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Watch out for bollworms

Old World bollworm may be a severe threat to agriculture in North America

Asian soybean rust, aflatoxin in corn and Palmer amaranth are but three examples of disease and weed species that have made huge news in the U.S. but haven’t yet crossed the border into Canada in a big way.

It makes it difficult to issue a credible alert about a new pest threat called Helicoverpa armigera or Old World bollworm (OWB). It’s a little like forecasting a severe storm, in fact. If you don’t warn people, they can be caught off guard with disastrous results. If you do warn them, and if the storm doesn’t appear, they can stop listening to future alerts.

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Scientists are already concerned that their alerts about soybean rust, aflatoxin and Palmer amaranth are producing yawns, not action.

Despite the alerts when Asian soybean rust landed in the Gulf States late in 2004, for instance, soybean growers in Eastern Canada rarely see the disease, and when they do, it’s usually so late in the season that there’s little if any damage.

Similarly, aflatoxin remains mostly a concern in drought-prone regions of the western Corn Belt in the U.S. and Palmer amaranth is having a tough time establishing itself against more prevalent species such as Canada fleabane and giant ragweed.

Now there’s OWB, the latest South American import which could have an impact on Canadian growers, depending on weather conditions from year to year.

According to Dr. Bill Hutchison, an insect specialist at the University of Minnesota, H. armigera is a lepidopteran species, similar to the European corn borer (ECB), and poses a wide range of concerns for growers as well as for entomologists, advisers and retailers.

OWB is a broad-based threat because of its diverse dietary preferences including corn, soybeans and wheat, along with sorghum and cotton, and even tomatoes and lettuce.

In all, OWB can feed on nearly 200 plant species in at least 45 families, according to a report from the Florida Department of Agriculture and Consumer Services.

Hutchison, as well as researchers with the U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) and universities in Australia and Brazil, issued a report in the spring of 2015 on the moth’s advance. Among other findings, researchers estimated OWB could cause $843 million a year in crop losses, just in its optimal climatic area. If it migrates beyond those borders, total losses could soar to near US$80 billion.

But could bollworm have that kind of impact in Ontario and Eastern Canada? That depends, says Hutchison, pointing to one pest assumption that did turn out to affect Ontario: the western bean cutworm has become a major economic pest in the province.

Another consideration, says Hutchison, is how often Ontario grain and sweet corn growers are forced to deal with corn earworm (Helicoverpa zea) — a very close relative of bollworm that can also migrate considerable distances.

“(Earworm) is well known to not likely overwinter in Ontario, and must therefore reach your growers each summer by long-distance migration, which it actually does quite well and consistently,” says Hutchison. “And it reaches your corn in late July to early August, when much of your corn may still be tasseling and silking and it is most attractive to corn earworm.”

In September 2014, OWB was detected in the U.S. territory of Puerto Rico. In response, extensive trapping was conducted on the island with the capture of 193 additional moths to date. With the expected establishment of the pest in the Caribbean, there is an increased risk for natural movement of OWB into the continental U.S.

Three OWB moths were trapped in Florida in early summer 2015, but no additional moths have been seen since then.

Once OWB is confirmed as “established” in the southern U.S., the risk to Ontario growers will depend on the number and magnitude of migratory flights northward each summer. With Asian soybean rust in early 2005, it was said that it would take one severe storm system born in the Gulf of Mexico to “blow up” rust spores into the U.S. Midwest and Ontario. Although that has yet to happen with any severity with soybean rust, the bollworm moths are capable of flying to high altitudes, particularly if and when food is scarce in one region.

“Because of migration, they indeed have the potential to easily cover the entire growing region of a given state or province,” says Hutchison.

Identification and control issues

What makes OWB particularly daunting in an if-and-when scenario is the difficulty that goes with identification and the fact that the species has developed resistance to most chemical pesticides. On a purely visual level, bollworm is identical to corn earworm in all stages of its life, and is differentiated only by dissection or genetic identification. The worms of the bollworm are also similar in appearance to the tobacco budworm and some species of armyworms, although those can be distinguished by colour patterns.

Based on what growers in Europe, Africa and Asia have encountered, insecticides do not work very well on bollworm: the species is capable of developing resistance very quickly.

The other concern with a North American arrival is the pest’s potential “hybridization” with corn earworm. The hybrid moths would produce larvae that could be difficult to manage, as they would appear to be corn earworm, yet potentially carry higher levels of resistance to insecticides or Bt corn. And that would complicate options for rapid identification and managing the pest.

One measure that seems to show some level of control comes out of Australia, where Bt cotton has proved useful. It’s believed (but has yet to be confirmed) that Bt technology used in corn hybrids and soybean varieties may also prove valuable.

About the author

CG Production Editor

Ralph Pearce

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