insecticides and miticides used to manage them have. Prior to the 1990s, greenhouse producers could expect two to three new active ingredients to be introduced for use in greenhouse production systems each year. In addition, there were fewer issues associated with registering pesticides than there are now. Consequently, greenhouse producers had a number of pesticides that were broad-spectrum in activity by killing nearly all insect and/or mite pests encountered during crop production and had long-residual activity. Because of that, fewer applications were required, which may have reduced selection pressure on insect and mite pest populations.
All of these pesticides have been discontinued or withdrawn for use in greenhouse production systems due to new federal and state laws and regulations enacted by the Environmental Protection Agency. However, some are still commercially available for use in greenhouses including acephate and methiocarb. Two major issues associated with these ‘older’ pesticides was that almost all were (and are) highly toxic to humans and wildlife, based on low LD50 values, and they were a hazard to the environment (e.g. ground water contamination).
The trend associated with increased pesticide regulations continues today. In addition, the costs (approximately $300 million dollars) and time investment (seven to 10 years) in developing and registering new active ingredients has resulted in fewer new active ingredients entering the marketplace for use in greenhouse production systems. As a result of the stringent regulations, ‘newer’ registered pesticides are more narrow-spectrum regarding target insect and mite pest activity than previous pesticides. Consequently, these ‘newer’ pesticides, oftentimes referred to as ‘selective,’ are less toxic to humans, based on high LD50 values, and have less impact on the environment due to lower residual activity compared to ‘older’ pesticides. Examples of these pesticides include: bifenazate (Floramite), cyantraniliprole (Mainspring), flonicamid (Aria), pyridalyl (Overture), pyrifluquinazon (Rycar) and pymetrozine (Endeavor).
However, pesticides with less residual activity or persistence need to be applied more frequently, which can place selection pressure on insect and mite pest populations; which leads to the development of resistance.
For instance, many western flower thrips, populations have developed resistance to spinosad (Conserve) and certain leafminer and twospotted spider mite, populations have developed resistance to abamectin (Avid) due to continual reliance or overuse of these pesticides. Consequently, these pesticides are used less often than before due to their reduced efficacy. In addition, they are included into rotation programs that involve using one mode of action within one generation before switching to another mode of action in the subsequent generation.
The excessive use of neonicotinoid systemic insecticides, before the issue affiliated with their supposed impact on pollinators, increased the likelihood of resistance developing as was shown by the Q-biotype of the sweet potato whitefly. The Q-biotype developed resistance to imidacloprid (Marathon), thiamethoxam (Flagship) and acetamiprid (TriStar). In addition, the Q-biotype developed resistance to two insect growth regulators: buprofezin (Talus) and pyriproxyfen (Distance). This demonstrates the need to avoid relying on or over-using the newer pesticides with similar modes of action.
Bactericides and fungicides
The use of modern complex chemistries started in the late 1960s with the introduction of systemic fungicides, including products that we now know to be carcinogens or unacceptably dangerous.
I started working in ornamental horticulture in the mid-1970s when I began graduate school and the products commercially available for control of diseases were limited. One of the first products I worked on at the University of Florida in 1980 was chlorothalonil (Daconil and others). Dithiocarbamates (FRAC M3 — Dithane and Protect) and benomyl (Benlate) were already mainstays used in the production of many crops. In the next 14 years, a wide variety of other fungicides were introduced with many still in use today. This was the same time that metalaxyl (Subdue) and fosetyl-Al (Aliette) were under development and their use in ornamentals was increasing.
Newer classes of conventional fungicides including SDHI’s [FRAC 7 — flutolanil (Prostar)] and DMI’s (FRAC 3 — triademifon (Strike)] were being evaluated for use in ornamental crops. In the early 1990s, strobilurins (FRAC 11) were being tested and registered on ornamentals, including: kresoxim methyl (Cygnus, which is no longer available), azoxystrobin (Heritage) and trifloxystrobin (Compass). Simultaneously, there was the introduction of FRAC 12 (fludioxonil). The movement away from using very broad-spectrum fungicides to utilizing more directed fungicides with minimal environmental side effects led to the introduction of many new products.
In the past 10 years, we have seen a plethora of products being registered in the ornamental market from a multitude of FRAC 3, FRAC 7 and FRAC 11 groupings (as well as many premixes of 7-11), as well as, the introduction of new FRAC groups. For example, FRAC 49 has a single active ingredient (OXTP — Segovis) with one of the longest residual activities of any fungicide.
By the mid-1970s, resistance in bacterial pathogens (especially Pseudomonas and Xanthomonas) to antibiotics (streptomycin sulfate) and copper were well-known in many crops, including ornamentals. In the early 1990s, we started observing resistance development issues in certain species of Pythium, especially to metalaxyl (Subdue), as well as, a number of fungicides used for Botrytis (including thiophanate methyl and to some degree iprodione). Thus, the development of new products for these key pathogens was a major focus.
The increased registration of biopesticides (many with organic certification) in the past few years is accelerating rapidly. Biopesticides are often associated with Bacillus spp. (Cease, Stargus and Triathlon), Trichoderma spp. (Obtego and RootShield) and a few novel organisms like Ulocladium (BotryStop) with newer formulations having extended shelf lives and effectiveness when used according to label directions. Copper products now include a number with organic certification (Camelot O, Kalmo and Nordox), as well as Kocide 3000 and Phyton 27. In addition, plant extracts (Ecoswing, Regalia and Regime) are being registered and marketed, in addition to true biologicals. At this point, greenhouse producers can choose to use organic products for some crops and some diseases. However, wide-scale use of biopesticides is still in its infancy for disease control. A truly integrated approach using conventional products and biopesticides simultaneously is the most common approach used by greenhouse producers to manage plant diseases.
The next article in the series will discuss the insect and mite pests and plant pathogens that are prone to develop resistance to pesticides and the reasons why.