Alfalfa Stubble Spray?  Insecticide REI, PHI, Residual and … Integrated Pest Management

Eileen Cullen, Extension Entomologist, UW Entomology Department

Every other week during the growing season, UW-Extension agriculture county agents and UW-Madison field and forage crop specialists have an opportunity to meet via teleconference and share information on insect, weed and plant pathogen pests, agronomy updates, and crop conditions throughout the state. On a recent call several county agents noted that they have visited with growers and CCAs who related concern about an apparent increase in adoption of prophylactic insecticide treatment of alfalfa stubble. The concern expressed was that while some individuals may routinely treat alfalfa stubble with insecticide, before regrowth has started and in the absence of insect scouting data, this practice does not follow the principles of IPM and the additional applications may increase selection pressure for insect resistance development.

Integrated pest management (IPM) of insects requires correct identification, timely scouting data, quantification of population levels, and if necessary according to economic thresholds, cultural or chemical control. While treatment of stubble soon after cutting could be justified in cases where insect pests are preventing regrowth, such a treatment decision would be based on regrowth condition, insect scouting, identification and population assessment. The concept prophylactic treatment without such data in order to obtain residual control for “incoming” pests for a pre-defined period (for example, 21 days) is not a valid IPM approach. UW Extension and grower/CCA concern is justified as alfalfa stubble treatments are likely to increase the number of applications to the stand within a season (whether a pyrethroid or organophosphate) and in turn increase selection pressure for insect resistance development. Treatment cost is another factor to consider.

REI: All pesticide products used in the production of agricultural plants have a prescribed-time re-entry interval (REI) for worker protection. REI is stated in hours on the label, usually 48-hour, 24-hour or 12-hour depending on the product signal word. For the most part, entry into treated areas during the prescribed time REI is not allowed.

PHI: The preharvest interval is the elapsed time between a pesticide application and harvest of the crop.  The PHI is defined explicitly in days. PHI on an insecticide label is where we typically see 21, 30 or in some cases a fewer or greater number of days. PHI depends on the specific pesticide label you are following, and the crop to which it was applied.

Residual: Foliar insecticide residual activity is essentially the period following treatment and initial mortality of the insect population present at application during which insect suppression continues to be effective to a relative degree. Residual activity is different from contact or systemic activity, rather this is the residual effect of insecticide on treated surfaces encountered by surviving or newly arrived insects after treatment. Typically, in insecticide efficacy trials a pre-treatment count is taken, experimental treatments applied, and post-treatment insect counts are compared to the untreated check plots 4, 7, and 14 days after treatment (DAT) and up to 21 or 28 DAT. Residual activity can vary and several inter-related factors are involved (e.g., insecticide active ingredient, formulation, temperature, UV light intensity, susceptibility of insect treated, pre-treatment population density, percent control achieved, to name a few).

The point of this brief note, is simply a reminder and second of the concerns of UW-Extension county agents, growers and CCAs. IPM protocol and UW recommendations do not advise routine treatment of alfalfa stubble immediately after cutting. Residual activity should be distinguished from well defined, (and label law) PHI’s stated on labels in days. An insecticide treatment decision, application timing and post treatment monitoring of treatment efficacy against the pest and impact on predators and parasitoids are based on field scouting data in an economically profitable and environmentally sound IPM program.