It may lack the drama of a lion taking down an antelope on the Serengeti, but that small beetle scuttling through your garden is just as predatory when it comes to going after weed seeds.
“Finding some of these bugs is not bad at all,” said Eric Sideman, organic crop specialist with the Maine Organic Farmers and Gardeners Association. “One of the founding principles of organic gardening is taking care of the soil and a spinoff of that is not harming the critters in the soil [because] they are all part of the ecosystem.”
In the case of the ground beetle Harpalus rufipes, they are especially helpful to Maine gardeners and farmers as seed predators.
“The ground beetle seed predators rank up there in my top four ways to manage weed seeds,” said Dr. Eric Gallandt, professor of weed ecology and management at the University of Maine. “In agriculture systems the key to weeds’ success is their ability to scatter their seeds on the surface of the soil [and] when those seeds are on the surface, the seed predators are able to get at them more easily.”
The more easily the ground beetles are able to help themselves to a meal of weed seeds, the better for for controlling the spread of those weeds.
University of Maine weed ecology doctoral candidate Sonja Birthisel looked at ground beetle seed predation in her masters thesis and found that where the beetles are allowed to roam and feed, fewer weeds are often observed.
“These beetles are something that naturally occur in the ecosystem that are good for farmers and gardeners,” Birthisel said. “I think they are kind of cute [and] they eat a fair number of weed seeds and prey on other pests like aphids and cutworms.”
The ground beetles — each about an inch or so long with black bodies and reddish legs — seem particularly happy when there is vegetative cover under which they can eat and move around, Birthisel said.
“If you are a farmer or a gardner, there are many reasons to grow a cover crop,” she said. “Even though you are not getting a crop out of it, it is useful for the soil and great for the beetles.”
Giving the beetles an environment in which they can thrive means a gardener or farmer will have that many more which will feed on weed seeds.
Given the beetles’ voricious seed appetite, it is understandable. Birthisel said, if growers fear the insects may turn their attention to the seeds that the growers want to sprout.
“One thing farmers and gardeners will ask me is, ‘so, these seed predators are great, but will they eat my crops?’” she said. “Usually not [because] the seed predators want to eat what’s on the surface and going down to get the seeds planted beneath the surface is just not worth the effort.”
Instead, she said, they are perfectly content to make a meal out of the weed seeds that naturally fall to the ground’s surface.
In studies looking at the impact these beetles can have. Gallandt said results are impressive, if a bit inconsistent.
“When we have put a bunch of seeds out on the soil and watched the beetles, there have been removal rates that are impressive,” he said. “But those can be misleading.”
In one study, Gallandt said, a 1-square-meter cage was placed in a field to prevent any animal or insect from eating the seeds within that cage.
“We looked at the density of the weed seeds inside the cage compared to the seeds outside the cage,” Gallandt said. “We did that for three years and in two of those years we saw no difference with seed removal but in one of those years there were 46 percent fewer weed seeds outside compared to the inside of the cage, so when weed seed predation does happen, there can be a substantial reduction in weeds.”
Any number of things can prevent the beetles from dining on the weed seeds, including seeds working their way below the soil due to rain or human practices or due to predation on the beetles themselves.
“I have done some work on who is eating these beetles,” Birthisel said. “Nobody had really studied what kind of animals were eating them.”
So, Birthisel decided to devise a pretty creative with a very hands-on experiment in which she trapped, bound and staked out beetles and then watched — with motion capture cameras — to see what predators ate them. It was raccoons and mice that came along to to enjoy a meal of beetle — served up almost on a silver platter.
To do so, Birthisel had to leash the beetles. With the help of her assistant — a former Eagle Scout who knew a thing or two about knots — Birthisel was able to pre-tie loops in the fishing line and then slip the loop over the head and first set of the beetles’ legs.
The other end of the fishing line was tied off to a tether made from a bent paperclip jammed into the soil.
“We know this was not an accurate reflection of nature because these beetles are really quite mobile but our setup did allow them a bit of movement and the ability to burrow into the soil,” she said. “It was a novel method and a little kooky, but it worked.”
When not staked out like the goat attracting the T-Rex in the movie Jurassic Park, ground beetles seem to do a pretty good job avoiding predation.
“Ground beetles are very common and I spot them all the time running around my garden,” Sideman said. “In gardening we say that we use ‘many hammers to control weeds’ and tractor cultivation, crop rotation or using a hoe are the big hammers and predatory beetles are among the hammers that can also help you.”
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