Insecticides and miticides are still used to alleviate problems with insect and mite pests in greenhouse production systems. However, continual reliance on insecticides and/or miticides can lead to resistance developing in insect and/or mite pest populations. Therefore, greenhouse producers need to understand the mode of action of designated insecticides and miticides to develop rotation programs designed to mitigate/delay resistance. Mode of action is how an insecticide or miticide affects the metabolic and/or physiological processes of an insect or mite pest. The mode of action of all insecticides and miticides can be found in the Insecticide Resistance Action Committee (IRAC) document entitled, “IRAC Mode of Action Classification Scheme,” which is available at irac-online.org
IRAC mode of action groups 9 and 29
This article discusses the IRAC mode of action groups 9 and 29, which are generally referred to as “selective feeding blockers.” The three selective feeding blocker insecticides available for use in greenhouse production systems are: pymetrozine (Endeavor: Syngenta Crop Protection; Greensboro, North Carolina), flonicamid (Aria: FMC Corp., Philadelphia, Pennsylvania), and pyrifluquinazon (Rycar: SePRO Corp.; Carmel, Indiana). Although all three insecticides were originally placed into group 9 (9A — pymetrozine and pyrifluquinazon; and 9C — flonicamid), flonicamid has been moved into group 29 due to differences in binding to specific receptor sites. In general, both groups act on chordotonal (stretch receptor) and sensory organs in the body of insects that are responsible for hearing, motor coordination and gravity perception.
Pymetrozine and pyrifluquinazon (IRAC Group 9) are considered chordotonal organ TRPV channel modulators. These active ingredients disrupt the gating of Nan-lav TRPV (transient receptor potential vanilloid) by binding to channel complexes in chordotonal stretch receptor organs that are essential for sensing and movement. In addition, feeding and other behaviors may be disrupted in target insect pests. Flonicamid (IRAC Group 29) is considered a chorodotonal organ modulator with an unknown target site. The active ingredient inhibits functioning of chordotonal stretch receptor organs that maintain senses (e.g. balance). Flonicamid (Group 29) differs from pymetrozine and pyrifluquinazon (Group 9) in that flonicamid does not bind to the Nan-lav TRPV channel complex.
The basics: selective feeding blockers
In general, selective feeding blockers (or inhibitors) are a group of insecticides that have a broad or physical mode of action that prevents insects from feeding by interfering with neural regulation of plant fluid intake in the mouthparts. These insecticides may modify behavior by inhibiting or disrupting stylet penetration into plant vascular fluids (phloem sieve tubes), which prevents insects from obtaining nutrients. This results in starvation.
Selective feeding blockers are active on certain phloem-feeding insects that are problematic in greenhouse production systems; these include aphids and whiteflies. Selective feeding blockers have activity against the young and adult stages, and they rapidly inhibit feeding. For example, aphids will stop feeding within hours, although they will remain alive for two to four days. In addition, selective feeding blockers may inhibit transmission of viruses that are vectored by aphids. These insecticides have no activity on flies (Diptera), beetles (Coleoptera), or caterpillars (Lepidoptera). Selective feeding blockers have both systemic and translaminar activity (penetrate leaf tissues and form a reservoir of active ingredient in the leaf), and can provide up to three weeks of residual activity. Selective feeding blocker insecticides are less directly and indirectly toxic to bees and natural enemies.
The mode of action of selective feeding blockers is less prone to insects developing resistance in the short term. However, continued use of this mode of action for extended periods of time may eventually reduce the effectiveness of selective feeding blocker insecticides. For instance, there may be issues associated with cross-resistance (based on a single resistance mechanism conferring resistance to insecticides in the same chemical class and/or having similar modes of action) of group 9 insecticides and neonicotinoid (IRAC Group 4A) resistant insects because enzymes such as cytochrome P-450 mono-oxygenases are able to metabolize these insecticides. Therefore, greenhouse producers need to exercise proper stewardship and apply insecticides with different modes of action in between using a selective feeding blocker within a rotation program to avoid any issues associated with resistance.