Pest description and crop damage Early instar larvae are light green/yellow with dark stripes down the back; older larvae can be up to 0.75 inch long and are yellow/tan with a brown head. They feed inside mint rhizomes and on mint roots, from late July through September and early October in some years. This pest can severely reduce stands in most mint-producing areas.
Biology and life history This pest overwinters in the soil around mint roots as a pre-pupa in a cocoon, pupates in the spring, and emerges as a moth May through July. There is one generation per year.
Scouting and thresholds Sample mint after harvest in late August through mid-September, when most larvae are large enough to detect, but have not caused much damage.
Screen square-foot soil samples taken at the depth of the rhizomes. Record larval numbers, and treat when larvae number from two to four per sq ft depending on age and vigor of the field, other pests or stresses, and oil price.
Adult moths can be sampled using sweep nets and/or pheromone-baited sticky traps. Although economic thresholds for adult moth numbers do not exist, sampling can be used in combination with known issues in previous years to plan for in-season control of mint root borer eggs and caterpillars before damage occurs.
- azadirachtin (Neemix 4.5 IGR) at 0.012 to 0.044 lb ai/A. PHI 0 days. REI 4 hr. Some formulations are OMRI-listed for organic use.
- chlorantraniliprole (Coragen) at 0.045 to 0.098 lb ai/A. PHI 3 days. REI 4 hr. Retreatment interval 14 days. Do not exceed 0.2 lb ai/A or 4 applications per calendar year. May be applied by chemigation.
- chlorantraniliprole/thiamethoxam (Voliam Flexi) at 0.1 to 0.125 lb ai/A. PHI 7 days. REI 12 hr. Retreatment interval 14 days. Do not exceed 15 oz/A Voliam Flexi or 0.188 lb ai/A of thiamethoxam or 0.2 lb ai/A of chlorantraniliprole per season. Apply in at least 10 gal water/A (ground) or 5 gal water/A (aerial). Do not use an adjuvant.
- ethoprop (Mocap EC or 15G) at 3 lb ai/A (6 lb ai/A if nematodes also a problem). PHI 225 days. REI 48 to 72 hr. Make only one application whether preplant or post-harvest per growing season (either preplant, or after last harvest of the growing season). Apply in 20 to 30 gal water/A. Broadcast over the field and incorporate into the soil to a depth of at least 2 to 4 inches, during or immediately following application by mechanical means, including by rotary tiller, rotary hoe, springtooth harrow, or by double discing, or by immediate application of 1 to 2 inches of overhead irrigation. Repeat irrigation before soil dries. Restricted use pesticide.
- thiamethoxam (Actara) at 0.023 to 0.047 lb ai/A. PHI 7 days. REI 12 hr. Retreatment interval 14 days. Do not apply more than 0.188 lb ai/A per season.