Alfalfa Weevil In Manitoba

The alfalfa weevil (Hypera postica) has been in Manitoba since at least 2001 and may have been here for a considerably longer period of time. Where it was initially found was in the southern part of the province, close to U.S. areas where the weevil was previously established. This was important because those U.S. regions already had many of the natural enemies of the weevil already present that generally holds weevil populations in check.

About three or four years later producers started noticing weevils in areas where they had not previously been encountered, notably the Interlake region. The new arrival of this pest went unaccompanied by its natural enemies and the results were devastating to many growers as the weevil, essentially a new pest to those growers, razed fields to the crowns. Damage was variable even within the same region but in many cases was so severe that multiple pesticide applications did little to control the pest and damage resulted in complete loss of production for that year. The sole good news that could be offered was that even though feeding went right down to the crown of the plant, it didn’t kill the plants and re-growth was observed in August after the weevils had finished feeding.

Alfalfa weevils are difficult to manage for a number of reasons. They overwinter outside fields in many cases so you cannot scout early nor do a burn to take care of the overwintering adult stage. The adults move into the field in the spring and the adult female lays eggs in the stems over a lengthy period of time. This prolonged egg laying results in a prolonged period of egg hatch, so not all weevils are hatched at the same time. The result is that you can have fully grown, mature larvae ready to pupate while more eggs are giving rise to first instar larvae. This is why a single insecticide application was sometimes not very effective. It certainly worked well on those larvae that hatched early but their brothers and sisters followed later and resumed the damage that their kin had started earlier.

MAFRI has been monitoring populations of weevils over this period of time and it seems history has chosen to repeat itself, albeit in a positive fashion this time. If you review the literature describing earlier outbreaks of alfalfa weevil in other parts of North America there is a period of time, about three to five years, where the weevil is an extremely devastating pest. After that period of time the natural enemies of the alfalfa weevil have had sufficient time to build up their numbers and the numbers of the weevils topple. In 2008 it was not unusual to find alfalfa fields where the number of weevils (larvae and adults) per sweep approached 50. In 2010 those numbers topped out at only 8 weevils per sweep. Essentially the pattern observed in other outbreaks in other regions has replayed itself in Manitoba.

Most of those natural enemies are small parasitic wasps that are generally unobserved by producers. The two most common species are Bathyplectes curculionis and Bathyplectes anurus. In addition a soil borne fungus may be present. We haven’t tested to see if it’s around, but in years experiencing heavy rainfall (sound familiar?) Zoophthora phytonomi sporulates and infects the larval stage of the weevil causing death within days of infection.

We already have detected a trend that is positive for producers: the timing in many fields of the best spray application is the same as that for Lygus bugs, just prior to putting the leafcutting bees out in the field. Going forward we suspect that the weevil will become something of an afterthought in most years.

From the August 2010 edition of CROPS E-NEWS