Agriculture: Cole Crops Pest Management Guidelines

Diamondback Moth

  • Plutella xylostella
  • Description of the Pest

    Diamondback moth larvae (caterpillars) grow to be up to 0.3 inch long. They are wider in the middle and taper at both ends. The two prolegs on the last segment form a distinctive V-shape at the rear end. When disturbed, the caterpillars wriggle rapidly or attach themselves to a leaf by a silken thread and drop off, similar to the behavior of leafroller caterpillars. Mature diamondback moth caterpillars are smaller than other caterpillars that are common in cole crops.

    The caterpillars mature in 10 to 14 days during warm weather and spin a loose cocoon and pupate on leaves or stems. Female adults (moths) lay tiny, roundish eggs singly on the undersides of leaves. Eggs are difficult to find.

    Diamondback moth may be active throughout the year, especially in coastal areas. The pest is most numerous in spring to early summer and in the fall.

    Damage

    Larvae feed mostly on outer or older leaves of plants. Young stages rasp the undersides of the leaves, creating damage with a characteristic "window paning" appearance, in which the upper surface of the leaf remains intact and becomes transparent. Older larvae chew small holes or feed at the growing points of young plants and chew floral stalks and flower buds.

    Diamondback moth infestations are most serious when they damage the crowns or growing points of young plants or Brussels sprouts. This injury can severely stunt growth. Sometimes the caterpillars bore into flower buds or broccoli and cauliflower heads, causing contamination and economic injury. Injury to leaves alone is not usually serious, except when the wrapper or cap leaves of cabbage are injured.

    Management

    Keep records of diamondback moth while monitoring for other caterpillars. Natural enemies and insecticides applied to control other pests might keep the diamondback moth under control in most fields in California. Keep in mind that broad-spectrum pesticides applied for other pests can sometimes increase diamondback moth infestations by eliminating natural enemies.

    Cultural Control

    Rotate out of cole crops so that they do not grow in the same field in consecutive years. Plant all cole crops in a given field at the same time, rather than staggering planting dates between adjacent blocks of the same field. Blocks with differently aged cole crops enable diamondback moth to move through the field and cause sustained, continuous damage.

    Biological Control

    Natural enemies often control diamondback moth in California. In Southern California, an ichneumonid wasp, Diadegma insularis, is the most common parasitic wasp that attacks diamondback moth. Trichogramma pretiosum, another type of parasitic wasp, may also attack the moth eggs. Predators such as ground beetles, spiders, syrphid fly larvae, and true bugs can also play an important role in biological control of diamondback moth. Naturally occurring microbial diseases are not known to effectively control this pest. Consider planting insectary plants to attract natural enemies of diamondback moth.

    Organically Acceptable Methods

    Use biological control in an organically certified crop, as well as sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad.

    Monitoring and Treatment Decisions

    Twice per week during the following periods, sample fields for caterpillars and record the number of diamondback moth larvae separately from other species:

    • during the seedling stage or right after transplanting,
    • at thinning, and
    • just before heading.

    In cabbage, regularly monitor wrapper leaves for damage after heading. Adults frequently migrate in from fields being harvested or disced, so carefully check border rows if numbers were high in adjacent fields.

    No economic thresholds have been developed for diamondback moth in California. Insecticide application may be necessary if there is significant feeding on growing points.

    Diamondback moth has developed resistance to multiple insecticides with different modes of action. Therefore, it is especially important to rotate insecticide modes of action to manage this pest. Do not apply more than two insecticides with the same mode of action within a 10- to 14-day period.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
     
    A. SPINOSAD
      (Entrust SC)# 1.5–4 fl oz 4 1
      (Success) 1.5–4 fl oz 4 1
      MODE-OF-ACTION GROUP NUMBER1: 5
      COMMENTS: Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days after. Use the Entrust SC formulation of spinosad for organic production. Check with organic certifier to determine which products are organically acceptable.
     
    B. SPINETORAM
      (Radiant SC) 5–10 fl oz 4 1
      MODE-OF-ACTION GROUP NUMBER1: 5
      COMMENTS: Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days after. Control improved with addition of an adjuvant.
     
    C. CHLORANTRANILIPROLE
      (Coragen) 3.5–7.5 fl oz 4 3
      MODE-OF-ACTION GROUP NUMBER1: 28
      COMMENTS: Foliar application; use with an effective adjuvant for best performance. Diamondback moth has developed resistance to this active ingredient in some areas. Local populations may remain susceptible; in such cases it is important to maintain that susceptibility by using practices to prevent resistance development, including the rotation of insecticide modes of action every 10 to 14 days. Furthermore, do not apply to any generation of diamondback moth more than twice, or apply more than twice within a 30-day period (see the label for further information). Use higher application rates within this range for heavier infestations, larger or denser crops, or extreme environmental conditions such as rainy weather or high temperatures.
     
    D. EMAMECTIN BENZOATE
      (Proclaim) 2.4–4.8 oz 12 See comments
      MODE-OF-ACTION GROUP NUMBER1: 6
      COMMENTS: PHI is 7 days for head and stem cole crops, 14 days for leafy cole crops.
     
    E. INDOXACARB
      (Avaunt) 3.5 oz 12 3
      MODE-OF-ACTION GROUP NUMBER1: 22A
      COMMENTS: Add a wetting agent to improve coverage.
     
    F. METHOMYL*
      (Lannate LV) Label rates 48 See label
      MODE-OF-ACTION GROUP NUMBER1: 1A
      COMMENTS: Not registered for use in broccoflower, kohlrabi, mizuna, mustard spinach, or rape greens. Application rate for diamondback moth in most crops is 1.5–3 pints per acre; consult the label for more information. Add a wetting agent to improve coverage. Adversely affects natural enemies of other pests.
     
    G. METHOXYFENOZIDE
      (Intrepid 2F) 12–16 fl oz 4 1
      MODE-OF-ACTION GROUP NUMBER1: 18
      COMMENTS: For suppression only.
     
    H. BACILLUS THURINGIENSIS ssp. KURSTAKI
      (Condor WP)# 1–2 lb 4 0
      MODE-OF-ACTION GROUP NUMBER1: 11A
      COMMENTS: Not registered for use on broccoflower (cavalo), mizuna, and mustard spinach. Diamondback moth may be resistant to Bacillus thuringiensis in some areas. If such resistance is not yet a problem, this insecticide is useful as long as it is rotated with other modes of action in order to prevent resistance development. Check with organic certifier to determine which products are organically acceptable. Label strongly recommends an approved spreader-sticker for application in cole crops.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
    1 Rotate insecticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    # Acceptable for use on organically grown produce.
    * Permit required from county agricultural commissioner for purchase or use.
    Text Updated: 12/20
    Treatment Table Updated: 12/20