Friday, September 13, 2013

Ground beetles are helpful beetles.

Over the past three years my doctoral student, Johan Pretorius, has been busy measuring the ecological services (i.e., benefits) provided by some of the insects commonly found roaming on the ground in sugar beet fields. We are working on sugar beet because the Western Sugar Cooperative has been very supportive of this work and it is a vital commodity for producers in western Nebraska. This work could (and hopefully will) be done in other crop settings and systems, but this is where we have started. Through our experiments and sampling, we have found that ground beetles (Carabidae) are the most prevalent group of arthropods within these western-Nebraska systems (sugar beets following maize). Other researchers have had similar observations with ground beetles being highly abundant in their agricultural systems as well. We have found about 40 species that reside in our sugar beets fields! The reason this is a very interesting and exciting find is that in recent years researchers have found that many ground beetle species provide a number of benefits to agricultural systems (if properly managed). All of these species have various micro-habitat, seasonality, and food preferences. Some species are strict seed feeders while others are voracious carnivores. This raises a couple questions: What are they doing in beet fields? How might their number be encouraged?

Some ground beetle are generalist predators and will feed on other ground beetles. It's thought that agricultural ground with higher residue (like sugar beet fields with minimum tillage following a maize crop from the previous year) provide greater habitat and shelter for more ground beetle species. That is, some of the smaller ground beetles that might otherwise be fed on by the larger predators have a place to hide. This, in turn, might free up time for those generalist predators to attack other insects (e.g., pest insects). At least, that could be one hypothesis. Others suggest that reducing tillage conserves habitat within the soil for the larvae of these ground beetles (they larvae are also soil dwelling and less mobile than adults) and thus increases their numbers within a field. The two ideas are probably not mutually exclusive.
Part of our studies have been focused on how tillage influences ground beetle habitat. However, we have also been very interested in the functional ecology of this group. Through our studies, it turns out that these beetles eat some of our pests insects common to sugar beet (e.g., the sugar beet root aphid) as well as some of our most tough-to-control weeds (e.g. kochia, foxtail, lambsquarters, and baryard grass). The video shows one of our bioassay choice tests. The beetles in the petri dishes are being given a choice between the seed of the four above-mentioned weed species. In this video you can see Harpalus eraticus (labeled as HE) and Amara carinata (labeled as AC). Some of the beetles are clearly more actively moving that others. We found Harpalus eraticus to be a voracious seed feeder; a single individual consumes as many as 20 Kocia seeds in a 12-hour time period! A single Kocia seed can produce a plant that will in turn produce tens of thousands of seed. So, the impact potential of this one species might be huge! With our growing problems with herbicide-tolerant weeds, our understanding of the role of ground beetles in agriculture and what we might do to encourage their numbers seems of growing importance.