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Apr 7

Cereal Leaf Beetle threat can be fought by introducing wasps

Posted on Apr 7 By: Taryn Dickson

The parasitic wasp will lay its eggs into developing beetle larvae, which will then hatch from the inside of the beetle, killing the host.  Credit: University of Alberta
It has cousins that attack asparagus, cucumbers and potatoes, but these little insects may be interested in your cereal crop next. The cereal leaf beetle—Oulema melanopus L.—native to Eurasia made its North American debut in southwest Michigan in 1962. Fifty years later, it’s concerning Alberta farmers.

This four-to-six millimetre bluishblack and red adult beetle was first discovered in Alberta in 2005.

“Populations have been increasing year-by-year, and the patchy infestations seem to be worse as we move east from Lethbridge,” said Dr. Lloyd Dosdall, associate professor in Agricultural Entomology at the University of Alberta.

“In the United States, it’s a different story,” said Dr. Héctor Cárcamo, research scientist in Insect Pest Management for Agriculture and Agri-Food Canada (AAFC). “Here, we haven’t seen the populations exceed the economic threshold, which is one larva per flag leaf.”

One larva per flag leaf can cause a five to six bushel loss of grain per acre, but Cárcamo said Alberta is not even close to those numbers.

While any cereal crop can harbour these beetles, Cárcamo said the first cereal that is available for the insect to feed on, such as winter wheat or earlyplanted barley, would be the crop to show damage first.

The larvae can consume up to 10 times their body weight in plant biomass, leaving translucent strips at the site and golden/brown lower leaves.

The mature cereal leaf beetle is red, blue and black and has been found in Alberta since 2005. Its North American origins have been traced to southwest Michigan in the early 1960s.  Credit: University of Alberta
Camouflaged in their own fecal matter, they continue to grow and molt until mid- to late July, when they drop to the ground to pupate. Three weeks later, the adult beetle will emerge, feed briefly and prepare to overwinter in haystacks, within fields.

But don’t get your insecticides out just yet—this little pest didn’t arrive alone. Field studies reveal that populations of a parasitic wasp (Tetrastichus julis) specific to this beetle have also been found in fields.

According to Cárcamo, the parasitic wasp is both a beneficial and an effective natural enemy that can keep up with the pest populations.
“This tiny little wasp lays her eggs into the developing beetle larvae,” said Dosdall. “When the wasp larvae hatch from these eggs, they feed on the cereal leaf beetle from the inside out for twoand-a-half to three weeks, until they emerge from the beetle host, killing it in the process.”

It may be hard to see the beetle’s eggs, so Dosdall recommended scouting fields in late May to early June in order to spot emerged larvae.

“The best way for farmers to know if they have the cereal leaf beetle, is to go into their field on a calm day in late May to early June—as the larvae will hunker down on a windy day—and look on the flag leaf for feeding larvae,” said Dosdall. “The larvae will be quite visible if they are causing significant damage.”

The larvae always leave linear damage behind.

The beetle, seen here in its larva stage, can consume up to 10 times its body weight in plant biomass, leaving clear strips at the site and brown and gold patches on the lower leaves.  Credit: University of Alberta
“We are doing a lot to help re-distribute the parasitoid (wasp),” said Dosdall.
“We have independently discovered infestations of cereal leaf beetle in farmers’ fields, then brought in parasitized larvae to emerge and attack the farmers’ beetles.

“Once crops have the parasitoid, they should be good to go.”There are also a variety of other management strategies farmers can employ to prevent and manage this pest.

“A good sweep net goes a long way,” said Dosdall. “Sometimes they are hard to see on the leaves, but if we take a sweep through the crop, we get the larvae in the net.”

Dosdall advised against spraying insecticides.

“Spraying tends to do much more damage than good because it kills the parasitoid, too,” said Dosdall, adding that there are currently no registered chemicals for this pest.

Cárcamo suggested practicing conservation tillage or no tillage in order to conserve the parasitic wasp, which overwinters in the fields a few centimetres below the soil surface. He also advocates educating yourself on the economic threshold associated with this pest.

“It’s good if you can become a bit of an entomologist, to learn the biology of the insect and its natural enemy,” he said.

For more information, check out the full cereal leaf beetle article at Prairie Crops and Soils: www.prairiesoilsandcrops.ca. Dosdall and his associates are also available to assist those who think they have the pest: (780) 492-6893 or lloyd.dosdall@ualberta.ca

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