Blog written by Angela M. Mech, Matthew P. Ayres, and Daniel A. Herms. Read the full paper here.
When we hear the word ‘Alien’, most of us conjure up images of extraterrestrials: terrifying, sharp-toothed, drooling, ugly creatures from outer space that are out to destroy us. The truth of the matter is that our fear regarding these aliens is not completely irrational – we don’t know what they would want, we don’t know how dangerous they could be, and we don’t know if we have any weapons that could stop them if they did turn out to be dangerous. When it comes to alien (non-native) herbivorous insects, we have the same fears for the same reasons. We don’t know if they will wipe out our native plants or permanently alter our ecosystems, we don’t know how much they will cost us, and we don’t know if we’ll be able to stop them. This fear exists even though we know that only a small percentage (~15%) of non-native insects actually damage our forests. One of the greatest challenges we face is that we don’t know whether a newly established insect will be harmless or a high-impact invader that causes widespread tree mortality. Our fears may be alleviated if we could somehow predict how much damage a non-native insect would cause before it even gets here. Towards that end, we need to understand what factors drive insect impact. In other words, what ingredients are in the recipe for destruction?
Approximately 450 non-native herbivorous insects have established in North American forests (Aukema et al. 2010). Since the late 1800’s, researchers have been trying to find a recipe that predicts their impact, with most hypotheses assuming that evolutionary history is probably one of the main ingredients. In our study, we evaluated multiple ingredients as potential predictors of insect impact, including those related to the evolutionary relationships between insects and trees. We examined four main categories: 1) insect traits such as the number of eggs laid, feeding method, and reproductive strategy; 2) host traits such as tree growth rate, wood density, and shade tolerance; 3) evolutionary history between the novel and coevolved hosts (i.e., how many million years ago the non-native insect’s new host and native host shared a common ancestor), and; 4) evolutionary history between the non-native insect and the species that coevolved with the new host tree (i.e., what is the closest insect relative that coevolved with the novel host?).
For our study, we focused on non-native insects that feed exclusively on conifer trees (conifer specialists). Conifers made a good model system because they are well studied (including evolutionary history) and of great importance ecologically and economically; 58 non-native insects in North America are considered conifer specialists. We found that divergence time to the most recent common ancestor between the insect’s native and new conifer host was a strong predictor of insect impact. For example, sap-feeding conifer specialists are more damaging on trees that diverged ~12-17 million years ago, with a 27% chance that they will cause widespread tree mortality, but that probability drops to as low as 1 in 650 million as the trees become more distantly related. In regards to host traits, we found that trees that were both shade tolerant and drought intolerant have a higher chance of experiencing mortality from a non-native insect than trees without these traits (26% versus 1.4% respectively). We also found that the chances of the non-native insect causing high impact was ten times lower if there was a North American insect in the same genus already utilizing the host tree. Surprisingly, there was no relationship between insect traits, singularly or in combination, and the probability that a non-native insect would be high-impact. Our research is the first that we know of to quantify the risk that a non-native insect will become a high-impact pest based on evolutionary history.
Most recipes typically have more than one ingredient, and we hypothesized that the same was probably true with predicting impact. In fact, we found that combining the three predictive models into a single model increased the strength of predictability beyond any of the individual models. From random selections of insect species that were high impact and not high impact, our composite model correctly identified the high-impact species 91% of the time. For example, one of the most damaging non-native conifer specialists in North America is the balsam woolly adelgid (Adelges piceae), which is causing mortality of a number of native fir species. These insect-tree combinations have the following category traits: 1) North American fir trees are shade tolerant and drought intolerant, 2) North American fir trees diverged from the European silver fir 13.6 – 16.8 million years ago, and 3) none of the North American fir trees coevolved with an Adelges species; the perfect recipe for mass destruction.
Our model provides a practical means for managing environmental security in the age of globalization. We can now better predict the impact that a non-native forest insect would have before it invades North America. Specifically, our combined model predicts whether a non-native, conifer specialist insect will have a one in 6.5 to a one in 2,858 chance of becoming high impact. This could have practical value in our struggles to limit high-impact non-native invasions. For example, we can now produce more quantitative and reliable watch lists of potential new pests, which could let us prevent their arrival in the first place. This knowledge will hopefully be a powerful weapon to help keep the next high-impact alien insect from becoming our next nightmare.
Aukema, J. E., McCullough, D. G., Von Holle, B., Liebhold, A., Britton, K., & Frankel, S. J. (2010). Historical accumulation of nonindigenous forest pests in the continental United States. Bioscience,60, 886-89. doi: 10.1525/bio.2010.60.11.5