Pesticides used in horticultural cropping systems are generally divided into two categories: conventional and selective (alternative). However, it is often difficult to distinguish between the two categories because, depending on your perspective and bias, a selective pesticide may be considered conventional, and vice versa.
Conventional pesticides generally are those that typically belong, although they’re not limited to, the chemical classes organophosphate, carbamate, pyrethroid and neonicotinoid. But what really is a conventional pesticide, and how can it be distinguished or differ from a selective pesticide?
Additionally, what about terms like “biopesticide,” “biorational” and “reduced-risk pesticide.” Sometimes, these, as well as selective pesticides, are also referred to as “soft” pesticides. All of these terms, particularly “biopesticide,” have been used to separate certain insecticides from the more conventional types.
So what are biopesticides? They are pest control materials that are placed into several distinct classes:
1. Microbial pesticides (or myco-insecticides). This group consists of a microorganism as the active ingredient. Bacterium, fungus, virus, protozoa or related organisms (e.g., spinosad) all qualify. These are highly selective materials with activity against specific target insect or mite pests.
2. Plant-derived pesticides or plant-derived essential oils (botanicals). These are primarily obtained by steam distillation or by other processes from plant leaves, flowers or seeds. Botanicals may include azadirachtin (derived from the neem tree, Azadirachta indica), pyrethrin (derived from the flower of Tanacetum cinerarifolium) and rotenone (derived from both Derris spp., and Lonchocarpus spp.).
Plant-derived pesticides or plant-derived essential oils act as insect growth regulators, work on the central nervous system or have activity on the mitochondria electron transport system. Many types of plant-derived essential oil pesticides exhibit a broad-spectrum activity against insect and mite pests due to multiple modes of action, such as antifeedant, molting and respiration inhibition, growth and fecundity reduction and cuticle disruption. In addition, these materials have been reported to act on the octopamine pathway within the central nervous system.
3. Biochemical pesticides. These naturally occurring substances are designed to control or regulate insect pest populations by non-toxic mechanisms, such as pheromones (e.g., sex, aggregation and alarm), mating disruption, monitoring and lure-and-kill strategies.
4. Plant-incorporated protectants. These are substances plants produce based on genetic material that is incorporated into plants to render them immune or tolerant of insect and mite pests.
Examples of materials that may be classified as biopesticides include Bacillus thuringiensis, avermectins (abamectin), spinosad, azadirachtin, pelargonic acid, salts of fatty acids and mycoinsecticides (e.g., Beauveria bassiana, Metarhizium anisopliae and Paecilomyces fumosoroseus).
Some of the beneficial characteristics of biopesticides include safety to non-target organisms and mammals; short-residual activity; safety to workers/applicators and less directly toxic to natural enemies (e.g., parasitoids and predators). There are, however, a number of issues associated with biopesticides, including short shelf-life, inconsistent field performance, limited pest spectrum, slower acting (speed of kill) and limited persistence (residual activity).
The fact that some biopesticides are naturally derived doesn’t mean they are safe to humans. Although the short-residual activity is considered a benefit, this means multiple applications are required. This may increase selection pressure on pest populations, possibly leading to resistance.
When resistance is mentioned or stated as a potential reason for failure to control or suppress a given insect or mite pest population, it is generally directed toward conventional pesticides. There are numerous scientific-based publications and trade magazines that refer to insect and mite pests developing resistance to conventional pesticides, and that biopesticides (and even selective pesticides) are immune to insect and mite pest populations developing resistance.
This is absolutely unfounded. First, insect and mite pests are unaware of the differences between conventional and biopesticides. The primary purpose of insects and mites (and other organisms) is to develop whatever means necessary to survive and evolve in order to sustain existing populations. In addition to biopesticides, there are the selective (alternative) pesticides, which may include insect growth regulators, insecticidal soaps (e.g., potassium salts of fatty acids), horticultural oils (e.g., petroleum and neem-based), selective feeding blockers and beneficial bacteria and fungi, as well as other microorganisms. These types of pesticides are presented in the Selective Pesticides sidebar (see above).
The Truth About Biopesticides
Generally, it takes relatively longer (although this depends on the frequency of application) for an insect or mite pest population to develop resistance to a selective pesticide or biopesticide. This is because most selective pesticides and biopesticides have broad or non-specific modes of action, which means these compounds are active on either multiple target sites in or on the insect or mite pest body. Or, the active ingredient attacks a variety of enzymatic or metabolic systems.
As already mentioned, a common claim or misperception published in both trade magazines and scientific-based literature is that biopesticides and selective pesticides are “less susceptible” to resistance. This is not entirely untrue, either. The consistent use of any pesticides or pesticides with similar modes of action will likely lead the development of resistant individuals in pest populations. So, regardless of whether a pesticide is considered a conventional or a biopesticide, insect and mite pest populations may develop resistance due to selection pressure. This is associated with the frequency of applying pesticides and dosage used (rate response).
Why Pests Build Resistance
There are a variety of reasons why insect (and mite) pests may develop resistance to selective (alternative) pesticides or biopesticides. For example, buprofezin (e.g., Talus) has demonstrated to be less effective against the greenhouse whitefly, which may be associated with the “high” volatility of the active ingredient resulting in inadvertent selection pressure. Thus, there is an increased probability of resistance. Furthermore, greenhouses, and even nurseries, tend to retain or restrict insect and mite pest populations with minimal migration of susceptible individuals. This results in the population interbreeding and “passing on” traits to its next generation, which will enrich the gene pool with resistant genes. This increases the proportion of resistant individuals in the population.
Still, it is important to note that this varies depending on the time of year and openness of the greenhouses and nursery. In addition, some insect and mite pests may not disperse very far, which means that concentrated populations of resistance individuals are continually present and may be exposed to pesticide applications. And if pesticides with similar modes of action are used and applications are made on a frequent basis, this will, in all likelihood, allow for more resistant individuals in the population – regardless of whether a conventional or biopesticide is involved.
It is critical to be aware of misleading information that insect and/or mite pests cannot develop resistance to certain pesticides – in this case, selective pesticides or biopesticides – because it really doesn’t matter what the pesticide is. Insect and mite pests don’t read entomological literature.
Regardless of the pesticide, always exercise proper pesticide stewardship. What is most useful to greenhouse and nursery producers is understanding the biology and lifecycle of insect and mite pests. This will help in the timing of applications and avoid exposing pest populations to pesticides with similar modes of action.