Mode of Action: Insecticides

The final installment of the mode of action (MOA) series focuses on insecticides that target energy production in the cell, biological insecticides, feeding inhibitors, soaps and oils.

Most of the insecticide modes of action covered in this column are not as straightforward as the classes previously discussed in the May and July issues. To keep from sounding like a cell biology class, the MOA discussions will be brief.

Selective feeding blockers
Pymetrozine (Endeavor) and flonicamid (Aria) are both selective feeding blockers (MOA group 9 A & B respectively). Insects exposed to these insecticides stop feeding. It may take a few days for the pests to die, but they are no longer feeding and damaging the plant. These two products are directed at different target sites so they can be used in the same rotation program.

Microbial disruptors
Microbial insecticides containing Bacillus thuringiensis (Bt) toxins are classified as microbial disruptors of the insect midgut membranes (MOA group 11). There are two Bt strains registered for ornamental crops. Bacillus thuringiensis var. israelensis (Gnatrol) is effective against fungus gnats and Bt var. kurstaki (DiPel) is effective against leaf feeding caterpillars. Bt occurs naturally in the soil, and these bacteria produce protein crystals that are toxic to some insect groups. The protein destroys the insect’s gut, which causes the insect to stop feeding and allows bacteria to infect the insect. Death usually occurs within a few days.

Lipid biosynthesis inhibitors
Spiromesifen (Judo) is a tetronic acid (MOA Group 23). Spiromesifen inhibits lipid biosynthesis, which prevents insects from maintaining proper water balance. This causes the treated insects and mites to dry up and die. Spirotetramat (Kontos) is also a member of this class. Spirotetramat applications result in decreased lipid contents, growth inhibition of younger insects and reduced ability of adult insects to reproduce.

Impacting energy production
There are four groups of insecticides (Groups 12, 13, 20 and 21) that impact energy production in the insect’s cell. Energy, or adenosine triphosphate (ATP) is broken down when energy is needed. Without ATP cells, insects do not have energy to function.

Organotins (MOA Group 12) inhibit mitochondrial ATP synthase. In simpler terms they disrupt ATP formation in the mitochondria so no energy is produced. Fenbutain-oxide (ProMite) is the only organotin registered for ornamental production.

It is important to understand how insecticides work so you can develop an effective resistant management program.Pyrroles (MOA Group 13), like organotins, target the mitochondria. Pyrroles disrupt the production of ATP. This causes cell death, and ultimately the pest dies. Chlorfenapyr (Pylon) is the only representative of this group registered for ornamentals.

Acequinocyl (Shuttle O) (MOA Group 20A) and hydramethylnon (Amdro Pro) (MOA Group 20B) target energy production in the Krebs cycle. The Krebs cycle is one of the processes during cellular respiration where the mitochondria makes ATP.

MOA Group 21A consists of pyridaben (Sanmite), which belongs to the pyridazinone class, and fenpyroximate (Akari) which is in the phenoxypyrazole class. Insecticides and miticides in Group 21A inhibit mitochondrial electron transport. Since no ATP is produced, the cells run out of energy.

Miscellaneous insecticides
There are a number of insecticides that are not placed into a specific insecticide group since they do not target a specific site. These include insecticidal soap, oils, and insect pathogens.

“Oil” is a vague term referring to a number of different types of products. Oils can be distilled from petroleum (horticultural or mineral oils) or extracted from plants and animals. When applied to the plant, a thin film of oil covers the target pest and the insect dies by suffocation.

Oils and insecticidal soap must be sprayed onto its target pest. The fatty acids in the soap disrupt the structure and permeability of the insect cell membranes. This causes the contents of the cells to leak and the insect quickly dies. Soaps may also remove the protective waxes that cover some insects, causing death through excess loss of water.

Soaps and oils are generally effective against soft-bodied insects such as aphids, scale crawlers, mealybugs, spider mites and whiteflies. Small larvae of some caterpillars and sawflies may also be controlled. Dormant oil sprays are also used against over-wintering eggs and scales.

Attacking fungi
Fungi that attack insects are known as entomopathogenic fungi. Beaveria bassiania (Naturalis-O and BotaniGuard) is the most widely use entomopathogenic fungi used in the ornamental industry. But there are new pathogens entering the ornamental market including Paecilomyces fumorosoroseus (PFR-97) and Metarhizium anisopliae (Met52). These products contain fungal spores. The spores adhere to the host, germinate and produce enzymes that attack and dissolve the cuticle. This allows the fungus to penetrate the cuticle and grow into the insect’s body. The fungus will eventually overtake the insect body. Many insects will change color as the fungus grows in their bodies. By the time the fungus is visible on the outside of the body, the insect is dead.

Unknown MOA
There are a couple of products that scientists have not been able to determine their exact mode of action. These include pyridalyl (Overture) and bifenazate (Flora-mite). Most likely over time these products will be placed into new classes.