So Many Shot Hole Borers: New Research Charts Four Nearly Identical Species
By Jiri Hulcr, Ph.D., and Jackson Landers
When an insect spends most of its life in total darkness, it doesn’t much matter what color it is. So, it comes as no surprise that so many species of bark and ambrosia beetles maintain the same brown hue as they slowly tunnel through wood and feed on a fungus that they carry with them into their trees. This similarity of appearance has been taken to an extreme in what has turned out to be a cryptic species complex. What was once referred to as the “tea shot hole borer” is actually four distinct species who all look almost exactly the same.
The tea shot hole borer first attracted notice from North American entomologists when it appeared in Florida in 2012, appearing harmless. But the beetles were first described as an economically significant pest in Sri Lanka in 1968. In 2009, the beetles were found eating through avocado and street trees in Israel. Then, in 2012 in California, with a bang, avocado trees were being attacked and killed. Given the value of global avocado crops and tea plants, entomologists had to start taking a closer look at these beetles.
Many taxonomists eventually came around to the idea that they were looking at three identical species rather than one: the tea shot hole borer (from southern Southeast Asia), the Kuroshio shot hole borer (originating in the Pacific Islands), and the polyphagous shot hole borer (presumed native to northern Southeast Asia). A 2017 paper authored by Richard Stouthamer and his team from the University of California, Riverside, first designated those three clades and established common names for them.
A species complex is a group of species that are closely related to each other and may have only recently diverged from a common ancestor. The various species of Galapagos finches described by Charles Darwin in his Origin of Species are a well-known example. For a species complex to be considered “cryptic,” it means that those individual species look so similar that they have been mistaken for one another.
Cryptic species complexes among insects have been increasingly discovered as genetic tools have become cheaper and more accessible to more scientists. As the genes of more of the world’s millions of species of insects are sequenced, we are likely to discover more of these cryptic species hidden in plain view.
This situation of the shot hole borers has presented a real problem to the people who manage trees threatened by woodboring beetles. What beetle have they found in their particular avocado grove or shipping container? What fungus does it carry? Is it an established native or invader, or does a quarantine need to be implemented?
Our team, led by graduate student Demian Gomez, decided to look for ways of distinguishing between the presumed three species based on morphology. A microscope is far more accessible to most avocado growers than a PCR and sequencing apparatus. To look for correlations, we first obtained 89 specimens from 26 locations in 11 countries. We then measured over 100 morphological characters and genotyped them to see what could be learned. Our findings were published in late November 2018 in Insect Systematics and Diversity.
It turns out that characteristics including pronotum and elytra dimensions and the presence of protibial socketed denticles correlate somewhat with the various species.
Emphasis, however, on “somewhat.”
We found that common characteristics allow generalizations to be made about the various groups of beetles, but enough exceptions remain that there is no set of physical measurements to be taken that will establish the species with total accuracy. DNA testing will still often be required to be completely certain as to what species of little brown beetle has been found.
If multiple samples can be gathered from a single population, a species identification based solely on morphology would have higher confidence as the number of samples increases.
Along the way, we were surprised to find that our genotyping identified a hitherto unknown fourth beetle in this cryptic species complex: Euwallacea kuroshio, which we believe to be native to Japan, Indonesia, and Taiwan, and introduced to Mexico and California. Thus, we concluded the Euwallaceafornicatus species complex to comprise four species (with suggested informal names in parentheses):
- E. fornicatus (tea shot hole borer clade a)
- E. fornicatior (tea shot hole borer clade b)
- E. whitfordiodendrus (polyphagous shot hole borer)
- E. kuroshio (Kuroshio shot hole borer).
Our samples included specimens from museums around the world gathered by bark beetle pioneers, finding that some of them matched our morphotypes and the geographics of our clades. These older samples give us greater confidence in our determinations as to where each species is and is not native to. They enabled us to resurrect old, sunken taxonomic names and assign them to genotypes. A reverse taxonomy.
Beyond the satisfaction of improving taxonomy for its own sake, establishing the clades and origins of these beetles may eventually have more practical applications. In the case of future outbreaks of these beetles as pests attacking stressed trees, it may be necessary to identify specialized natural enemies of each species.
Insect Systematics and Diversity
Jiri Hulcr, Ph.D., is an associate professor of forest entomology at the University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS). Website: www.ambrosiasymbiosis.org. Email: firstname.lastname@example.org. Jackson Landers is strategic science communicator at the UF/IFAS Emerging Threats to Forests Research Group. Email: email@example.com