Monitoring

• Populations of this pest can build quickly, especially in warm temperatures. Therefore – SCOUTING FOR DIAMONDBACK IS ALWAYS RECOMMENDED.
• Use pheromone traps placed at field edges to estimate timing and activity of adults
• Move across the field, pulling a leaf from a plant with each step
• Pull ten leaves, examine them, then pull an additional ten leaves. Repeat in different areas of the field. Choose leaves that are not too small and not too big. Leaves should be mature and unfolded but not old or discolored
• Additionally, examine stems and developing heads if the crop is within a few weeks of harvest (see last bullet point, below)
• Look for:
  • Evidence of ‘windowpaning‘ – a distinctive feeding pattern that leaves the epidermis of the leaf intact – this creates translucent ‘windows’ in the leaf tissue. However, be aware that the windows ‘fall out’ as they dry and age, so try to note holes in leaf tissue even without the panes
  • Larvae (and note their developmental stage). Larvae are tapered in shape and tend to ‘wriggle’ backwards when disturbed. Mature larvae are about 0.5 inches (12-13mm) long
  • Developmental stage of the crop. If broccoli or cauliflower buds are expanding and elongating, and mature larvae are present, they will crawl up into the elongating heads and form pupae. This is called contamination and can lead to load rejects by processors.

Management

• Natural enemies (predators, parasitoids, entomopathogens) have good impact and keep populations regulated. However, this process is cyclical and works better in some years than others.
• Other factors (winter temps, summer heat waves, overwintering population pressure, uncontrolled weeds used as alternate hosts, etc.) affect DBM populations and should be considered.
• If control is needed, be aware that this species is very difficult to control due to a high level of resistance to many – and possibly most – commonly used insecticides. They are resistant to most insecticides? Really? YES, really. The Int’l Insecticide Resistance Action Committee (IRAC) says it best, so I have placed their quote below.
• Growers and applicators MUST rotate chemistries based on the product’s mode-of-action, or group number. Yet, because of resistance and how quickly it can develop, adequate control is difficult even when rotating chemistries. Here is a pamphlet (.pdf link) that describes this concept well.

“P. xylostella is resistant to most insecticide classes including newly introduced mode of action
chemistry. The proliferation of generations combined with destructive feeding behaviour
requiring numerous sprays contributes to the rapid development of insecticide resistance”

IRAC Educational Poster, Feb 2021