Mint XXXI-3

Mint Flea Beetle

Sue Blodgett & Bill Grey

Flea beetle, adult

Identification (life cycle and seasonal history)

Mint flea beetles overwinter as eggs in the soil near the crown of mint plants.  Eggs hatch in early spring and larvae, move to and feed on small mint roots and later tunnel into rhizomes. Flea beetle larvae are white, slender and worm-like with a shiny pale brown head and reach ¼ inch in length when mature.  Larval development is completed in early summer. Pupation takes place in the soil and is completed in about 3 to 4 weeks. Adults begin emerging from the soil during mid-summer. The adult is a very small beetle about 1/8 inch long, with brownish-yellow bodies and darker, reddish-brown heads. The hind legs are long and thickened for jumping, hence the name "flea beetle".

Winged females seldom disperse by flight and males cannot fly. At harvest, adults disperse slowly into adjacent fields to lay eggs in the soil near a mint crown or uncut plant until freezing temperatures occur.

Plant Response and Damage

Both adult and larva can cause damage to mint, however, larval damage is of greater concern.  Adult damage is characterized by leaves that appear to be peppered with small round holes.  In the spring, larval feeding causes severe damage to roots resulting in slow regrowth, spotted stands, and reddish plants that are stunted and stressed.  However, these symptoms are also typical of stress caused by symphylans, nematodes or water stress.

Management Approaches

A degree day-based method, used to predict the occurrence of mint flea beetle life stages, was developed by Oregon State University researchers.  A 41^oF base temperature was used to calculate the degree-day requirements of each life stage.

Larvae are detected by soil sampling after 500 to 600 degree-days have been accumulated. A 5 X 7 inch soil sample including roots to a depth of 4 inches is collected for each 1-2 acres with a minimum of 25 samples per acre. Soil samples can be screened in the field or placed in Berlese funnels to collect the larvae.  If you are using Berlese funnels, soil and rhizomes should be separated 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. An average of 0.5 to 1.0 larvae per soil sample may cause injury to mint (Oregon State University).

A sweep net can be used most efficiently in the early morning to sample adults from dry foliage after 1500 to 1600 degree-days have been accumulated.  Oregon State University recommends sampling at least five sites for every 20 acres taking 10 to 20 sweeps per site and calculating the average number of adults per sweep.  A treatment threshold has not 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.

Table1.  Degree-day requirements for mint flea beetle life stages using a 41^oF base
temperature, calculated from January 1.  Oregon State University.

Stage	Cumulative DD
1^st instar	405
2^nd instar	575
3^rd instar	775
Prepupae	1045
Pupae	1370
Adult	1555

Cultural Controls

Planting flea beetle infested rootstock is one of the principal ways new mint flea beetle infestations are established.  Purchase and plant rootstock that is certified pest free.

Biological Control

Parasitic nematodes 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 60^oF in the spring and after about 750 degree-days have been accumulated. Early spring applications may not be effective because of cold soil temperature, small size of larvae, and their concealment in rhizomes.  Cool spring temperatures and the lack of adequate irrigation water in the spring may prevent the usefulness of early spring applications.  See label for specific application guidelines. 

Chemical Control

Apply insecticides after adults emerge, usually in early July.  There are no insecticides registered for the control of mint flea beetle larvae.

Product list for Flea Beetle:
Pesticide	Product per acre	Preharvest Interval, Remarks
Actara	1.5 – 3.0 oz	7 day.  Do not exceed 8oz/A/season.  Do not use less than 10 GPA by ground.  Do not apply by air. Allow at least 14 days between applications
parasitic nematodes	3 billion nematodes/A	Apply at night with supplemental irrigation.
^1Label allows chemigation, ^2 Generic active ingredient, several formulations available, see labels for rates ^R Restricted use pesticide
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

Categories: Mint, Insects, Flea Beetles

Date: 06/06/2006