Ground Beetles as Weed Seed Predators

As diurnal animals, we are generally unaware of the slew of animal activity that occurs during the night. Even if we were to venture out in the dark, we still wouldn’t be able to detect much. Our eyes don’t see well in the dark, and shining a bright light to see what’s going on results in chasing away those creatures that prefer darkness. We just have to trust that their out there, and in the case of ground beetles, if they’re present in our gardens we should consider ourselves lucky.

Ground beetles are in the family Carabidae and are one of the largest groups of beetles in the world with species numbering in the tens of thousands. They are largely nocturnal, so even though they are diverse and relatively abundant, we rarely get to see them. Look under a rock or log during the day, and you might see a few scurry away. Or, if you have outdoor container plants, there may be a few of them hiding out under your pots with the pillbugs. At night, they leave the comfort of their hiding places and go out on the hunt, chasing down grasshoppers, caterpillars, beetle grubs, and other arthropods, as well as slugs and snails. Much of their prey consists of common garden pests, making them an excellent form of biological control. And, as if that weren’t enough, some ground beetles also eat the seeds of common weeds.

Harpalus affinis via wikimedia commons

Depending on the species, a single ground beetle can consume around a dozen seeds per night. In general, they prefer the seeds of grasses, lambsquarters (Chenopodium album), pigweeds (Amaranthus spp.), and various plants in the mustard family (Brassicaceae). The seeds of these species are small with seed coats that are easily crushed by a beetle’s mandibles. Providing suitable habitat, avoiding insecticides, and minimizing soil disturbance (i.e. reducing or eliminating tillage) are ways that healthy ground beetle populations can be encouraged and maintained. Ground beetles prefer dense vegetation where they can hide during the daytime. Strips of bunchgrasses and herbaceous perennials planted on slightly raised bed (referred to as beetle banks) are ideal because they provide good cover and keep water from puddling up in the beetles’ hiding spots.

The freshness of weed seeds and the time of year they are available may be determining factors in whether or not ground beetles will help control weed populations. A study published in Weed Science (2014), looked at the seed preferences of Harpalus pensylvanicus, a common species of ground beetle that occurs across North America. When given the choice between year old seeds and freshly fallen seeds of giant foxtail (Setaria faberi), the beetles preferred the fresh ones. The study also found that when giant foxtail was shedding the majority of its seeds, the density of beetles was on the decline, meaning that, at least in this particular study, most of the seeds would go uneaten since fewer beetles were around when the majority of the seeds were made available. Creating habitat that extends the ground beetles’ stay is important if the goal is to maximize the number of weed seeds consumed.

Harpalus pensylvanica via wikimedia commons

Of course, the seeds of all weed species are not considered equal when it comes to ground beetle predation. Several studies have sought to determine which species ground beetles prefer, offering seeds of a variety of weeds in both laboratory and field settings and seeing what the beetles go for. Pinning this down is difficult though because there are numerous species of ground beetles, all varying in size and activity. Their abundances vary from year to year and throughout the year, as do their food sources. Since most of them are generalists, they will feed on what is available at the time. A study published in European Journal of Entomology (2003) found a correlation between seed size and body mass – small beetles were consuming small seeds and large beetles were consuming large seeds, relatively speaking.

Another study published in European Journal of Entomology (2014) compared the preferences of ground beetles in the laboratory to those in the field and found that, in both instances, the seeds of field pansy (Viola arvensis) and shepherd’s purse (Capsella bursa-pastoris) were the preferred choice. The authors note that both species have lipid-rich seeds (or high “energy content”). Might that be a reason for their preference? Or maybe it’s simply a matter of availability and “the history of individual predators and [their] previous encounters with weed seed.” After all, V. arvensis was “the most abundant seed available on the soil surface” in this particular study.

Pterostichus melanarius via wikimedia commons

A study published in PLOS One (2017), looked at the role that scent might play in seed selection by ground beetles. Three species of beetles were offered the seeds of three different weed species in the mustard family. The seeds of Brassica napus were preferred over the other two by all three beetle species. The beetles were also offered both imbibed and non-imbibed seeds of all three plants. Imbibed simply means that the seeds have taken in water, which “can result in the release of volatile compounds such as ethanol and acetaldehyde.” The researchers wondered if the odors emitted from the imbibed seeds would “affect seed discovery and ultimately, seed consumption.” This seemed to be the case as all three beetle species exhibited a preference for the imbibed seeds.

Clearly, ground beetles are fascinating study subjects, and there is still so much to learn about them and their eating habits. If indeed their presence is limiting the spread of weeds and reducing weed populations, they should be happily invited into our farms and gardens and efforts should be made to provide them with quality habitat. For a bit more about ground beetles, check out this episode of Boise Biophilia.

Further Reading:

Selections from the Boise Biophilia Archives

For a little over a year now, I’ve been doing a tiny radio show with a friend of mine named Casey O’leary. The show is called Boise Biophilia and airs weekly on Radio Boise. On the show we each take about a minute to talk about something biology or ecology related that listeners in our local area can relate to. Our goal is to encourage listeners to get outside and explore the natural world. It’s fascinating after all! After the shows air, I post them on our website and Soundcloud page for all to hear.

We are not professional broadcasters by any means. Heck, I’m not a huge fan of talking in general, much less when a microphone is involved and a recording is being made. But Casey and I both love spreading the word about nerdy nature topics, and Casey’s enthusiasm for the project helps keep me involved. We’ve recorded nearly 70 episodes so far and are thrilled to know that they are out there in the world for people to experience. What follows is a sampling of some of the episodes we have recorded over the last 16 months. Some of our topics and comments are inside baseball for people living in the Treasure Valley, but there’s plenty there for outsiders to enjoy as well.

Something you will surely note upon your first listen is the scattering of interesting sound effects and off the wall edits in each of the episodes. Those come thanks to Speedy of Radio Boise who helps us edit our show. Without Speedy, the show wouldn’t be nearly as fun to listen to, so we are grateful for the work he does.

Boise Biophilia logo designed by Sierra Laverty

In this episode, Casey and I explore the world of leaf litter. Where do all the leaves go after they fall? Who are the players involved in decomposition, and what are they up to out there?

 

In this episode, Casey gets into our region’s complicated system of water rights, while I dive into something equally complex and intense – life inside of a sagebrush gall.

 

In this episode, I talk about dead bees and other insects trapped and dangling from milkweed flowers, and Casey discusses goatheads (a.k.a. puncture vine or Tribulus terrestris) in honor of Boise’s nascent summer celebration, Goathead Fest.

 

As much as I love plants, I have to admit that some of our best episodes are insect themed. Their lives are so dramatic, and this episode illustrates that.

 

The insect drama continues in this episode in which I describe how ant lions capture and consume their prey. Since we recorded this around Halloween, Casey offers a particularly spooky bit about garlic.

 

If you follow Awkward Botany, you know that one of my favorite topics is weeds. In this episode, I cover tumbleweeds, an iconic western weed that has been known to do some real damage. Casey introduces us to Canada geese, which are similar to weeds in their, at times, overabundance and ability to spawn strong opinions in the people they share space with.

 

In this episode, I explain the phenomenon of marcescence, and Casey gives some great advice about growing onions from seed.

 

And finally, in the spring you can’t get by without talking about bulbs at some point. This episode is an introduction to geophytes. Casey breaks down the basics, while I list some specific geophytes native to our Boise Foothills.

 

2018: Year in Review

Another year has passed, which means it’s time again for a Year in Review. As I have done in the past, I am including links to a selection of posts that came out in 2018. Most of these posts are part of ongoing series. You can find the rest of the posts from 2018 in the Archives widget on the right side of the screen (or at the bottom of the page if you are viewing this on a mobile device).

Among many memorable happenings this year, the one I feel most compelled to highlight here is a short radio show that a friend and I started doing on Radio Boise, our community radio station. The show is called Boise Biophila, and each week Casey O’Leary and I spend about a minute each talking about something biology or ecology related with the goal of encouraging people to get outside and take a closer look at the natural world around them. After the shows air, I put them up on our Soundcloud page for all to hear for years to come. You can follow us there, as well as on our Facebook page.

Ficus carica via PhyloPic

In the spring of 2018 we set up a Donorbox account, which is a simple way for people who enjoy Awkward Botany and want to see it continue to give us a little financial encouragement. We received several donations at that time, and we are very grateful to those that contributed. But just like public radio or any other organization that would like to receive your support in the form of money, we continue our plea. If Awkward Botany means something to you and you feel compelled to share some of your hard-earned dollars with us, we are happy to receive them and promise to put them to good use.

Donate

Money aside, a major contribution you can make to the success of Awkward Botany is to share it with your friends. Spread the word in conversation, through the postal system, over the phone, or through one or more of the myriad social media platforms. However you choose to share is none of our business. We are just happy that you do.

You are also encouraged to follow our various social media pages: Twitter, Tumblr, and Facebook. Above all, keep reading. We have lots more posts in the works for 2019, and we wouldn’t want you to miss out on the fun. Our appreciation for plants and the natural world is a constant, and we hope you will continue to share in our botany nerd revelry throughout the coming year.

Fragaria vesca via PhyloPic

Book Reviews:

Botany in Popular Culture:

Tiny Plants:

Field Trip:

Eating Weeds:

Two-parters:

Guest Posts: