Carrot weevil control

Carrot weevil remains the most important insect pest to many Ontario carrot growers, and the amount of damage seems to be increasing in recent years. Carrot weevil females overwinter in the soil and plant debris and lay their eggs in cavities in the crown of the carrots. These eggs develop and hatch into larvae and begin to feed on the carrot root. Feeding damage from the carrot weevil larvae causes unmarketable tunneling near the crown of the carrot and can account for significant losses to commercial growers.

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Figure 1. Carrot weevil damage

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Figure 2. Carrot weevil larvae feeding damage on young carrot roots

Increases in the amount of damage have been seen in the Holland Marsh growing region of the province and some growers have experienced significant losses. One factor contributing to this increase is the carrot weevil's development of resistance to Imidan (phosmet). Imidan has been the primary method of control for the last 25 years and signs are pointing to reduced efficacy.

Research on this subject was done by University of Guelph grad student Zach Telfer under Dr. Mary Ruth McDonald in 2016. In lab trials comparing carrot weevils collected in the Holland Marsh to a lab strain with no previous exposure to Imidan, the Holland Marsh carrot weevils had a reduced susceptibility to Imidan. At the highest rate tested, Imidan only killed 12% of the Holland Marsh weevils compared to 80% of the susceptible strain. These results highlight the need for rotational partners with new modes of action to provide control while preventing further resistance developing.

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Figure 3. Two adult carrot weevils in a Boivin trap. A carrot piece attracts the adult weevils and the open slats provide optimal shelter for the carrot weevils to remain until counted.

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Figure 4. Carrot weevils in trap

A second factor that could be leading to an increase in carrot weevil damage is the change in the carrot weevil biology. Carrots are being damaged by carrot weevil outside of the normal periods of activity. This could be an adaption by the carrot weevil to a more prolonged period of egg-laying (oviposition period) or there could be the development of a second generation of carrot weevil. This second generation has not been previously reported in Ontario but it does occur in Quebec and other carrot growing regions in the US. The graph below illustrates how based on the Degree Day accumulations for 2016, a 2nd generation of carrot weevil is likely occurring in Ontario causing additional feeding damage.

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Figure 5. Degree Days (base 7 degrees C) accumulated over the 2016 growing season and potential life stages of the carrot weevil. Graph courtesy of Zach Telfer (IPM Coordinator, Muck Crops Research Station, University of Guelph)

Scouting and management

Current IPM recommendations are based off of Boivin trap counts. Boivin traps consist of a carrot piece surrounded by slats of wood, sandwiched between two pieces of plywood (see below). The carrot attracts adult carrot weevils while the slats of wood provide an optimal home for the weevils until collection. Current thresholds of 1.5 and 5 weevils/trap, counted cumulatively, will justify an insecticide spray at the 2nd and 4th leaf stage of the carrot respectively.

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Figure 6. Boivin weevil trap placed at the border of a carrot field

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Figure 7. Disassembled Boivin weevil trap. A base, top, middle section with wooden slats, carrot piece, flag and rubber band

Matador 120 EC / Silencer 120 EC (lambda-cyhalothrin) and Rimon (novaluron) along with Imidan (phosmet) are registered for control or reduction in damage of carrot weevil. Rimon works differently than Imidan or Matadar/Silencer as it is an insect growth regulator that in order to be effective must be either absorbed by the eggs or ingested by the larvae. Rimon stops the carrot weevil from developing to the next life stage so this product does not control adults and good application coverage is key to ensure maximum uptake.

See table below for rates, PHI and re-entry information for currently registered carrot weevil products.

Research is planned for 2017 to determine the optimal application timing for Rimon, evaluate new products as well as update the current IPM recommendations. Stay tuned.