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Bad Bugs and Bad Trees

Is This Pair a Match Made in (Tree of) Heaven?

By T'ai Roulston

Arboretum Curator

You've likely heard of spotted lanternflies by now -- the newest exotic pest to threaten U.S. agriculture. First detected in Pennsylvania in 2014, they are now confirmed in New York, New Jersey, Delaware, Maryland, Connecticut, Massachusetts and now Virginia,  including Clarke County.  (We have yet to detect them in the Arboretum, but we're on the lookout). As adult insects in autumn, they are not subtle: These inch-long bulbous beasts, sporting black polka dots on pinkish-gray wings, cluster on plant stems by the hundreds.They have spread quickly and caused governments to quarantine merchandise moving in and out of infested areas. Frederick County, just to our west, and Winchester have had such a quarantine in place since May 2019. 

My goal here is not to repeat all the information you can find through university extension offices. Instead, I want to explain two common things we hear about spotted lanternfly biology that individually are straightforward but when put together seem puzzling. 

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First, spotted lanternflies are broad generalists. They are known to feed on more than 70 species of plants, including economically important crops, making them a widespread threat to agriculture. They have already caused substantial damage to grapes in the U.S. and are known pests of apples, peaches, and hops as well.

The other thing you may have heard is that spotted lanternflies strongly prefer the invasive tree of heaven ( Ailanthus altissima). In fact, they prefer this tree so much that the main control strategy for spotted lanternflies is a combination of removing most tree of heaven nurse trees while treating the remaining ones with systemic pesticides to kill the spotted lanternflies that find them.

So, are spotted lanternflies generalists that are primed to cut a broad swath through our crops, street trees and native plants, or are they specialists that mainly attack a noxious weed tree we'd like to kill anyway? Apparently both, but how does that work? 

	Richard Gardner, Bugwood.org

In late spring,  lanternfly eggs hatch  into wingless black and white insects that crawl to a nearby plant and start sucking the sap out of plant stems. At this stage, they have a broad diet and can be found on many different kinds of plants. They continue to feed on a variety of plants through their first three molts (until July) but become much choosier as they reach the fourth growth stage (fourth instar). At this point, they become bright red, and show a strong preference for tree of heaven. What happens if they can't find tree of heaven? They look for other plants and eat them instead. 

In our region, the next favorites seem to be black walnuts, maples, willows and grapes, which they can feed on abundantly in later developmental stages. How well they do on these other hosts, however, is not entirely clear. It is challenging to monitor the entire life cycle of spotted lanternfly to associate feeding choices with reproductive outcomes. As yet, no one can rear the pest from egg to adult to egg again in the laboratory to score success, so much detail awaits that breakthrough. But a recent study suggests an intriguing way in which tree of heaven, even if not required for development, could greatly improve spotted lanternfly performance and support larger populations.

Richard Gardner, Bugwood.org

Soorim Song at the University of California-Berkeley, along with colleagues in South Korea and Poland, studied the effects of host plant choice (tree of heaven versus willow) on the palatability of spotted lanternflies to Korean insectivorous birds in 2018 ( Scientific Reports #8, article 16831). Birds are often major predators of plant-feeding insects, so dense aggregations of large, slow-moving insects are likely to attract attention. The authors wondered if it was more than coincidental that spotted lanternflies shift from a broad to a narrow diet at the same time that the insect's coloring changes from an inconspicuous black and white to a very conspicuous bright red. Tree of heaven is known to contain toxic defensive compounds called quassinoids, and red coloration in insects is often associated with toxicity, so maybe spotted lanternflies were advertising their unpalatability to birds. The birds might sample one, have a bad experience, and remember to avoid the red things on the tree next time. 

The authors carried out three experiments to test whether spotted lanternflies become unpalatable by feeding on tree of heaven, all using the omnivorous bird  Parus minor (Oriental tit). First they showed that margarine balls (a bird treat) were less likely to be eaten by wild birds if they contained ground-up spotted lanternflies reared on tree of heaven. This shows the birds don't care for typical spotted lanternflies, at least with their margarine, but not that the effect is mediated by tree of heaven. Next, they worked with hand-reared birds that had never tasted spotted lanternflies and showed that these birds also preferred pure margarine, but that they were more deterred by spotted lanternflies reared on tree of heaven than those reared on Korean willow. They concluded that something in tree of heaven makes the insects less susceptible to bird predation. 

Finally, they showed that one compound in tree of heaven (ailanthone), if applied to sunflower seeds at bird feeders, deterred wild birds from visiting those feeders. In this way, even if it turns out that tree of heaven is unnecessary for spotted lanternfly development, the choice of host plants may leave them more -- or less -- susceptible to predators. 

At present, no one knows the palatability of spotted lanternflies reared on other plants when offered to North American birds. Let us hope our birds like them. And let us really hope that grape vines and hops plants produce especially tasty bugs that birds devour.