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The adult mint stem borer is a black weevil, 1/8inch long, that begins flying in mid-May to early June. Mating begins soon afterward, and mating pairs may be found into late June. After mating, the female chews a hole in the mint stem near the soil line and deposits a single egg in the wound, usually laying only one egg per stem. Each female is capable of laying up to 100 eggs per season.

Larvae hatch by late June and begin feeding on the soft tissues around the egg deposition sites. They then penetrate to the centers of the stems and usually burrow into the rhizomes. Some larvae may burrow upward into the stems. The main feeding and larval development period lasts 3 to 4 weeks and extends into late July, although some larvae may be found inside the plants as late as October. The earliest pupae are found at the ends of their feeding tunnels inside the rhizomes or stems during late July. Adult weevils emerge from the pupal cases in 2 weeks.

One generation per year occurs in most fields, but a partial second generation may occur in late fields. The late season immature stages do not survive the winter unless they complete development and overwinter as adults. Fifty to 60 percent of the adult population overwinters inside hollowed out mint plants; the remaining adults survive in soil debris.

Host Plants

In Idaho and eastern Oregon, the mint stem borer infests all species of commercially grown mint including peppermint, spearmint and citrata. In addition, several weeds such as wild mint, goldenrod and Kochia are hosts of this insect. Other crops grown in the Idaho and Oregon areas are not affected.

Damage and Symptoms

Although adult weevils feed on stem tissue, their damage is insignificant. The larvae are much more damaging as they hatch from eggs, begin to feed near the egg laying sites and ultimately bore inside to hollow out the stem.

Stems of infested plants weaken near the soil line where the eggs are laid and the resulting larvae feed on the plant tissue. A general weakness of the infested plants then occurs with the early visible symptoms of yellowing, wilting and lodging. Young mint plants are often killed back to the soil line, but new shoots emerge from the rhizomes. In established stands where numerous shoots emerge from a mat of rhizomes, only those stems arising from infested rhizomes show symptoms. Secondary bacterial decay is often associated with larval feeding areas.

Healthy, vigorously growing mint usually does not show symptoms or yield loss despite high numbers of adults in the field. Mint fields 3 to 5 years old or mint that is under stress (water, fertilizer, root disease, etc. ) is most affected by mint stem borer infestation. The weevil prefers spearmint and typically causes slightly greater damage in spearmint fields than in peppermint or citrata fields.

Spread

The weevils can move from field to field in several ways. Adults are strong fliers and during late spring can readily move several hundred yards in short flights. The weevils can also be spread when infested plant material or root stock is moved to new areas.

Sampling for Mint Stem Borer

Adults--A standard sweep net is useful for detecting adult weevils in a field and for determining when adult activity begins (mid-May to mid-June). However, mint stem borers tend to stay low on the plant so hey are not readily picked up in a net. Adult weevils an best be caught in sweep nets between 10 a.m. and I p.m.

Wood lath stakes coated with stickum can be placed in mint fields to detect adult mint stem borers. Three foot-long, unpainted wood stakes should be placed in mid-May to detect early flights. Stakes should be checked and serviced twice weekly through midJune. Positive identification of the trapped beetles by an expert is necessary because several commonly trapped insects can be confused with mint stem borers. Sweep net and sticky stake sampling are inefficient methods of assessing adult populations, but if done consistently, they provide an indicator of adult activity that allows growers to spray for aphids and other pests at the same time the adult mint stem borers are active.

Larvae--To assess the actual infestation of larvae in mint plants, take random samples of 10 mint plants each. Cut the mint plants just below the soil line and examine the centers of stems and rhizomes for feeding damage or larvae. Count only those plants with larvae present or actual feeding damage, not those with root rot or other disease symptoms. Sample several areas in each field and total the results to accurately estimate the percentage of plants infested in each field. The best time for stem sampling is late June through July.

Economic Impact

In most instances the mint stem borer is not an economically important pest, that is, it does not cause sufficient damage to justify insecticide treatments. Without question the insect causes damage, but damage to mint stands as a whole has been minimal. In Idaho studies, only a few fields have been found with significant mint stem borer damage, with at most 25 percent of the stems showing the apparent borer symptoms of serious wilting, yellowing or death. These fields were also under severe fertilizer or disease stress, and many plants died without evidence of mint stem borer damage.

Mint stem borer larvae seem to cause the greatest damage among stressed plants. Conversely, young, healthy mint stands tend to incur only light infestations in spite of heavy adult breeding populations in the mint canopy. Older mint fields nearing the end of production may be finished by a severe mint stem borer infestation if stressed by other factors. The most damaging infestations have occurred in 4 to 5 year old fields that should have been rotated out of mint production the previous year.

Control

The best way to control the mint stem borer is to avoid stressing the mint plants. Irrigation and fertilization must be properly managed to avoid excesses or deficiencies regardless of the age of the stand. Crop rotation is another key to maintaining healthy, vigorous mint and preventing mint stem borer damage. Although we cannot recommend a specific crop sequence, we suggest growers plow out the mint when production becomes mediocre and replant to a non-mint crop for at least 2 years.

Weed control during mint producing years is important since mint stem borer larvae can develop in wild mint, goldenrod and Kochia. Ditch banks, roadways and waste areas must also be kept free of these hosts if mint stem borer populations are to be reduced or eliminated during years of non-mint production. When planting or replanting fields to mint, growers should make every effort to select root stock from certified fields or from fields known to be free of mint stem borers.

Insecticidal control of the mint stem borer is difficult because the larvae are concealed and the adults are exposed only briefly before they begin laying eggs. In some years, adults are active and mating as early as mid-May, and in other years activity may be delayed into early June. Mint growers may wish to time their aphid or looper sprays to coincide with adult mint stem borer activity as determined by sticky stake and/or sweep net sampling. Aphid/looper sprays are moderately effective at killing adult mint stem borers. No insecticides are registered specifically for the mint stem borer.

No insecticides, systemic or other, are effective in controlling developing mint stem borer larvae inside the stems or rhizomes. Systemic and residual insecticides applied to soil or foliage in small test plots at even 10 times the recommended rate for other crops failed to reduce the mint stem borer population. Therefore, we do not recommend insecticidal control once the larvae are inside the stems.

If plants were infested with mint stem borer larvae the previous year, insecticide use to control adults can be justified in:

1. Fields used for root stock increase.

2. Fields where mint roots will be dug and used for replanting in new fields, especially if the mint is to be sold and replanted out of state.

3. Newly planted mint (baby mint) if the field was formerly in infested mint and the field has not been rotated to a non-mint crop.

Summary of Control Recommendations

1. Positively identify the pest. Mint stem borer larvae can be confused with mint root borer larvae although the seasonal occurrence of the two pests is quite different.

2. Rotate the field out of mint when production declines. Don't try for an extra year when the stand has declined due to disease and insect damage.

3. Avoid stressing mint plants. Maintain optimum fertilizer and moisture conditions.

4. Maintain proper weed control, especially of the larvae's alternate hosts: Kochia, goldenrod and wild mint. Control should include waste areas, ditch banks and roadways.

5. Eliminate goldenrod, Kochia and volunteer mint during years of non-mint production.

6. Before planting fields to mint, select certified root stock from noninfested fields.

7. If applying foliar sprays for control of aphids or loopers in fields also infested with adult mint stem borers between mid-May and mid-June, spray during the warmer periods of the day. In most years, sprays applied between June 7 and June 10 are most effective. Apply sprays at high pressure and gallonage and with drop nozzles to get the spray around the bases of the plants.

8. Do not attempt to use foliar sprays, soil insecticides or granular systemics to control mint stem borer larvae already in the stems. No insecticides are registered for this use nor have any been found effective.

9. Do not apply foliar sprays to blooming mint that is adjacent to honey bee, leafcutting bee or alkali bee areas.

The Authors--Craig R. Baird is Extension entomologist at the University of Idaho Southwest Idaho Research and Extension Center at Parma. Darrell G. Bolz is Canyon County Extension agricultural agent at Caldwell. Guy W. Bishop is emeritus research professor, formerly at the Southwest Idaho R&E Center at Parma.