HPIPM:Mint Mint Root Borer
Author: Sue Blodgett & Bill Grey
Identification (life cycle & seasonal history)
Mint root borer adult emergence begins in June and continues through summer until early August with peak emergence in July. Eggs that resemble tiny fish scales are laid along leaf veins on both sides of the leaf. Larvae are white to cream colored with a light brown head. First instars feed briefly on leaf surfaces before dropping to the soil surface and tunneling into rhizomes. Larvae feed in rhizomes and mint roots for the remainder of the summer, overwintering as a prepupae in an earthen cell and pupating in the spring. There is a single generation each year.
Plant response and damage
Larvae feed inside root rhizomes and roots causing stand reduction and weakened plants; larval damage increases susceptibility to winterkill.
Management approaches
When adult moths are detected in a field, by sweeping foliage with a net or through the use of pheromone traps. Soil samples should be collected as soon as possible after harvest (usually late August). Mint root borer adults may be confused with false celery leaftier adult.
Two soil samples each measuring 1 sq ft and 2-3 inches in depth should be taken for every 2.5 acres, with a minimum of 25 samples per field. Samples should include soil and mint rhizomes. Soil samples may be hand sorted or screened in the field or Berlese funnels used to recover larvae. Rhizomes should be inspected for damage by larvae if samples are inspected in the field. Treatment is justified if an average of 2 - 3 larvae per square foot sample (OSU-IPPC).
Biological Control
Parasitic nematodes have been found to control mint root borer larvae. Typically nematodes are best applied through sprinkler irrigation. See label for specific application guidelines.
Parasitic nematodes should be applied in the evening or at night because the nematodes are vulnerable to ultraviolet light. Soil should be pre-wet with an irrigation of at least ¼ inch before injecting the nematodes. Immediately following injection, ½ - 1 acre-inch of sprinkler irrigation should be applied to the field. Nematodes may also be sprayed on the field in the evening with a ground sprayer calibrated to deliver at least 50 gallons of water per acre, followed immediately with ½ - 1 acre inch of irrigation. Carefully follow the directions on the label to insure proper handling and injection of the parasitic nematodes.
Cultural Control
Post-harvest tillage has been shown to significantly reduce mint root borer. Keith Pike, Washington State University describes tillage methods for mint root borer (In Mint Root Borer in the Pacific Northwest, PNW 322). Strip tillage combined with chlorpyrifos application prior to mid-September provided 90 per cent or more control in furrow-irrigated mint fields. In sprinkler-irrigated fields, plowing and double disking mint fields in late October or early November or in the spring during February or March provided 80 per cent or more control. However, tillage should be avoided when fields are infested with Verticillium wilt since tillage is a major method of disease spread.
Chemical Control
Insecticide should be applied after harvest, but if delayed, effectiveness decreases.
Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.
