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.
- chlorantraniliprole (Coragen, Coragen 2ee) at 0.045 to 0.098 lb ai/a. PHI 3 days. REI 4 hr. Do not exceed 0.2 lb ai/a or 4 applications per season. Retreatment interval 14 days. May be applied by chemigation.
- chlorantraniliprole/thiamethoxam (Voliam Flexi) at 0.1 to 0.125 lb ai/a. Do not use an adjuvant. Retreatment interval 14 days. PHI 7 days. REI 12 hr. Do not exceed 15oz/a Voliam Flexi or 0.188 lb ai/a (thiamethoxam) or 0.2 lb ai/a (chlorantraniliprole) per season. Apply in at least 10 gal water/a.
- chlorpyrifos (Lorsban Advanced) at 1.9 lb ai/a (postharvest). PHI 90 days. REI 24 hr. Most effective when applied as chemigation treatment in late August through early September. Pre-irrigation of dry soils is essential to control. If you apply Lorsban as a broadcast spray, immediately follow with overhead irrigation (1 acre inch). Dry soil, strawload, organic matter, and charcoal residue greatly reduce Lorsban's effectiveness. One postharvest application allowed per season. RESTRICTED USE IN OREGON.
- 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. After last harvest, evenly broadcast 15G or EC over soil surface and immediately incorporate to a depth of 2 to 4 inches using rotary tiller or hoe, harrow, double disk or with 1 to 2 inches of irrigation water followed by another 1 to 2 inches when the soil becomes moist but not dry. Make only one application whether pre-plant or post-harvest per growing season.
- parasitic nematodes (BioNem-C, Becker Underwood) at 1 to 1.5 billion per acre. Apply in late August or early September when larvae are small. Application with irrigation water is essential for nematodes to be effective. OMRI-listed for organic use