||The Mint Stem Borer in Idaho
Craig R. Baird, Darrell G. Bolz and Guy W. Bishop
University of Idaho College of Agriculture
Current Information Series No. 808, 1990
Cooperative Extension System
Agricultural Experiment Station
The mint stem borer (Pseudobaris nigrina) is a relatively new pest in the
mintgrowing areas of Idaho and eastern Oregon. In the mid1970s, this small weevil infested
commercial peppermint and spearmint fields in southwestern Idaho, mainly in Ada and Canyon
counties. It has since been found in Payette County and across the Snake River in Malheur
Description and Life Cycle
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.
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.
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
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
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.
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.