BUG TALK: Tiger beetle is a hostile hunter in the agricultural world
Northern New York’s organic vegetable farms are hubs of insect activity.
Chirping cricket choruses rise above a buzzing backdrop of bumble bees, busily pollinating cucurbit crops. Staccato spurts of irrigation sprinklers punctuate the din. Swirling flurries of cabbage white butterflies erupt from shin-high kale forests, looping across the garden like the last flakes to fall in a snow globe. Such soporific droning belies the primordial aggression of an underfoot feeding frenzy.
One of the most hostile hunters in the agricultural expanse is the tiger beetle, a type of ground beetle in the family Carabidae. These coruscant carabids dash across sandy soil in search of caterpillars and smaller beetles.
Tiger beetles move so swiftly, their brains cannot process their surroundings quickly enough for the beetle to continuously hone in on prey. The beetles often pause, to rediscover the locations of their targets, and then reengage.
When entomologists Rewicz and Jaskula studied the tiger beetles Cicindela hybrida and Calomera littoralis, they noticed that C. hybrida most frequently consumed caterpillars, whereas C. littoralis consumed a roughly equal portion of caterpillars and other ground beetles. Their work was published in 2018, in the journal PeerJ. While C. hybrida is likely to benefit farmers by protecting crops from caterpillars, the role that C. littoralis plays in agricultural settings is unclear. By consuming other ground beetles, it might be diminishing the ecosystem services provided by these insects.
One example of an ecosystem service provided by ground beetles would be the predation of slugs. Slugs are harmful to salad greens and radishes. The ground beetle Pterostichus niger Schaller (Coleoptera: Carabidae) is one noteworthy consumer of slug juveniles and eggs. The presence of this beetle leads to synergistic effects when it coexists with the ground beetle Pterostichus melanarius Illiger, meaning that when these beetles are present together, the impact they have on slug populations is greater than one would expect if they added the individual effects of each beetle species.
The synergism may be due to the tendency of P. melanarius to kill slug eggs, whereas P. niger can control the mobile stages. Although P. melanarius can harm pest control efforts by eating spiders, which are predators of multiple insect pests, ground beetles tend to be more useful as slug deterrents when the beetles are present in a diverse community.
The trend that multiple ground beetle species are more useful than a single species is also demonstrated in weed control. For the control of weeds in grain systems, ground beetles in the genera Harpalus and Amara consume weed seeds as adults and larvae. Beetles in the genus Harpalus provision underground tunnels with weed seeds, so their larvae within them have food.
Although these beetles are regarded as free labor on the farm, ground beetles in the genus Zabrus can eat grain seeds, rendering them pests. The status of Zabrus as a crop consumer complicates the narrative that diverse ground beetle communities are necessarily better than less diverse systems; however, weed control is still generally improved by having more than one ground beetle species on a farm.
Ground beetles can enhance the predatory impact of other beneficial insects. For example, when ladybird beetles hunt aphids, the aphids will sometimes drop off the plant to avoid capture.
Upon hitting the ground, they normally escape the canopy-dwelling ladybird beetles, but they become prey for soil-dwelling ground beetles. Many canopy-dwelling ground beetles are nocturnal, whereas some ladybird beetles are diurnal. The presence of both ground beetles and ladybird beetles allows for continuous predation of aphids. This service is helpful to the farmer, because aphids are bugs that can harm plants by sucking nutrients out of them. They also excrete a sugary liquid called honeydew, which can lead to sooty mold on the leaves.
Due to the beneficial nature of many ground beetles, some farmers are trying to coax them onto cultivated lands. Beetle banks are one strategy for accomplishing this goal. By constructing berms or strips of native plants to act as habitat for the beetles, ground beetle community richness and abundance can be increased. Such beetle banks have been implemented at the University of Southampton, in the United Kingdom, where groundbreaking beetle research takes place. The trend is catching on in the United States, where the Xerces Society is providing more information about constructing these habitats. An additional benefit of the beetle banks is that they can support pollinators. If banks are used, then efforts must be taken to ensure the beetle bank plants do not contaminate cropland with whatever weeds are growing in the bank.
(Shane Foye was a volunteer at Cornell Cooperative Extension of Essex County over the summer.)