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Mint Root Borer in the Pacific Northwest

K.S. Pike, C.R. Baird, and R. E. Berry.
Washington State University Extension Service, Pullman.
PNW 322
1988

Introduction

Mint root borer, Fumibotys fumalis (Guenee), is one of the most serious insect pests of commercial peppermint in the Pacific Northwest. It attacks crops in the major peppermint areas of Washington, Idaho, and Oregon, except central Oregon.

Injury to peppermint occurs as larvae bore and feed in mint rhizomes, generally between mid - July and late October. The damage severely weakens mint, making plants more susceptible to winter injury. In western

Oregon, damaged plants are more susceptible to injury caused by flaming in the fall to control verticillium wilt and in the spring to control mint rust. Infested fields show reduced vigor and poor regrowth in the spring; declining yields and shortened stand life follow.

Description

The adult mint root borer is a light brown pyralid moth with distinctive line markings on the wing . Its wing span is 3/4 inch. Eggs are flattened and oval in shape, 1/15 to 1/16 inch in length and generally light colored. They darken just before hatching. The mature larva, which may be found in the rhizome or the soil, is dirty white in color, 1/2 to 3/4 inch in length, with 3 pair of legs and a brown head capsule. The pupal stage is encased in a silk - lined soil case called a hibernaculum, which resembles a small teardrop - shaped soil clod, 3/4 inch in length.

Life Cycle

The life cycle of the mint root borer appears on the facing page. The insect overwinters as a diapausing prepupa within the hibernaculum. Hibernacula are located within the upper inch of soil, and are frequently attached to root hairs or rhizomes. Peak emergence of adult moths occurs in late July, but some moths emerge as early as mid - June and as late as mid - August. Adults live 7 to 10 days, during which time they mate and females deposit eggs in rows of 3 to 7 on the upper surface of leaves.

Eggs hatch into larvae in about 10 days. New larvae feed for 2 to 4 days on the leaves, then drop to the soil to feed on and in mint rhizomes. Feeding lasts 2 to 3 months, and may extend into late October in western Oregon or early November in other production areas. After larval development is completed, the insect leaves the rhizome to form its overwintering hibernaculum. One generation occurs annually.

Host Plants

Peppermint is the main host for the mint root borer. It has been found occasionally on spearmint, and in one instance involving Scotch spearmint in south - central Washington, the insect caused significant damage. Native spearmint has never been damaged by the borer.

Detection of Mint Root Borer and Evaluation of Infested Fields

Although mint root borer is widely distributed in the Pacific Northwest, it will not infest all fields or will not infest them equally. To determine if the insect is present, check fields for adults during late July when moth activity is at its peak. This is best accomplished by sweep - net sampling or by carefully observing the insect at rest on the foliage. Recognize, however, that other day - flying pyralid moths --orange mint moth, Pyrausta orphisalis Walker, and false celery leaftier, Udea profundalis (Packard) --found in mint fields resemble mint root borer moths. Orange mint moth is beneficial, and false celery leaftier is of little or no economic importance to mint. No controls are needed for these insects. The above key and illustrations on the facing page show differences in the adult moth stages. For information on the beneficial effects of orange mint moth on mint, see WSU XB0982, Orange Mint Moth: A Beneficial Insect in Washington's Commercial Mints.

If adult mint root borers are found during summer sampling, an infestation of mint root borer larvae will probably follow. However, it is difficult to predict the level of infestation and associated feeding injury. To determine if control is necessary, conduct follow - up soil and rhizome sampling for borer larvae in mid - to late August after harvest in Washington and Idaho; or from the last week in August to the first 2 weeks of September in western Oregon.

To sample, take two square foot soil samples for every 2 to 3 acres. Sample depth need not be more than 3 inches, or not deeper than the mint rhizomes. During sampling, examine soil as well as rhizomes for larvae, since larvae move between rhizomes during feeding. Take soil samples around areas where plants are weak, not in areas where plants are dead. Screen soil samples in the field or place them in Berlese funnels to remove the larvae. In some production areas, you may also recover root weevils and symphylans during sampling. For more information on insect soil sampling and Berlese funnels, see OSU EC 1079, Sampling for Soil Insect Pests.

__________________________________________________________________
KEY
to the Common Pyralid Moths of Mint in the Pacific Northwest.

1. Wing color, generally light
brown........................................    Go to 2
la. Wing color, generally dark
brown and orange....................    ORANGE MINT MOTH
2. Submarginal line on wings
broken or dotted; small,
distinctly outlined donut
marking on forewing..............    FALSE CELERY LEAFTIER
2a. Submarginal line on wings
solid; small donut marking on
forewing lacking......................    MINT ROOT BORER

Guidelines for Borer Control

Research and field experience indicate that in general, if the number of mint root borer larvae averages 2 or more per square foot in peppermint, the insect should be controlled. This is particularly true if the field will remain in production for one or more years.

Control Approaches

Four approaches to mint root borer control are listed below. Not every approach is suited for all farms or locations. Consider the methods carefully before deciding which will work best for you.

1. When planting new fields, use peppermint root stock free of mint root borer, or at least root stock taken from Lorsban 4E treated fields. If native spearmint, which is not affected by mint root borer, is an economic and agronomic option to growing peppermint, grow native spearmint.

2. If the number of borer larvae averages 2 or more per square foot, treat fields with Lorsban 4E, at the rate of 2.0 pounds active ingredient per acre, using a broadcast foliar application in the late summer or fall after harvest, and sprinkler irrigate the field within 24 hours, using a minimum of 1 - acre inch of water. By sprinkler incorporating Lorsban immediately, compared with delaying, or using no irrigation, you can minimize insecticide loss through dissipation. The 1 - inch minimum water requirement provides maximum downward movement of Lorsban in the soil; Lorsban is not easily leached by irrigation (see Pike and Getzin 1981).

Using sprinkler irrigation following treatment as described will generally require treatment in strips coordinated with the movement of irrigation across fields. Some growers combine application and irrigation by applying Lorsban 4E to peppermint through a sprinkler irrigation system. This may simplify the application/ incorporation of Lorsban ; however, the approach on mint has not been experimentally verified and is not label approved; it is labeled for this type of application on selected other crops. See the Lorsban 4E label, also "Lorsban 4E Insecticide Center - Pivot Irrigation Guidelines," The Dow Chemical Company, 1984.

3. If the number of borer larvae averages 2 or more per square foot, treat fields with Lorsban 4E at the rate of 2.0 pounds active ingredient per acre by broadcast foliar application in late summer or fall after harvest. Immediately incorporate Lorsban into the soil, using strip tillage as illustrated in Diagram 1. To strip till after harvest, you may need to irrigate the soil first. During tillage, lower the rear apron on the tiller to smooth and level the soil. Follow with a packer wheel. Locate new tilled strips over the old beds. The best roots generally are established in these old furrows after late summer irrigation. Cut furrows, if needed, in the tilled strips.

The strip tillage process, in addition to incorporating the Lorsban , physically destroys some larvae. However, strip tillage alone following harvest is not an adequate treatment for borer. The process also aids in reducing soil compaction, may stimulate or rejuvenate mint stands, may facilitate herbicide weed control, and is useful in reestablishing or alternating irrigation furrows. The Lorsban tillage treatment, timed immediately after harvest, provides on the order of 90% or more control, and prevents or limits much borer damage that would otherwise occur in the fall. The treatment has been especially helpful to growers in south - central Washington and to some Idaho growers. It has led to significant oil yield gains in research plots; however, it may not be the best approach for all growers. It may not work well in sandy soil and it is not recommended in western Oregon because of the danger of spreading verticillium wilt and the absence of irrigated furrows. We recommend that you test this practice first on small plots before adopting it fully.

4. In western Oregon, plowing and discing 4 - 6 inches deep in the fall (late October or early November) or in the spring (February or March) reduce mint root borer adult survival by more than 80%. However, because this practice may spread verticillium wilt, it is only recommended every two or three years, and only in fields with low incidence of verticillium wilt. Growers may observe additional benefits from plowing and discing, such as stand rejuvenation, suppression of some weed species common groundsel and false dandelion - -redistribution of nutrients, and the establishment of uniform pH levels in the upper 4 to 6 inches of soil (see Talkington, 1993 and Talkington and Berry, 1986)

Acknowledgments

Research leading to development of this bulletin was supported in part by the Washington Mint Commission, the Idaho Mint Commission, the Oregon Mint Commission, the Oregon Essential Oil Growers League, and the Mint Industry Research Council. Photographs in this publication are courtesy of the following: Ken Gray, Entomologist, Oregon State University Extension Service...mature mint root borer larva, page 3; overwintering prepupa, pupa, and hibernaculum, page 4; mint root borer and false celery leaftier adult moths, and mint root borer eggs, page 5. R. E. Berry, Entomologist, Oregon State University...feeding damage of second and third instar larvae, and second instar larva entry hole, page 3. K. S. Pike, Entomologist, Washington State University Irrigated Agriculture Research & Extension Center, Prosser...orange mint moth adult, page 5. The life cycle of the mint root borer was drawn by Leslie Boydston. Diagrams 1 and 2 were based on drawings by Leslie Boydston.