MINT FLEA BEETLE MANAGEMENT

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Mint flea beetle continues to be a pest in central and western Oregon in the spring. This is the time of year when root damage caused by the larvae of the mint flea beetle can readily be seen. Spring regrowth of mint injured by this pest is slow and characterized by spotty stands and reddish plants that are stunted and stressed. These symptoms are also typical of other factors, such as water stress or damage from symphylans, root weevils, or nematodes.

Recognition of adult flea beetle feeding damage on mint leaves is the easiest clue to determine if this pest is present. Feeding is interveinal, and appears as many small "shot holes" through both leaf surfaces. A sweep net can be used in late June and July to monitor for adult mint flea beetles (usually after the accumulation of 1500 to 1600 degree-days after January 1, using a base temperature of 41ºF). The small yellowish-brown beetles are most efficiently sampled early in the morning on dry foliage. Sample at least five sites for every 20 acres taking 10 to 20 straight line sweep net samples through the foliage at each site. Record the total number of beetles in the net at a given site and then divide by the number of sweeps taken at that site to obtain the average number of adults per sweep. No treatment threshold has been developed for adult flea beetles, but yield reductions have been observed when an average of 5 to 10 adults are found per sweep sample.

SAMPLING

To sample for larvae, begin taking soil samples in late May and early June or after 500 to 600 degree-days have been accumulated. Take a minimum of 25 samples per field. Every 1 to 2 acres take a 5 by 7 inch sample of soil and roots to a depth of 4 inches. This can be done with a small shovel or hollow metal core sampler. Soil samples can be screened in the field or placed in Berlese funnels to collect the larvae (see Morris, 1990 and Shields et al., 1981). If you are using Berlese funnels, we recommend separating the soil and rhizomes to enhance recovery of larvae. In the field, carefully inspect the soil and roots for small cream-colored larvae from 1 to 2 mm long with brown heads. They can be found in black or brown "tracks" within tissue just below the surface of the root. Occasionally they will be in the soil near the root, or protruding from the root itself. An average of 0.5 to 1.0 larvae per sample may cause injury to mint (Morris, 1990).

No insecticides are registered to control mint flea beetle larvae. However, the use of Vydate (oxamyl) to control nematodes in May can provide control of mint flea beetle larvae if applied when the larvae are in the first and second instars -- usually after the accumulation of 500 to 600 degree-days.

PARASITIC NEMATODES

Spring applications (late May or early June) of parasitic nematodes at a rate of 3.0 billion juvenile nematodes per acre may provide control of mint flea beetle third instar larvae, prepupae, pupae, and teneral adults. Applications should be made in the evening through sprinkler irrigation after soil temperatures exceed 60ºF in the spring and after about 750 degree-days have been accumulated (Morris, 1990). Early spring applications may not be effective because of cold soil temperature, small size of larvae, and their concealment in rhizomes. In areas East of the Cascade Mountains, cool spring temperatures and the lack of adequate irrigation water in the spring may prevent the usefulness of early spring applications. See also the information about using parasitic nematodes to control strawberry root weevil and mint root borer.