It is important that officials responsible for phytosanitary protection, Congressional oversight committees, and stakeholders have access to key trade and pest data as well as independent analyses of them in order to evaluate programs’ effectiveness.
But we don’t have such access … and existing analyses cannot be used to detect trends.
My focus is on tree-killing insects and diseases, but these constitute a small fraction of the total number of all plant pests that have become established in North America since Jamestown was settled. According to Aukema et al. (2010), approximately 450 non-native insects have colonized forest and urban trees. This is about 17% (less than one-fifth) of the total of 3,540 non-native insects established in North America according to Yamanaka et al. (2015). The larger number includes ones apparently causing negligible harm, along with a significant proportion of insects and diseases affecting row crops.
What could we learn from comparisons of data on introduced tree-related vs. overall plant pests? Could we uncover new pathways? Identify more effective approaches to phytosanitary protection?
Unfortunately, neither published studies nor USDA/APHIS’ data allow comparisons and tracking of trends in pest establishment.
For example, a study by Work et al. (2005) estimated that during the late 1990s, approximately 10 new phytophagous insects were established each year. The authors considered all phytophagous insect pests, not just tree-killing pests; but they did not include pathogens or insects that feed on dead wood (e.g., termites).
The Work team’s number is about four times larger than the estimated rate of establishment provided by Aukema et al. (2010), which estimated that approximately 2.5 new tree-killing insects and pathogens became established each year from 1860 to 2006. The Aukema study did not attempt to track establishments of all pests that use arboreal hosts. On the other hand, it did include pathogens. So the two studies’ findings are not truly comparable.
In its 2009 Implementation Plan for Section 10201 of the Food, Conservation and Energy Act of 2008, APHIS reported that between 2001 and summer 2008, 212 plant pests had been reported as new to the United States – an average of 30 new pest establishments detected each year. This estimate does include pathogens … but not insects that do not feed on living plants. So it is not comparable to the Yamanaka study. Still, the APHIS figure is 12 times higher than the Aukema et al. estimate for tree-killing pests.
I am unaware of a publicly available estimate for more up-to-date establishment rates.
An internal USDA APHIS database was made available to me. It lists about 90 new species of plant pests (of all types, ranging from insects to nematodes to fungal pathogens) with populations that were detected in the U.S. during the four-year period 2009 – 2013. The rate of detection of “new” species established during this four-year period was approximately 22 per year. This establishment rate is higher than the estimate of approximately 10 new phytophagous insects per year during the late 1990s put forward by Work et al. — not surprising since taxa other than insects are included. However, this estimate is lower than the 30 new pest introductions each year estimated by USDA APHIS for 2001-2008.
So what is the current rate? How has the establishment rate been affected by changing volumes of imports over this 20-year period (imports rose until 2008, then fell because of the Great Recession)? How has the reported number of new establishments been affected by changes in monitoring program criteria and funding levels?
Do the databases include sufficient information about dates of probable establishment, likely pathways of introduction, etc., to allow a more complete analysis of at least the new insect species?
I have not seen the database compiled by Yamanaka’s team so I don’t know.
The USDA database from 2009-2013 does not specify the probable pathways by which these pests entered the United States. I have concluded that the viruses, fungi, aphids and scales, whiteflies, and mites were probably introduced via imports of plants, cuttings, or cut foliage or flowers. These pests number 37 – or 41% of the total.
The database on tree-killing insects and pathogens compiled by the Aukema team includes both date of probable introduction and likely pathway; and articles by this team discuss trends in introduction rates. Thus, Liebhold et al. 2012 reports that approximately 69% of the pests in the database were introduced via the trade in live plants. This figure is one-third higher than the proportion I calculated from the USDA database (which, I remind you, includes all plant pests, not only those that attack trees).
Many of the pests associated with imports of plants in the Liebhold study were introduced decades ago, before the U.S. adopted phytosanitary regulations. Does the difference in the proportion of pests associated with plant imports in the 2009-2013 period compared to the earlier period covered by Liebhold et al. reflect a reduced risk from this pathway as the result of tighter regulations and shifts in the market? I doubt anyone can say – beyond the acknowledged increase in wood-borers associated with wood packaging.
Without better, and more readily available, data, we won’t ever be able to answer key questions. It is urgent that APHIS make available its data on trade volumes, pest interceptions, newly established pests, etc., for analysis by academics, other agencies, and stakeholders. And certainly it would be helpful if both APHIS and other researchers used more consistent approaches so to make possible longitudinal studies that can disclose trends.
Sources:
Aukema, J.E., D.G. McCullough, B. Von Holle, A.M. Liebhold, K. Britton, & S.J. Frankel. 2010. Historical Accumulation of Nonindigenous Forest Pests in the Continental United States. Bioscience. December 2010 / Vol. 60 No. 11
Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live Plant Imports: the Major Pathway for Forest Insect and Pathogen Invasions of the US. www.frontiersinecology.org
Work, T.T.; McCullough, D.G.; Cavey, J.F.; Komsa, R. 2005. Arrival rate of nonindigenous species into the United States through foreign trade. Biological Invasions7: 323-3
Yamanaka, T., N. Morimoto, G.M. Nishida, K. Kiritani, S. Moriya, A.M. Liebhold. 2015. Comparison of insect invasions in NoAm, JP and their Islands Biol Invasions DOI 10.1007/s10530-015-0935-y
Posted by Faith Campbell