The Alfalfa Weevil Watch — Scout, Shake and Count

Opinion

This spring I listened in on the Saskatchewan Ministry of Agriculture’s Alfalfa Weevil Webinar.  I assume I’m not the only farmer out there who had little knowledge of the alfalfa weevil before the webinar, let alone any idea of how much of a problem it is becoming. Let’s just say that if you have yet to scout for alfalfa weevil, I highly recommend you prepare yourself to do so now, especially if you’re in southern Saskatchewan, where, according to the Saskatchewan Ministry of Agriculture, there are fields requiring immediate attention.

Alfalfa weevils damage plants by chewing on developing buds, skeletonizing leaves, stunting growth and reducing bloom, according to Scott Hartley, provincial insect specialist with the Saskatchewan Ministry of Agriculture. This feeding will result in a silver sheen and can severely decrease the yield of first-cut hay.

Scouting should begin mid-June at the latest, though it can prove difficult, as Lorne Klein, regional forage specialist with the Saskatchewan Ministry of Agriculture explains, “Whenever there’s a disturbance, the larvae drop down to the ground.”

Don’t let the challenge bug you.

Of all the scouting methods I’ve discovered, I think the most practical method is outlined by forage management specialist Andre Bonneau.  It’s as easy as…

1. Collect 30 stems from an M-pattern scout, immediately placing them in a 5 gallon bucket. Do this with minimal disturbance, to prevent larvae from dropping off early (the best way to do this is to bend the plant into the pail then cut it or pull it out of the ground).
2. Shake stems to dislodge larvae into pail
3. Count larvae (excluding 1^st and 2^nd instars, or those under 3mm in length) and divide by thirty to get larvae/stem

Once you’ve collected the insects, numbers and mosquito bites, compare your findings to economic thresholds outlined on page 442 of the 2013 Guide to Crop Protection. As most alfalfa crops are now over 40 cm (16 inches) in height, control options should be considered if you find two or more larvae/stem.

So it appears you’re over the economic threshold that best applies to you. Now what?

Research out of the United States and some anecdotal evidence out of Saskatchewan suggests grazing can set back the larval populations.  “In one case, a producer had extreme populations,” says Klein, “he decided he would turn the hay field into pasture.  He grazed it two years, and I was there in the spring of the second year…we could not find a weevil.”  Still, Klein cautions that there is no research yet to document timing, intensity and duration of grazing for alfalfa weevil control.

The most common method of control is actually to cut the crop early, which will kill many larvae. But the decision of whether to cut or use chemical control options is yours, and it likely depends on the number of acres with high alfalfa weevil populations, weevil staging, crop staging and how many free labourers (children) you have.

If you are considering spraying, please oh please do not forget about pre-harvest intervals, which range from 0-20 days. Chemical options for alfalfa weevil control (and suppression) are listed on page 444 of the 2013 Guide to Crop Protection.

Now, off you go! Tweet me pics!