It may be a small pest, but alfalfa weevil has been making a big impact across Western Canada and may still be running below some forage producers’ radar.
The main feeding period for the pest runs from mid-June to mid-July — making it a primary concern for first-cut alfalfa in areas where the pest is present.
Adult weevils are five mm long and brown with a darker brown stripe extending down the back. The adults overwinter in crop debris or under alfalfa crowns.
In spring, the weevils lay eggs in alfalfa stems; when larvae hatch a few weeks later they start eating holes into leaves and new growth. Light damage can be detected by a ragged damage appearance on the leaves.
When populations are high enough, an entire affected field can take on a silvery hue from the skeletonized leaves, now absent of green foliage with only the leaf midribs remaining.
Just as you can’t put the genie back in the bottle, once an introduced pest gets established in a new area, you almost never can go back.
History and spread
According to Julie Soroka, an entomologist with Agriculture Agri-Food Canada in Saskatoon, the alfalfa weevil arrived in Utah around the turn of the century, likely in a load of Italian furniture that was packed with alfalfa for the long bumpy journey. By 1954 it has marched north and arrived in southeastern Alberta and southwestern Saskatchewan in the Milk River and Maple Creek regions.
For the first half of the century the pest was a solely a western North American problem. Then, Soroka said, “in the 1960s a second introduction of alfalfa weevil occurred along the eastern seaboard of North America. That population stayed along the east, creating two distinct biotypes in North America.”
The two populations were relatively stable — especially the Canadian population, limited to the original area, moving a bit east and west but staying beneath the 51st parallel. In the 1990s, however, the alfalfa weevil started moving east, though still south of the Trans-Canada Highway. By 2000, the pest had reached Manitoba and it started moving north.
Today, only the northernmost agricultural regions of the Prairie provinces are free from the insect.
The problem with an introduced pest is it tends to arrive without the predators and parasites that keep it in line back home. In the 1910s, USDA introduced biocontrol agents: tiny wasps that lay eggs which hatch and grown inside the larvae stage of the weevil. When the second biotype established in the east the USDA released 11 different biological control agents, Soroka said.
In eastern North America alfalfa weevil is fully controlled by these predators and parasites. The weevil is still present, but at such low levels that it no longer causes economic damage to alfalfa crops.
In 1954, the first year the pest was discovered in Canada, one type of parasitic wasp was found along with the pest in Alberta — but the parasite’s control of the weevil since then has remained patchy. Insects are sensitive to a variety of factors. Weather that does not mimic their home territory enough rarely establish successfully. If it is too cold, too dry, or too hot the pest may survive — but not those that prey upon them.
Given the success of biological control in eastern North America, entomologists remain hopeful a solution exists for western North America.
Some parasites are present, and Soroka found in 2012 as many as 20 per cent of the alfalfa weevils were infected in some Saskatchewan fields.
John Gavloski, an entomologist with Manitoba Agriculture, Food and Rural Initiatives at Carman, said he will be looking for a parasite wasp that is particularly successful in the east. “In most years they can keep alfalfa weevil below economic thresholds.”
Gavloski wants to know if the wasp is already present at low levels and could be augmented with additional releases. If the parasite is absent in some regions but successful in others, then additional releases may help control the pest in those areas.
About the parasite, Gavloski said, “we don’t know enough about its limitations as to how far it could go.”
Spray the weevil, kill the parasite?
The importance of these parasites to growers is such that controlling alfalfa weevils with insecticides when they are below the economic threshold may do more harm than good.
A number of products are registered for use on the Prairies against alfalfa weevil, including Malathion, Imidan, Lagon, Cygon, Matador, Silencer and, in seed crops, Decis. But by killing the weevil, they may kill the parasites which would have emerged and built up the local populations and thus reduced the severity of future weevil populations.
The simplest method to control the weevil in alfalfa hay crops is to cut the alfalfa when it’s reached the bud or early bloom stage. Cutting is especially effective if a grower uses an aggressive crimp on a mower conditioner — compared to a sickle or swather mower that gently lays the alfalfa without damaging the weevil.
Controlling the weevil with cutting doesn’t tend to impact parasite levels the same way insecticide applications do, since the infected weevils tend to drop off the plant, lay on the soil surface and are not damaged by mowing. The wasps are still able to go through their full life cycle and build up local populations.
Scientists across the Prairies are working to provide growers with additional tools to better manage the alfalfa weevil. This year they will monitor the growth stages of the alfalfa weevil and temperatures to develop a growing degree day model. If growers can access an interactive map, better control may be achieved.It is easier to scout a field regularly with the additional support of a tool that shows when alfalfa should be present based on temperatures.
That said, “the alfalfa weevil is not on all hay growers’ radar,” Gavloski added.
This was confirmed in a recent webinar presented by the Saskatchewan ministry of agriculture. It found in 2012 many hay producers were waiting for 15 per cent bloom before cutting — and the alfalfa never bloomed. Alfalfa weevil feeding delays or prevents flowering. Growers were waiting for a bloom stage that was never going to arrive, and the longer they waited, the bigger the alfalfa weevil problem became.
“Producers have to get out of their trucks, into the field and know what is going on,” Soroka said.
Lorne Klein, regional forage specialist with the Saskatchewan ag ministry in Weyburn, said growers “should be checking the fields two times a week. If you have a high number of fields and find that it is taking too much time then growers should focus on the fields with a high percentage of alfalfa and the fields that are three years and older.”
The economic threshold may be lower in 2013, with the high price of hay making the loss of production more critical. Getting an accurate understanding of pest populations is the first step to making an informed decision.
— Stuart McMillan writes from Winnipeg on weather and agronomic issues affecting Prairie farmers.