Some growers are interested in combining natural enemies — biological control agents, such as parasitoids and predators — with pesticides, which in this case include insecticides and miticides. The goal is to regulate arthropod (insect and mite) pest populations and avoid or minimize plant damage. Combining the two methods may also lead to a reduction in pesticide applications and, thus, fewer problems associated with pesticide resistance. It is important to understand, however, that some pesticides may have direct or indirect effects on natural enemies after exposure.
Pesticides And Natural Enemies
The impact of pesticides is not solely based on direct effects like mortality. It is also not uniquely based on contact exposure or odors that influence behavior, which affect the ability of natural enemies to locate prey (hosts) on plants.
Pesticides may have indirect effects, sometimes referred to as sub-lethal or chronic effects. These effects consist of inhibiting host foraging ability, reproduction, development time, mobility and longevity. Furthermore, parasitism/predation rates and prey consumption may be inhibited, significantly reducing the performance of natural enemies. The susceptibility of natural enemies to pesticides varies depending on the life stage (eggs, larvae, nymphs or adults) that was exposed.
Pesticides may, in general, alter the fitness of natural enemies leading to long-term effects on populations. Altered fitness would influence the natural enemies’ effectiveness in regulating arthropod pest populations. There are a number of biological and behavioral factors pesticides may indirectly affect.
Reproduction: The number of eggs laid by females, duration of the oviposition period and egg survival may be negatively affected. Any indirect effects on reproduction may lead to outbreaks of arthropod pests.
Longevity: Both immature and adult longevity may be severely impacted, which could result in increased duration of the life stages.
Development Rate: Development may be prolonged or reduced. In either case, this may interfere with synchronization of the natural enemy with the designated prey. It could also influence the number of prey present.
Sex Ratio: One sex (male or female) may predominate, or the sex ratio may be altered such that there is an excess of one sex. The latter would reduce the number of progeny (young) in the next generation.
Behavior: Some pesticides — especially the pyrethroids — may have repellent activity that alters or disrupts the ability of natural enemies to locate prey. This allows the prey to escape, attack and potentially continue damaging plants. Additionally, due to the mobility of natural enemies (adult parasitoids and adult and immature predators) this may actually lead to increased exposure to pesticide residues. The increased exposure could then decrease movement or flight activity.
Furthermore, these residues may be harmful if consumed during grooming or when feeding on contaminated prey. This may alter prey quality and reduce acceptability of prey to parasitoids or predators. Any exposure to pesticide residues may in some form reduce foraging behavior, which decreases the ability of natural enemies to locate prey especially those that are distributed in patches among a crop (i.e. spider mites). This decreases the frequency of encountering prey, resulting in a reduction in the number of prey parasitized (for parasitoids) or consumed (for predators).
Mobility: Pesticides may influence mobility by reducing the number of encounters with prey. This negatively affects foraging ability and results in decreased mortality of insect or mite pests.
Indirect Effects Of Systemic Insecticides On Natural Enemies
There are unsubstantiated claims that systemic insecticides and natural enemies are compatible. Systemic insecticides, especially when applied to soil or growing medium as granules or drenches, and any metabolites are generally considered to be safe or non-toxic to natural enemies. This is due to minimal direct exposure. In some cases, however, there may be indirect effects, and depending on the feeding habits of predators and parasitoids, the two may be incompatible.
Predators may be indirectly affected when they feed on plant tissue as a supplemental food source. For example, predatory bugs such as Orius spp. may feed directly on plant tissues as a supplemental food source during certain stages of development, potentially ingesting lethal concentrations of systemic insecticides. Other bugs, like ladybird beetles, feed only on prey, so they are less likely to be indirectly affected by systemic insecticides.
Parasitoids may be indirectly harmed by plants treated with systemic insecticides as a result of feeding on extra-floral nectaries (sugars and amino acids) containing the active ingredient. Behavior and survivorship may be influenced by systemic insecticides when the active ingredient is translocated into pollen or nectar, indirectly affecting parasitoids during the feeding process.
Any odors or volatiles associated with some insecticides (especially those with extended residual activity) may invoke an avoidance behavior in natural enemies. This impacts their effectiveness as biological control agents. The duration of any indirect effects may be influenced by the systemic insecticide and persistence or residual activity. Furthermore, the active ingredient may not actually be responsible for any effects, but any metabolites produced during the conversion process may be indirectly harmful to natural enemies.
Systemics And Leaf Guttation
The active ingredient of a systemic insecticide may move through the leaf edge to the outside leaf surface through guttation. Guttation is a natural occurrence in turgid plants in which liquid droplets associated with xylem (water-conducting tissue) fluids accumulate at the margins of leaves or leaf tips. This occurs when the fluids are forced out of stomata-like pores or hydathodes, which are located in the epidermis, after exposure to moist growing medium and high relative humidity.
The droplets that form when exposed to low growing medium moisture content and dry ambient air conditions tend to contain more concentrated solutes due to excessive water evaporation. This phenomenon may occur in the early morning or evening. Guttation fluid may contain a variety of mineral elements and organic compounds. The quantity exuded varies from several droplets per leaf to many milliliters. The composition of the liquid may also vary from nearly pure water to dilute solutions containing organic and inorganic solutes and minerals. Any residues may be left by evaporation of guttation fluids.
Guttation fluids may contain concentrations of the active ingredient of systemic insecticides, which are subject to volatilization, leaving residues that may be toxic to natural enemies, including parasitoids and predators. Guttation is a means by which systemic insecticides move from the inside to the outer leaf surface. Any evaporation of water from the guttation fluids may leave deposits on the tip of the leaf surface, which could impact natural enemies indirectly during the grooming process. The translocation and accumulation of insecticides in guttation fluid is clearly evident, although the impact on natural enemies requires further investigation. Moreover, any toxic metabolites may be excreted from leaves.
The toxicity of guttation fluids may be associated with the water solubility of the systemic insecticide and concentration present in guttation fluids, which is related to the application rate. For example, parasitoids may drink from guttation fluids and inadvertently ingest some of the active ingredient; however, the effects of this ingestion are unknown at this time. Studies with bees, which have intense drinking behaviors, have shown that they can be killed or indirectly affected after consuming guttation droplets from plants treated with systemic insecticides.
This overview of pesticide compatibility with natural enemies addresses the complex nature of this interaction. Therefore, when attempting to integrate natural enemies with insecticides, it is important to be aware of any potential indirect effects, particularly those associated with systemic insecticides. In some cases, using systemic insecticides may not be compatible with natural enemies.