Search for a Narrow-range, Minimum-risk Insecticide for Sugarcane Borer Control

Linda Benedict  |  10/17/2016 8:24:19 PM

T. Eugene “Gene” Reagan and Kenneth Gravois

In 2014, more than 80 percent of all the tebufenozide (Confirm) insect growth regulator insecticide produced in the U.S. was applied to Louisiana sugarcane for control of the sugarcane borer. Use of this insecticide, which is considered narrow-range and minimum-risk, came about through an intensive evaluation by LSU AgCenter researchers. During the last half of the previous century, stalk borer control and use of insecticides in U.S. sugarcane led to many environmental concerns with fish and bird kills, numerous effects on nontarget and beneficial arthropods, as well as pesticide resistance. The review and termination of many effective insecticides for the sugarcane industry led researchers at the LSU AgCenter to search the world over for what turned out to be a new class of narrow-range, minimum-risk insecticides called the MACs or molting accelerator compounds. These insecticides affect larval growth of some Lepidopterous insects, particularly some in the Pyralid and Crambid groups. Tebufenozide (RH-5992) was later labeled by Rohm and Haas Company under the trade name Confirm and received the Environmental Protection Agency Green Chemistry Challenge Award in 1998.

Since the first evaluation of tebufenozide for sugarcane borer control in 1993, it became obvious to the AgCenter research team that the chemical was different from most other insecticides in how it worked. Most insecticides work by rapidly killing insects within a few hours after exposure. However, tebufenozide was shown to act only during the molt, which is when insects shed their exterior covering during growth. Larvae exposed soon after the molt showed no chemical effects for several days until after the next molt. In addition, from the many insecticide studies in 1994 and 1995 (many chemicals often causing significant two- or three-fold reductions of nontarget and beneficial insects), the only insecticide studies not suppressing nontarget crickets, beneficial spiders, and other selected arthropods was tebufenozide. Additionally, in classical biological control studies with borer parasitoids exposed to sugarcane leaves taken immediately from insecticide-treated, replicated field plots, tebufenozide was the only chemical that did not kill parasitoids.

The goal was to find an insecticide that would kill only the sugarcane borer without harming nontarget or beneficial organisms. For that reason, researchers pushed for the tebufenozide label in contrast to methoxyfenozide (RH 2485) Intrepid insecticide, which was toxic to many other insects as well as the sugarcane borer, and now has a few more labels. In addition, researchers expected the field toxicity rate would have to be higher for Intrepid with the sugarcane borer. Tebufenozide also is labeled and used for control of the codling moth and leaf miner in pome and stone fruits, but these and other uses are in great distances from sugarcane insect control.

Interactions of host plant resistance and insecticide use differ substantially depending on whether tebufenozide is used for sugarcane borer control or Mexican rice borer control in sugarcane. With the sugarcane borer, resistance of cultivars is based on early rind hardness (at the time early stage larvae enter the stalk) as well as leaf sheath appression (how tight the leaf sheath is to the stalk) together with several other factors. However, plant features involving Mexican rice borer-resistant sugarcane cultivars relate very much to the period of time the insect pest is exposed to tebufenozide or other insecticides on the surface of the stalk. Cultivars that allow the Mexican rice borer to enter the sugarcane plant more rapidly than other cultivars are more susceptible because they protect larvae from insecticide exposure (inside the stalk) as well as from some arthropod predators and parasitoids unable to maneuver past Mexican rice borer excrement that is packed in the tunnels. Tunnels of the sugarcane borer are far more open, allowing entrance of arthropod predators and parasitoids.

Insecticide use in the Louisiana sugarcane industry averaged approximately three annual applications before tebufenozide was introduced in 1993 to substantially less than one half of an application today. Much of that reduction is due not only to efficacy of tebufenozide, but also to a major enhancement of biological control involving tebufenozide conservation of beneficial arthropods throughout the sugarcane industry.

Low levels of insecticide resistance with tebufenozide have been detected in Louisiana. However, substantially less insecticide use for control of the sugarcane borer has led to reduced tebufenozide selection pressure. Additionally, because of pesticide resistance concerns, alternating tebufenozide with insecticides with other modes of action is recommended. Tebufenozide is at the center of integrated pest management control of the sugarcane borer in sugarcane in Louisiana. Without the availability of tebufenozide for Louisiana sugarcane growers, insecticide use will increase and sugarcane yields will decrease. Tebufenozide remains by far the most important insecticide for sugarcane borer control in Louisiana sugarcane.

T. Eugene “Gene” Reagan is the Austin C. Thompson Distinguished Professor of Entomology, Ecology and Pest Management and extension IPM coordinator. Kenneth Gravois is the Graugnard Brothers Professor and sugarcane extension specialist.

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T. Eugene “Gene” Reagan, entomology professor, and his research team received the Environmental Protection Agency Green Chemistry Challenge Award in 1998. Photo by Olivia McClure

History of Insecticide Use in Sugarcane for Sugarcane Borer Control

In the late 1950s, the organochlorine endrin was used. It killed everything in the field – borers, ants, spiders, snakes, birds and fish (with runoff). Because of fire ant suppression, borer problems increased, and insecticide resistance became a problem.

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Guthion and Azodrin (organophosphates) replaced endrin but caused problems with fish and bird kills. In 1992, the Environmental Protection Agency had a meeting in the LSU AgCenter over the 1990 and 1991 fish kills resulting immediately in severe restrictions (prescription use) on Guthion use in sugarcane, and finally, loss of the label three years later. Carbamates including Sevin were also labeled and used. Pydrin and other pyrethroids were then used, substantially impacting numerous nontarget arthropods and some beneficials. The only insecticides labeled for foliar spray in rice today are pyrethroids, and Louisiana rice producers still have no economic thresholds for stem borers.

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When these other insecticides for the Louisiana Sugarcane industry were restricted or taken away by EPA, LSU AgCenter scientists began collaborating with the Rohm and Hass Company research laboratory in 1992 at Philadelphia. It is interesting that with one of their first studies with tebufenozide, Rohm and Hass scientists had put the larvae on a back bench to discard, but they forgot to throw them out when it did not work. Then when the scientists returned several days later to discard the larvae, all of the anticipated LD-90 larvae were dead because they had molted again. That began the start of the environmental award for tebufenozide.

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