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Pea leaf weevil continues to spread

Research investigating best control methods.


November 22, 2007
By Bruce Barker

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10Dr. Bob Byers initially noticed the pea leaf weevil, the first serious insect pest of field peas, in 1997, with sporadic reports in the early 2000s. However, in 2006, pea leaf weevils were noticed at outbreak levels and around 10,000 acres of peas were reportedly sprayed in southern Alberta. With pea leaf weevil continuing to expand its range from southern Alberta northward across the Trans-Canada Highway and eastward into Saskatchewan, researchers are searching for the best ways to control it.

“We’re really only at the beginning stages of the research, with our major studies in the first of three years in 2007,” says research scientist Hector Cárcamo at Agriculture and Agri-Food Canada (AAFC) at Lethbridge, Alberta. Some of the research direction is based on work in other pea growing areas, such as in Idaho and Europe, where farmers have been fighting the pea leaf weevil for years.

Under Alberta conditions, the pea leaf weevil produces one generation per year. The adult pea leaf weevil over-winters in shelterbelts, alfalfa fields and other perennial legume stubble. In the spring, the adult flies to pea fields when the temperature reaches 15 degrees C. While they feed on several pulse crops like clover, lupins and alfalfa, the adults have a clear preference for peas and fababeans, where they reproduce. “They are very good at detecting pea crops. We’ve seen one pea field that had a five mile isolation and it still became infested,” says Cárcamo.

The adults feed on pea foliage, leaving a characteristic scalloped or notched appearance on the leaf edges. The females lay up to 3000 eggs over an extended period and Cárcamo says that according to studies conducted in the US in the greenhouse, a female was observed to have laid 3299 eggs. The eggs are scattered on the ground or laid in crevices near the base of pea seedlings. When hatched, the larvae feed on the nitrogen-fixing rhizobium nodules. Research in other countries has found that feeding larvae destroy virtually all the nodules, leading to poor nitrogen fixation and large yield losses.

Research aimed at preventing larvae feeding
Cárcamo’s research, with funding from Alberta Crop Industry Development Fund (ACIDF) and the Alberta Pulse Growers Association, is working to develop a better understanding of how pea leaf weevils develop on the prairies, how they affect the pea plants and how that knowledge can be used to develop an integrated pest management strategy.

One component is tracking the life cycle of the insect, to determine when the adult weevil appears in a pea crop, so that the best insecticide spray timing can be determined. As a new pest, there is some confusion concerning the best spray timing, with some growers spraying as soon as the weevils are observed in the field, with others waiting until the five node stage of the pea crop. The timing challenge though, is that the adult weevil arrives in the field in waves, so spraying too early means the later waves might be missed; and too late means the early wave of females might have laid their eggs before being controlled.

“Ideally, you need to control the females before they start laying eggs, but that can be difficult to do or predict,” explains Cárcamo, and adds that a goal for 2008 will be to improve the understanding of the timing and egg laying pattern of the female weevils – not an easy task since the eggs are less than half a millimetre and there are thousands in a square metre of soil.

Matador insecticide received Minor Use registration for pea leaf weevil control in 2007 and was the insecticide used in the trials. The Matador label recommends, “make first application after pea emergence but before the five to six node stage. Apply when adults are still present in the plants before egg laying begins.” Cárcamo is trying to refine that recommendation and his research looked at spraying the adults when the pea crops were in the two to three node stage, five to six node stage and seven  to eight node stage.

Integrated crop management specialist Scott Meers is also involved in the research and says that small plot work with insects is challenging because the insects can move from the control plot into the other treatments. This means the treatment effects can be masked and that indeed did happen in 2007.

“Part of the problem was that the infestation levels were so high in the plots that while we could see fairly obvious control with the insecticide, the yield results don’t look much different because the plots were overwhelmed by large numbers moving back in from unsprayed areas,” explains Meers. For 2008, Cárcamo plans on including field scale trials, with plots measuring 50 by 100 metres.

Under lower infestation levels, Cárcamo thinks spraying early would be better than spraying late, and he hopes that another component of the research will also help to answer that question. This component is looking at the amount of egg laying that happens at each growth stage. The researchers will collect soil samples, immerse the soil in water to float the eggs to the surface and then count the eggs – an extremely laborious process.

Economic thresholds have not been established for the prairies, with spray decisions being made using an Idaho threshold: if 30 percent (three in 10) seedlings have feeding damage on the terminal leaf at the two to three node stage, insecticide spraying is recommended.
Cárcamo cautions to go beyond the field edges when scouting for damage, as weevil feeding is usually heavier along field boundaries.

The research also looked at the effect of Cruiser seed treatment on weevil control. However, these plots were also overwhelmed by the large infestations of weevils, so the researchers are still trying to determine the effectiveness of the seed treatment. They need to analyze the degree of damage that was done to the nodules.

Syngenta Canada also conducted research trials with Matador and Cruiser. Technical manager of Seed Care with Syngenta, Ted Labun says the company is assessing research conducted in 2007 and experience from large scale commercial trials initiated by growers who used Cruiser Maxx Pulses, a combination of Cruiser and a complete fungicide package. He says that in 2007, Cruiser definitely reduced the amount of damage in the field when comparing 40 acres of treated versus 20 acres of untreated. Matador also worked well in reducing damage in the field.

“When we watched the impact of Cruiser on the adult, we observed them on their back, kicking their legs in the treated areas,” explains Labun. There is no doubt Cruiser has an impact on adult pea leaf weevils. However, more research is needed to understand what that means in terms of egg laying and the larval stage.

Cárcamo’s research also intersects with the seasonal activity of the insect. The goal is to understand when the larvae are active and feeding on the nodules. This research would help to clarify whether Cruiser works by controlling the adult weevils, or whether it has activity on the feeding larvae as well.

Yield loss studies are also looking at the affect of nitrogen (N) fertility on larval feeding. Cárcamo hypothesizes that with higher N fertility, there will be lower yield losses because the peas are not as dependent on the nodules for N nutrition. The belief is that the larvae are specialized to feed on only the nodules and do not feed on the roots.

Seeding dates also appear to be important, with early seeded pea crops being more susceptible since they act as a magnet for emerging weevils. This research will help to develop trap crop strategies, where the first several rounds could be seeded early, or a winter pea crop could be seeded on the first several rounds. Trap crops may have success in concentrating the adults, allowing more effective control with insecticides or with possible biocontrol methods in the future.

Researcher Ken Coles with the Southern Alberta Research Association had two trap crop field studies in 2007. The first used a winter pea trap crop. The winter pea was sown around the outside edge of the field with one pass of the seeder in the fall. In the spring, the inside of the field was sown with regular spring peas. “The theory is that the weevil adults will be drawn to the trap crop early in the spring, where it can be sprayed. This would hopefully mean that the rest of the field doesn’t need to be sprayed,” explains Coles.

The second approach was to use a spring field pea seeded very early around the outside of the field, approximately the week of April 25. This trap crop was treated with Cruiser. The remainder of the field was sown several weeks later during the week of May 19 without Cruiser.

Coles observed that there was definitely a concentration of feeding on the outside edge in the winter pea field. However, because populations of weevil were so large, there was enough weevil feeding in the rest of the field that the grower sprayed the entire field. Similar results were observed on the spring Cruiser trap crop, as well.

Cárcamo says that he hopes to find funding to start research into biocontrol with beneficial insects. He says that a screening program needs to be set up to look for parasitoids and if they are not found here, then Europe and Asia may hold potential. Observing economic threshold levels also helps to avoid spraying beneficial insects that may be occurring naturally here, which could help them eventually control the pea leaf weevil.

Like most insect pests, the research will likely produce not one magic bullet, but a series of integrated pest management strategies. While the three year study will not be completed until 2010, pea and fababean growers, in the meantime, should continue to scout their fields and consider spraying when economic thresholds are reached. Growers in areas where pea leaf weevil numbers were high in 2007 may also want to consider the use of Cruiser seed treatment if its Emergency Use permit is renewed for 2008. Using trap crops and staggered seeding dates may also help manage the pest.