Cutting alfalfa an early line of defense against weevil

Researchers are working on an interactive map to help producers assess potential for weevil infestations

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Many Saskatchewan forage growers dutifully waited for alfalfa to reach the 15 per cent bloom stage before cutting last year, but their patience went unrewarded.

Thanks to the alfalfa weevil, a nasty little pest that has been gradually spreading across the southern Prairies since its arrival here in the 1950s, they were waiting for a bloom that was never going to happen. All they got from the delay was a higher infestation of weevils.

Researchers and provincial entomologists say the alfalfa weevil is damaging Prairie alfalfa crops, yet the extent of the infestations are unknown because many farmers remain unaware it even exists.

“The alfalfa weevil is not on all hay growers’ radar,” said John Gavloski, an entomologist with Manitoba Agriculture, Food and Rural Initiatives.

The adult weevils, which are five mm long and brown with a dark-brown stripe extending down their back, overwinter in crop debris or under alfalfa crowns.

Each spring, they lay eggs on the alfalfa stems. In a few weeks, the newly hatched larvae start munching holes into leaves and new growth, which often results in the plant failing to flower.

“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,” said Julie Soroka, an entomologist with Agriculture and Agri-Food Canada. “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 solely a western North American problem. “Then 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,” she said.

The two populations remained relatively stable and isolated geographically until the 1990s, when the western weevils started moving east south of the Trans-Canada Highway. By 2000, the pest had reached Manitoba and started moving north. Currently, only the northern-most agricultural regions of the three Prairie provinces remain free from the insect.

The USDA has had success controlling the wasp by releasing parasitic wasps that lay eggs that grow inside of the weevil during its larval stage. “When the second biotype established in the East, the USDA released 11 different biological control agents,” Soroka said during an interview May 17.

The program has kept the weevil populations below levels that cause economic loss in the eastern U.S.

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 subsequent biological control by the parasite has remained patchy.

Given the success of biological control in eastern North America, entomologists are optimistic, however, that a biological solution exists for western North America. Soroka said in 2012 as many as 20 per cent of the alfalfa weevils were infected by parasitic wasps in some Saskatchewan fields.

Gavloski said he is checking Manitoba fields for the parasitic wasp that has proved successful in the East. He 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. “We don’t know enough about its limitations as to how far it could go,” he said.

The presence of the parasites makes for a tricky decision over whether to try controlling the weevil with insecticides.

Killing the weevil with insecticides also kills the parasites, which would have emerged, built up local populations and reduced the severity of future weevil populations.

Currently, the most effective method for controlling weevil populations in alfalfa is to cut the crop.

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.

Controlling the weevil with cutting does not affect parasite levels the same way as insecticides, since the infected weevils tend to drop off the plant, lay on the soil surface and are not damaged by mowing. That allows the parasitic wasps to complete their life cycle and build up local populations.

Researchers plan this year to monitor the growth stages of the weevil along with temperatures to develop a growing degree day model. The hope is to develop an interactive map that provides producers with a tool that shows when the weevils might be present, based on temperatures.

But they said it is also important for producers to monitor their fields.

“Producers have to get out of their trucks, into the field and know what is going on,” Soroka said.

Light damage can be detected by a ragged damage appearance on the leaves. When populations are high enough the entire field takes on a silvery hue from the skeletonized leaves, now absent of green foliage with only the leaf midribs remaining. The main feeding period is mid-June to mid-July, so it is of primary concern for first-cut alfalfa.

On a recent webinar Lorne Klein, regional forage specialist with the Saskatchewan government, 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.”

There are a number of insecticides registered for alfalfa weevils that are listed in provincial crop protection guides, but growers must remain aware of the harvest intervals following product application.

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.

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