Posts Tagged parasitic wasps
Those of you who have read this blog for a while may remember when I first wrote about the invasive non-native insect called Emerald Ash Borer here. This insect species is killing almost (maybe all) ash tree species in North America — no joke. It started in areas like Canada and Michigan, and has been marching steadily southward ever since. Its occurrence is widespread in North Carolina. Dr. Kelly Oten, Forest Health Monitoring Coordinator for the North Carolina Forest Service, told me that confirmed sitings are reported for every county around me. The closest infestation she knows of is about 10 miles north of my five acres.
I had read about an experimental program Dr. Oten’s office is using to combat the Emerald Ash Borer (EAB) — the release of a parasitic wasp species native to the same part of Asia where EAB evolved. I believe the wasps used in NC parasitize EAB eggs by laying their own eggs inside EAB eggs. A couple of wasp species that parasitize EAB larvae also exist, as described in a US Forest Service publication on EAB biocontrols here.
I assumed that my little five-acre patch of Piedmont would be too small for this experimental wasp release program, but a forester friend of mine encouraged me to give Dr. Oten a call, so I did. I was delighted to discover that Dr. Oten was interested in the stand of 37 mature Green Ash trees growing on the floodplain portion of my land. However, she cannot release wasps unless she is certain the EAB is present on my land, because the wasps will die without a food source.
I have seen no evidence of EAB damage in my ashes <knock wood>, such as crown dieback and a yellowing of leaves (Here’s a link to a PDF from the Canadian Forest Service containing everything you need to know about detecting EAB damage.), so Dr. Oten suggested that we set up a couple of EAB traps at the appropriate time. That time is early April, because that’s when EAB egg-laying occurs, and it is EAB eggs that the experimental wasps look for to parasitize.
On the morning of April 11, Dr. Oten arrived with two traps to hang on a couple of my ash trees. The traps are shipped flat in pairs that are stuck together by the sticky fly-paper like glue used to snag passing EABs. In the above photo, she has successfully pulled the two purple traps apart and has begun to fold the one she is holding so that the sticky glue is on the outside of the three-sided trap. The traps are purple, she told me, because research shows this color attracts EABs most effectively, perhaps, it is theorized, because young ash leaves often possess a purplish hue.
Besides the EAB-preferred purple color, traps also contain a bag of scent lure that is hung inside the trap. Dr. Oten is holding one of those bags in the above photo (click on any photo to see a larger version). The scent emulates the smell of an ash tree in distress. Many studies have confirmed that plants engage in sophisticated chemical warfare with their insect enemies. In many plants, when a plant is under attack, it emits a scent signifying its distress, which in turn stimulates nearby plants of the same species to begin producing chemicals that may help them repel invading insects. This doesn’t work for the ash trees with EAB, because North American ash trees did not evolve with this insect; thus, they have not developed any defenses against EAB attacks.
After she attached lure bags to the center of the EAB traps, Dr. Oten used the long extension pole in the photo to attach the traps to sturdy horizontal branches on two ash trees at opposite ends of my floodplain. This turned out to be trickier than you might think, because my canopy-size ash trees don’t possess many horizontal branches within reach of the pole. Dr. Oten’s first attempt to hang the trap was unsuccessful; the sticky trap fell into a stand of bladdernut shrubs, thus becoming adorned with bits of bladdernut leaves and flowers. Her second attempt with a different tree was successful.
In the second photo above, you can see bits of bladdernut leaf and flower stuck to the trap. Dr. Oten said this will not interfere with the trap’s effectiveness in luring EABs.
Next, we slogged through the mud to the far side of my floodplain, where Dr. Oten selected a second ash tree suitable for trap-hanging. This operation went more smoothly than the first, and the trap was soon hung.
Dr. Oten explained that EABs are actively flying and egg-laying in my area from early April until about June. She plans to return to my floodplain in about four weeks to inspect the traps for EABs. If she doesn’t see any, she will add fresh lure bags and return in another four weeks. If no EABs are found on the traps during that time period, it is less likely — but not impossible — that EABs have found my ash trees — yet. If she does find EABs stuck to the sticky glue on the purple traps, she will release parasitic wasps into that area.
Note that the wasps are not expected to stop the demise of my ash trees. My understanding is that the introduction of the wasps is part of a long-game biocontrol strategy that may, perhaps decades from now, yield benefits. It is an entomological shot in the dark, as it were.
For me, helping with this experiment is far better than the two alternatives available:
- Doing nothing but watch the ashes decline as the woodpeckers feast on their dying remains full of EAB larvae, leaving behind a floodplain almost fully devoid of its canopy tree cover.
- Having an arborist inject systemic poisons into the trees. Besides the exorbitant expense (37 60-70-foot tall ash trees), the poisons kill any insect that takes a bite out of treated trees. In his classic book, Bringing Nature Home, Douglas Tallamy notes the number of different insect species that rely on native trees for food. For ash species, his number is 150; that’s 150 different insect species that rely on ash trees as a food source. So if you poison your ash trees to prevent EAB invasion, you will also potentially poison at least 150 native species of insects that rely on ash trees. Further, those now-dead insects — mostly caterpillars — would have fed myriad species of nesting songbirds, which also will likely now die from starvation.
You cannot break one link in the chain of life without affecting every other link. I pray every day that humanity figures this out — and acts on that knowledge — before so many links are broken that the chain cannot be mended.