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Canola resists late-season flea beetles

Finding heavy infestations of flea beetles in late summer as canola matures can be frightening, yet until recently little was known about whether the insect could really cause economic damage at that time of year. During the drought years of 2002 and 2003, late-season flea beetle populations were high and growers were concerned about potential impacts on canola yields and quality. Researchers initiated a project in 2004 to determine the late-season feeding effects of flea beetles on canola. However, 2004 turned out to be one of the coldest, wettest springs in years, and flea beetle populations were at very low levels for the next several years.

“We continued the work in 2006 through to 2008 when we finally had enough flea beetles to do some field experiments,” explains Dr. Julie Soroka, research scientist with the Agriculture and Agri-Food Canada (AAFC) Saskatoon Research Centre. “We conducted three different experiments over each of the three years, including a natural infestation spray trial and two cage trials.”

The experiments included two canola seeding dates, an early date in the second week of May and a late date in the first week of June. For the natural infestation plot, researchers monitored the plots in late summer for natural flea beetle populations and then sprayed for control. “For the cage trials, we had to collect flea beetles, in some years from across the Prairies because of very low numbers, and then place a targeted number in the cages,” says Soroka. “Tall cage trials utilized metre square cages 1.5 metres high with a plant density of about 60 plants per square metre. Sleeve cage trials included one single plant covered with a sleeve cage.” Between 250 and 450 flea beetles per plant were placed in the tall cages and 100 to 150 flea beetles per plant in the sleeve cages in the late-summer trial. 

In the early-seeded canola there were no significant differences in any yield parameters between sprayed and unsprayed plots or infested and non-infested cage plots for the late-summer infestations. “In those plots, it did not matter how high the flea beetle populations were, seed yield wasn’t affected,” explains Soroka.

Damage rarely seen
The greatest difference in crop growth stages between early- and late-seeded plots occurred in 2008, when late-seeded plots were just beginning to ripen at the time of cage infestation. The results from the late-seeding experiments in 2008 showed no benefit to spraying in the natural infestation trial. “We did find yield loss in the tall cage trial, where the youngest plants at the translucent seed stage infested with about 350 flea beetles per plant resulted in a yield loss of 3.6 bushels per acre,” explains Soroka. “This was the only instance in the three-year experiment where we saw reduced seed yield, reduced seed weight and increased green seed levels. We did not see increased shattering that sometimes is attributed to late flea beetle feeding.”

The results show that in most cases late-feeding effects of flea beetles are rarely a concern and likely not the biggest worry for growers at harvest. “Two factors have to occur simultaneously in order for there to be damage – very young canola plants and very heavy flea beetle populations,” says Soroka. “These two conditions seldom occur at the same time. If there are very high late-season flea beetle populations, the summer probably has been hot and dry and the canola has very likely powered through to maturity.”

On rare occasions, if a grower ends up with a late green crop for whatever reason and under the right weather conditions, flea beetle infestation may be a problem. Susceptible canola plants have seeds in the lower pods that are full sized but have not yet turned green. Once they start to turn green brown or lime green, the pods are virtually impervious to flea beetle feeding. “However, even if a crop is susceptible, there may be little growers can do as the pre-harvest interval for chemical application may not allow for an application prior to harvest,” says Soroka. “We have not developed a late season economic threshold for flea beetles, although from the trials we know there would have to be more than 100 flea beetles per plant.”

Soroka emphasizes that the trial was conducted with flea beetles only, and the cage trials excluded any other pests. However, if growers are facing multiple pests, such as diamondback moth, bertha armyworm or lygus, the addition of late flea beetle infestations may be enough to tip the balance but is not likely the main problem. Again, observing the pre-harvest spray interval will dictate whether or not a spray application is possible.

So although researchers did not find any real problems stemming from late-season flea beetle infestations, it is still very important for growers to be monitoring and assessing the populations. “High flea beetle populations in the fall can be useful in predicting the potential of infestation the following spring,” says Soroka. “For growers, 90 percent of control efforts for flea beetles will be in the spring. Seeding early and ensuring good healthy stand establishment will usually help growers avoid any concerns over late-season flea beetle populations.”