They may sound tiny and harmless, but even when flea beetles don’t kill your seedlings, they can spread diseases like bacterial wilt and fungal blight. When these pestiferous plant-eaters arrive, you need to act quickly.
Talking about options for managing flea beetle in greens at the most recent Great Lakes Expo was Ashley Leach, Ph.D., assistant professor of specialty crop entomology at Ohio State. Specifically, she was looking at the crucifer flea beetle and the striped flea beetle, as management is similar between the two species.
Leach said these pests have a traditional lifecycle, laying their eggs at the base of the crops they prefer. One to three generations of flea beetles may emerge per year, based on climate conditions and the length of the growing season.
For example, in Ohio, growers may see population peaks in late May and late July. States with similar growing conditions will see similar trends.
It’s principally the adult beetles causing the shot hole damage in the leaves of greens, making them unmarketable. The adults are very mobile, so to monitor their populations, Leach said it’s best to use sticky cards.
“They have host preferences,” she explained. “It all depends on the volatiles the plants give off.” The Brassicas they most prefer are mustard greens, then bok choy, then collard greens. Lower on the list of preferred hosts are crops like broccoli and kale.
A heavy infestation of crucifer flea beetle on seedling broccoli. Photo courtesy of Whitney Cranshaw, Colorado State/Bugwood.org
Unfortunately, “most of what we know about these beetles comes from canola growers, who have different levels of the ‘acceptable’ number of holes per leaf,” Leach said. “But what can we copy-paste from them?”
She looked at canola growers’ insecticide programming, biological controls and cultural controls.
Specifically with insecticides, she wondered if there was anything better to use than pyrethroids. She and her team conducted a six-week trial, from May to July, to test out some different options. They found that Harvanta performed fairly close to the standard, but pyrethroids are still doing the best work.
For biological control, Leach’s team wanted to look at flea beetles’ soil dwelling stages (egg, pupa and larva). They hypothesized that entomopathogenic nematodes might work in the suppression of this pest population.
“We trialed Harvanta and pyrethroids with nematodes to test control in a six-week trial and saw only a slight reduction with nematodes alone,” Leach reported. “They did not synergize with the insecticides – but there is a lot of nuance here.” (Nematodes need moisture to thrive and this trial took place during summer 2023 under drought conditions.)
As for cultural control, Leach wanted to know if growers can use known host preferences to improve pest push-pull and decrease insecticide need. This simply means planting “sacrifice” crops to lure the beetles away from leafy Brassicas.
According to Montana State University, flea beetle damage to oilseed Brassica crops exceeds $300 million annually in North America, so the ongoing research in this field will benefit both those growing canola and those raising leafy greens for consumers.
by Courtney Llewellyn
