Tussock moth monitoring and control in blueberries

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The whitemarked tussock moth (Orgyialeucostigma) is distributed throughout eastern North America and can be a pest of blueberry fields in Michigan. This species feeds on hardwood and ornamental trees and is found in native habitats. When this insect moves into blueberry fields, growers may experience damaging levels of leaf loss on young bushes and contamination of fruit at harvest can cause economic hardship in mature fields. The larvae also have irritating hairs that can cause dermatitis in hand pickers, so it is important to control this insect before harvest. Early detection of this insect is the key to preventing economic loss.

Life-stages

The egg mass is white-cream colored with numerous 1-2 mm diameter eggs nested within white foam. This is usually wrapped inside a blueberry leaf and attached to a stem, or it may fall to the ground. Over 100 larvae can hatch from one egg mass, and these change from a light brown color when newly-hatched to multicolored as they grow. Full grown larvae have a bright red head with a yellowish body, a pair of upright pencil tufts of black hairs on the prothorax, and four white to yellowish brushlike tufts of hairs on the top of the body toward the head. After the first instar, larvae have a conspicuous red dot on segments six and seven. The adult moths vary considerably between the sexes: the female moth is a ¼-inch long white furry moth without wings, usually found inside a leaf surrounded by the hairs from her pupal case. The male is ¾-inch across its wings that are mottled brown, and it has feathery antennae.

Timing of development

A recently-completed study has revealed the phenology of whitemarked tussock moth in Michigan blueberry fields. This insect has two generations in southern Michigan with peaks of larvae on bushes in June and again in August, and peaks of moth activity in late June and late August. The whitemarked tussock moth overwinters in the egg stage within a large mass of eggs nestled in a hard foam that is wrapped inside a dry leaf. These are usually found on stems and are visible during pruning. The eggs hatch during bloom and larvae stay close to the egg mass at first, dispersing in the plant canopy after a few days. Larvae feed in the shady parts of the canopy until full grown in late June/early July. At this point, they pupate on the bush and emerge in July as adult moths. The female moth is flightless and stays on the leaf she pupated in, mating and laying eggs in this same position. The foamy egg mass hardens and young larvae emerge in late July through August to feed on the mature foliage, growing during the period of blueberry harvest. These larvae are the ones that cause the conflict with pickers and harvest.

Development of whitemarked tussock moth can be predicted using growing degree days. During a two-year project that included laboratory and field studies, we found that development of this pest was best predicted using a base temperature of 55oF. Egg hatch of the first generation was found to start at 208 DD base 55 (from March 1) while egg hatch of the second generation was found to occur at about 1100 DD base 55. In both years, egg hatch of the second generation in early July was found to start at 300-350 DD base 55 after the first male moths were trapped in pheromone monitoring traps. We are currently working with the Enviroweather team at MSU to integrate the findings of this project into their degree day prediction capability.

Monitoring

Early detection is important for management of whitemarked tussock moth so that the larvae are not full grown when attempting control. Using monitoring traps and bush scouting can help ensure that management actions are made at the best time to ensure a high level of control.

Using pheromone traps

Fields with a history of whitemarked tussock moth infestation should be monitored with two pheromone-baited traps, placed at the field edge and interior, and with regular scouting of bushes. This will allow growers to know where, when, and if a pesticide application is needed. To monitor, place traps in the region(s) of the farm with previous whitemarked tussock moth infestation during early June. Traps should be placed toward the wooded edge of a field if present and in the field center, and checked weekly. One pheromone lure can last all season, so no lure replacements are needed. Weekly catches of moths in the trap can be used to identify whether the infestation was controlled by the post-bloom insect management program, or whether there is still a population with the potential to cause problems during harvest. If high numbers of moths are still being trapped, fields should be carefully scouted for larvae during the pre-harvest period to determine whether chemical control is needed.

Scouting for larvae

To scout for larvae, look on the underside of leaves by turning over leaves with feeding damage and by looking in the center of the bush where the larvae prefer to hide. The large larvae are conspicuous due to their yellow, red, black and white coloration, but the smaller brown larvae may be more difficult to locate. Beware of the allergenic hairs and approach with caution!

Control

Because the first generation of larvae hatches during bloom, application of an effective fruitworm spray program during the early part of the season provides growers with good control of whitemarked tussock moth. Problems with whitemarked tussock moth tend to be worse in fields where no fruitworm sprays are applied, where they are applied without good coverage, or where the fields are weedy. Because larvae prefer the more dense and shady part of the bush, they can escape contact with insecticides if the fields are not effectively treated at the post-bloom timing.

Whitemarked tussock moths are naturally controlled by a complex of parasitic wasps that attack the larvae and by diseases. These natural enemies cycle through the pest population and help to cause the typical increases and crashes seen in whitemarked tussock moth populations. In years where the pest population is amplified, these wasps and diseases cannot suppress the pest population and intervention is needed to prevent larvae contaminating the bushes and fruit during harvest. Selection of a moth-specific insecticide such as Confirm or Intrepid can help conserve the beneficial insects.

Insecticides that are active on other moth larvae are generally effective against whitemarked tussock moth larvae (Confirm, Guthion, Sevin, Lannate, Asana, Danitol, Mustang Max). We also expect the new insecticides Delegate, Intrepid, and Assail to be effective insecticides against this insect. Broad-spectrum insecticides such as Guthion, Lannate, Asana, Danitol, and Sevin will provide fast control of this pest, killing young larvae in one to two days. The growth regulators Confirm and Intrepid take a little longer since they disrupt the molting of the larvae, providing control of young larvae in four to five days. We expect Assail and Delegate to be intermediate in their speed of control.

Because the larvae are sensitive to a broad range of insecticides, applications made for other pests such as fruitworms, blueberry maggot, or Japanese beetle are often able to control whitemarked tussock moth. However, the coverage of the bushes and the spray timing must be sufficient to bring the larvae into contact with the residue. It is important to understand that the larger the larvae become, the more difficult they are to control. This is another good reason to make sure the first generation is controlled in fields with whitemarked tussock moth infestation. This avoids having to attempt control at harvest time when bushes are bigger and laden with fruit, pickers are in the field, and re-entry and pre-harvest intervals make control much more challenging.
We thank MBG Marketing and the Michigan Agricultural Experiment Station for support of our tussock moth research

Tussock moth IPM program

  1. If the field had tussock moth problems last year, remove and destroy egg masses during winter pruning. Treat with a bee-safe insecticide at 300-500 DD base 55oF (mid-late bloom).
  2. Monitor tussock moth with a pheromone-baited traps placed in the field by early June. Check weekly and count tussock moths.
  3. If high numbers of male moths trapped, especially in interior field traps, treat field at 400-500 base 55 after first consistent moth catch (early-mid July). Beware of PHI and REI restrictions.
  4. Monitor bushes weekly from early July to harvest, to detect larvae while small (usually inside canopy).
  5. If larvae detected near to harvest, apply an effective insecticide with short PHI and with good coverage of the whole bush.

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