Tiny insects may save island’s black oaks

Over the last few years, homeowners and land managers have been watching the increasing insect damage to the island’s black oak trees. Last year, the Nantucket Conservation Foundation and the Nantucket Land Council teamed up to fund a student research project to study the oak gall wasp and parasitoids – tiny beneficial insects that prey on the gall wasp – to learn whether they may be mitigating the gall wasp’s impact on island black oaks ( Quercus velutina).

Many of the largest and oldest trees growing wild on Nantucket are black oaks, and this species is a key part of our island forests.Previous research by Monica Davis, then a Ph.D. student at the University of Massachusetts-Amherst’s Elkinton Entomology lab, led to the description and naming of the insect as Zapatella davisiae, and contributed information on the insect’s life cycle and possible origin (likely eastern North America).

The theory is that rapid population booms of beneficial parasitoid insects have brought the numbers of infesting gall wasps in Long Island – the earliest known infestation – to very low levels. The same pattern was observed in recent years on Martha’s Vineyard and then on Cape Cod as the gall wasp and its predators continued to spread throughout coastal forests.

But was this also occurring on Nantucket? Working in Joseph Elkinton’s UMass-Amherst lab, Cameron Smith-Freedman embarked on his senior-thesis research project in the fall of 2017, and this work has already yielded some interesting results.

By comparing densities of the harmful gall wasps in Nantucket branch samples to those collected on the Vineyard and the Cape, the status of the Nantucket infestation has become clearer. Each branch sample consists of this year’s growth at the tip, with prior years’ nodes (points of branching and growth) serving as a time series, offering a glimpse into the last several years as the gall wasp arrived and increased in numbers.

The branch nodes also contain a record over time of any beneficial parasitoids that may have been present and preying on the gall wasps. Elkinton suggests that the ramping up of beneficial parasitoids is likely what ultimately caused the decline of the pest elsewhere in the region, an ecological process known as “top-down regulation.”

It’s the same sort of situation as when gardeners introduce ladybugs or lacewings as beneficial insects to control aphids, but it’s something the ecosystem is doing on its own, without human intervention.

Due to the island’s isolation, the gall wasp infestation arrived later than in the surrounding region. Smith-Freedman now estimates arrival in 2011, with the first damage not becoming visible to observers until 2014, a typical scenario. At that time, Davis was still working to identify the insect and pinpoint its ecology, origin, and life history. That information is critical in deciding what, if anything, may be done to minimize damage from a newly-arrived insect pest.

Since then, gall wasps and their damage have increased on Nantucket, in home landscapes and in forests such as Masquetuck Reservation and Squam Swamp.

Over the space of just a few years, the insect was assigned a species name and it was confirmed that it was indeed the same insect that was causing infestations on black oak in Cape Cod, Martha’s Vineyard and on Nantucket.

Fortunately, Smith-Freedman’s latest data from Nantucket shows that we do have established populations of the beneficial parasitoid wasps. Using extracted DNA, the researchers worked to compare it to samples stored in a database called GenBank.

This method allows researchers to quickly and accurately identify insects to species compared to traditional methods. The DNA fragment works just like a barcode and is used to determine how a mystery organism is related to other creatures, or to identify it to species if it matches one already in the database.

At this point, Smith-Freedman’s data suggests that in 2017, beneficial parasitoids remained much lower on Nantucket than on Cape Cod or Martha’s Vineyard. But the pattern over time suggests that this year may mark the turning point, when the parasitoids will skyrocket in numbers and initiate a population crash in the harmful gall wasps.

The good news is that two species found to be important predators on the gall wasp are already here on-island and increasing in numbers, ready to lay eggs on the next generation of gall-wasp larvae – and hopefully bring the damage to our oaks down to a minimal level in the near future.

So what does this mean for our Nantucket black oaks? Some land owners have been treating their trees with injected herbicides, and it seems that another year or two of treatment may be enough to see them through.

Many of the trees have been weakened by several years of gall-wasp activity and some may not survive. But Nantucket trees often have unusual responses to disease or insect impacts, as seen with the beech bark disease which appears to have been present on Nantucket for decades without causing significant mortality in our gnarled old American beech trees.

Although injected (systemic) pesticide kills the beneficial parasitoids along with the gall wasps, there should be enough beneficial parasitoids in surrounding natural areas to regulate the gall wasps in the future. Homeowners who have been treating their most valued trees should stay the course for another year or two in hopes that the system is naturally “balancing” itself out as the beneficial insects increase and lower the numbers of the pests.

It should be noted that although infestations in home landscapes and particularly prized specimen trees can be managed with injected herbicide, large-scale treatment in natural areas is not economically feasible, and actually could slow the buildup of the beneficial wasps. This is definitely a case of “the enemy of my enemy is my friend,” as Smith-Freedman and Elkinton emphasize.

Also important to recall is the fact that oaks are ecological powerhouses, and their role is much greater than just producing acorns for wildlife. In entomologist Doug Tallamy’s study of native trees and shrubs, Mid-Atlantic oaks were found to support by far the highest number of Lepidoptera (butterflies and moths) of any tree genus studied. Systemic treatment with pesticides on a large scale would impact all of these species, which form the base of the food chain for many species of island birds.

Smith-Freedman presented his preliminary results at a talk in the Atheneum’s Great Hall April 23, and will return to collect more samples this summer to continue to study Nantucket’s ongoing oak gall wasp/parasitoid battle. Another snapshot in time will help answer the question of when we may see the end of the gall wasp boom.