Loopers are small, light green caterpillars with white lines that run the length of the body. The easiest way to identify them is to examine the number of pairs of abdominal prolegs: there will be 2 located close together, and 1 at the tip, as seen in the photo. Very small, vestigal ‘nubs’ may be located on the 3rd and 4th abdominal segments, but do not count these.
The cabbage looper overwinters as a pupa either in the soil or in trash near the base of host plants. Moths begin emerging in May, and adults lay eggs singly on plants from late May until June. Eggs hatch in about a week. Larvae feed for 3 to 4 weeks before forming a pupal cocoon that is loosely attached to leaves or brassica heads. There are multiple, overlapping generations per year.
Loopers are foliar feeders that cause small, irregular holes in leaf tissue. Holes tend to have ragged edges or be located along the leaf margin. Additional damage can occur when pupae become attached to broccoli heads. This contamination can lead to rejected loads at commercial processing plants.
Pheromone traps are used for sampling, and one lure is used for both target species. Cabbage loopers (CL) are more abundant than alfalfa loopers (AL) in this region, but relative damage varies depending on the crop.
A sweep net can be used to sample larvae. Move in a line across the field and take 10 sweeps from each of 10 sites. Watch a moment for ‘looping’ action, which occurs as the looper extends the front of it’s body and pulls the abdomen forward. For this pest, a presence/absence of each sweep is sufficient, rather than counting.
Be sure to take note of any larvae that appear creamy white, limp, or swollen, all signs of infection by NPV (see section below). NPV-infected individuals can be re-distributed to help aid spread of this naturally-occurring entomopathogenic virus.
History of outbreaks: In the Willamette Valley, there were booms of cabbage looper in 2008 and 2017, with peaks of 100 moths caught on average (per day per trap) and the abnormal activity continued season-long. In 2010 and 2016, there were elevated counts for some weeks, but most of the season was within range of historical averages. So far, 2019 has a similar trend to 2008.
Biological: A naturally-occuring nucleopolyhedrovirus (NPV) is considered the most effective natural enemy of looper populations. NPVs are usually species-specific (A. californica, T. ni). Like Bt (a baculovirus), NPVs have been commercially developed for some species, but efficacy of formulated products is largely unknown. The virus acts on gut cells and eventually causes the larva to rupture, dispersing the virus onto leaf material that other larvae will consume. So gross and cool, huh?
General predators such as ladybugs, syrphid larvae, and birds also feed on loopers. Egg parasitism by Trichogramma spp. can account for up to 35% mortality; a tachinid fly (Voria ruralis) attacks large larvae in the fall, and can be quite effective (>50% mortality).
Chemical: At-plant and foliar insecticides are important tools for commercial growers. Registered use options for homeowners and commercial growers alike are available in the PNW Insect Management Handbook.
The following pest model is provided by USPEST.ORG. Please consult that website for model parameter information, and be aware that degree day models are merely estimates of when pest activity is predicted to occur near Corvallis, OR. Click image for a direct link to the model.