Bats can help scientists track invasive insects

Identifying the arrival of invasive species before they become a problem takes some detective work. Environmental biologists are turning to environmental DNA (eDNA) to monitor species of interest in a similar way to how forensic investigators use traces of DNA at the scene of a crime. In bodies of water where invasive species like Asian carp and the American bullfrog are suspected, minute traces of DNA in a water sample can alert authorities before they are even seen. The Brown Marmorated Stink Bug (BMSB) is an invasive insect that is a pest to fruit producers.

To track BMSB, researchers at the State University of New Jersey, USA had the clever idea to let bats collect insects – in their mouths, as they normally do. When they finish digesting their meals, bats conveniently deposit a record of insects on their menu in the form of guano pellets in their roost, which can be collected and analyzed. The team recently reported in PLoS ONE that they indeed picked up traces of BMSB DNA in the droppings of the big brown bat and suggest using their method as an invasive insect monitoring tool.

For both the Northeastern U.S. and Ontario Canada, agricultural authorities have the BMSB on their unwanted lists. BMSB likes to eat agricultural produce like apples, tomatoes and corn. They poke their straw like mouths into fruits and vegetable and inject digestive enzymes which causes the fruits and vegetables to scar which doesn’t just look bad, but affects taste and texture too. I’m pretty sure I have bitten into apples with these scars but I didn’t know they were caused by BMSB until I found one in my house last December.

The BMSB I found in my house

When my kids wanted me to look up the bug’s identity on the internet I found it had the telltale double stripes, distinguishing BMSB from other stink bugs, on it’s antennae and was prompted by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) website to report my finding to them.

Black light traps attract BMSB only at certain times of year but they can only attract the insects from close proximity of 2-100m so miss a lot of these bugs. Visually looking for the bugs can be variable depending on the person doing it and sweeping with nets to collect bugs can damage produce. The New Jersey research team thought they could cast a wider and less disruptive net by taking advantage of the large foraging area of bats.

Many farms and orchards encourage bats to roost to help control insects. The researchers gathered big brown bat guano pellets once per week at three fruit orchards in New Jersey, where bats were roosting in the attic of a farmhouse, the loft of a packing house and roost boxes. DNA was extracted from these pellets and BMSB DNA was detected using Polymerase Chain Reaction (PCR), which is a method of making copies of target segments of DNA. Without PCR you would not detect BMSB DNA. Even with PCR, detecting BMSB DNA in bat guano is a feat. BMSB DNA would make up only a small fraction of DNA available from guano and after going through digestion and then sitting in poop would be of poor quality as well. Recent advances in PCR sensitivity have made this possible. Advances in DNA sequencing technology which make it quicker and cheaper also come into play because PCR also needs some knowledge of the DNA sequence in the segment for which you want to make copies. Adapting this monitoring system to pick up other insects would be easy to do.

While big brown bat guano can be used to detect the presence of BMSB in the area more work needs to be done to compare how this method stacks up against other conventional methods like black light traps, pheromone traps and timed visual counts. The team identified BMSB from bat guano eDNA earlier in the season than black light traps, suggesting it might be more sensitive. The high sensitivity of PCR can also be a curse because great care must be taken not to have contamination between samples. Training of personnel, time to do the work and purchasing of equipment and molecular biology materials to do the detection will have to be weighed against the benefits. At least big brown bats are doing part of the work for free and didn’t need any training or equipment.

More about PCR

DNA has two complementary strands stuck together. For PCR, samples are heated to 96^oC to melt the two strands apart. Then upon cooling to 60^oC, two short stretches of DNA sequence designed by the researchers to match only this particular BMSB segment of DNA, called primers, bind to the places where they match up. The primers each bind opposite strands and have front and rear ends facing opposite directions like two cars in opposite directions on each side of the street. Next an enzyme added to the reaction called a DNA polymerase, which normally does the job of copying DNA when cells divide, copies the DNA sequence from that primer onward. Think of an asphalt paver. After multiple cycles of heating up, binding primers and copying DNA, many copies of that DNA region between the two primers are created.

Here a form real-time PCR was used where an additional BMSB specific probe with a fluorescent label was added. It binds to the BMSB DNA down the road between the PCR primers and the dye gets activated only when the DNA polymerase enzyme runs through the fluorescent primer. A camera can detect the DNA copies being made as the PCR reaction proceeds. Not only is the detection of the dye very sensitive to low DNA levels, it cuts down on false positives. If PCR primers stuck non-specifically to non BMSB DNA and made some copies of any other DNA, they wouldn’t show up as false positives because the fluorescent probe wouldn’t get activated.

Bat image attribution: “bats roosting under bridge” by USFWSmidwest is licensed under CC BY 2.0

Reference

Maslo B, Valentin R, Leu K, Kerwin K, Hamilton GC, Bevan A, et al. (2017) Chirosurveillance: The use of native bats to detect invasive agricultural pests. PLoS ONE 12(3): e0173321.

https://doi.org/10.1371/journal.pone.0173321