Feature Article: Insect Contaminants of Peas

Alfalfa caterpillar

During the past week, I have received some calls about insect contaminants in peas and control options. Research by Drs. Whitney Cranshaw (now at Colorado State) and Ted Radcliffe (UofM), in the late 1970's provide one of the most comprehensive data sets regarding the impact of multiple arthropod (insects and spiders) species, and risk of contaminants (various insect body parts and life stages) in final product. As a legume early-season crop in the Midwest, this study confirmed over 60 species of arthropds (as well as a token slug and earthworm) occurring at least once as a contaminant on final inspection belts. However, the good news is that because of various washing/processing operations, only about 15 species occurred on a regular basis. Most of these 15 species include many moth and butterfly larvae or pupae. This year, the alfalfa caterpillar (Colias butterfly) larvae and pupae (chrysalis), and cabbage or celery loopers have been the primary concern (see photos).

Alfalfa caterpillar chrysalis (pupa)

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Cabbage looper

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Insect Contaminants

Of the many results of this work, a key recommendation was that, for most fields with moth or butterfly larvae present in the field, a single spray 10 days before harvest (typically with a pyrethroid) should provide adequate control. This is based on the estimated developmental rate of the larvae and pupae for each species. For example, the loopers and alfalfa caterpillar typically spend about 9-10 days in the pupal stage (assuming avg. temp. of 73F). Thus, ASSUMING this avg. temp. range for late May/early June in southern MN, the 9-10 day guideline may be adequate. However, with cooler temps. this year (until this week), the pupae were likely in this stage for a longer period of time, indicating that a single spray application probably should go on about 15 days before harvest. Also, even with good efficacy with the pyrethroids available (Capture or Asana), if a field has 40,000 larvae/ac, and we get 98% control, that still leaves 800 larvae/ac to become potential contaminants. Alfalfa caterpillar is a particular problem because the pupae will stick to the foliage and can be harvested, and can show up as contaminants; this is a good example as to why the insects need to be controlled in the larval stage before pupation. For this year, processors may need to adjust their spray schedule to 15 days before harvest; and possibly spray again 5 days before harvest if stink bugs have re-invaded the field. Also, review what can be done to improve washing/processing options in the processing plant. Another option is to add drag chains, or something, to shake the vines before they get harvested; this can help dislodge larvae and other insects prior to peas being harvested.

Reference: W.S. Cranshaw and E.B. Radcliffe. 1984. Insect contaminants of Minnesota processed peas. Tech. Bull. AD-TB-2211. Minn. Agric. Expt. Station, Univ. of Minn., St. Paul.