The vertical gardeners

Autumn is here and that means it’s time to harvest the last of the veggies in the garden and start putting the soil to bed for the winter. I find that gardening is a wonderful activity to stimulate thinking and is when a lot of “ah ha!” moments seem to blossom. Recently I got to thinking again about some very interesting and novel research going on here in Raleigh that focuses on some different gardening fanatics.

A sample of some of the morphological diversity in Xyleborini ambrosia beetles (photo by Jiri Hulcr)

Now most people know about leaf cutter ants and how they cultivate huge gardens of fungus to support their colony. But they are not the only insects to do this. Another group of miniature mycoculturalists are the ambrosia beetles. This is a somewhat-obscure group of insects that live just under the bark of trees, in a layer called the xylem (the layer in which water moves upwards in a plant from roots to leaves). The ambrosia beetles are a polyphyletic grouping of weevils, meaning that the lifestyle they share (living under bark and cultivating fungus) has evolved multiple times (in fact, at least 11 times) in individual species groups. This indicates that the inner tree-dwelling, fungus-gardening gig is a pretty successful one and was adopted anew several times. It’s also withstood the test of time. While human agriculture has been going on in some form or another for about 12,000 years, the association between ambrosia beetles and their cultivation of fungal gardens began over 60 million years ago. So you’d expect them to be “master gardeners”… which they are.

One of the tricks of the ambrosia beetle trade is morphological. Successful human gardeners are said to have a “green thumb”, but the ambrosia beetles have something better. They are outfitted with structures called mycangia that they use to transport the seeds of their fungus gardens (spores, actually). As I mentioned already, the fungus gardening lifestyle evolved multiple times in these beetles, so as a result there is some diversity in the construction and location of ambrosia beetle mycangia. Some some beetles have these spore-holding pockets next to their mandibles, some under their wings, and some with grooves on their backs to collect and carry fungus to their next home. In all cases, when the beetles leave a tree they carry with them an instant garden.

Left: A cross-section image of fungus-bearing mycangia of Xyleborus affinis which reside in their heads and sit at the base of their mandibles. Right: A dissected Xylosandrus germanus with the mycangial structure removed from the inside of the thorax. (photos by Jiri Hulcr)

The relationship between beetle and fungus is a mutualism and neither organism would survive without the other. But the exact species involved are less critical. In general, the beetles are able to cultivate and feed on different types of fungus and 95% of the beetle species tested show no preference for one fungal foodstuff over another. The common thread is the fact that all of these fungi decompose wood. So the same colony of beetle can cultivate multiple types of fungus just like we grow tomatoes, potatoes, peas and corn in our little garden at home. Other ambrosia beetles that lack mycangial structures and the ability to grow their own food will engage in raids on established gardens. Beetles that engage in mycocleptism (fungus theft) will bore into the galleries of another species and chow down on that colony’s crops.

In their native ranges, ambrosia beetles usually stick to attacking downed trees that are already dead or dying. The fact they feed on wood-decomposing fungi makes it possible for them to feed on several different species of dead trees. The beetles rely on olfactory cues (scents) to find and identify downed trees that they then use as nesting and gardening sites. Their role in spreading fungi from place to place and cultivating its growth is important to the process of wood decomposition in forests. However this neat little system appears to come undone when beetles are introduced to new habitats that have unfamiliar tree species and strange chemical cues (or a lack thereof). And this is where the story of ambrosia beetles takes a sour turn.

A native redbay tree (Persea borbonia) being attached by the exotic ambrosia beetle, (Xyleborus glabratus) near Savannah, Georgia. Note the pin-like extrusions of sawdust as a result of beetle boring activity. (photo by Bud Mayfield)

In 2002, the an exotic ambrosia beetle from Asia, Xyleborus glabratus, was found attacking redbay trees (Persea borbonia) near Savannah, Georgia. Redbay is native species that is common in maritime habitats along the southern Atlantic and Gulf Coasts. It’s a close relative of the plant that is the source of the cooking spice called “bay leaf” (P. nobilis) and can be used as its substitute in recipes. In its home range X. glabratus attacks only already-doomed trees, but all of the redbay that has been hit by this beetle in the South are live and otherwise healthy. That is until the beetles and fungi move in. Once they are attacked 100% of the trees die.

The range of tree species in the U.S. that are attacked by X. glabratus extends beyond redbay to include other members of the Laurel family (Lauraceae). The other trees that have been confirmed as vulnerable are some notable and iconic species like sassafras (Sassafras spp.), camphor (Cinnamomum camphora), pondspice (Litsea aestivalis, which is Federally endangered) and avocado (Persea americana). The avocado industry in Florida has gone into panic mode in an attempt to fend off the invasive beetles and ease the damage that is certain to come. In the meantime, the beetles are still spreading southward towards the avocado growing areas of South Florida, using the redbay trees and other Lauraceae as a bridge.

The range map of the invasive redbay ambrosia beetles (Xyleborus glabratus) and a malady that it vectors, laurel wilt disease, in the S.E. United States. Color codes of counties reporting the disease corresponds to the year reported.

One of the alarming statistics is that X. glabratus is only one of about 260 species of ambrosia beetle that is recorded as exotic in the U.S. Another startling figure is that invasive ambrosia beetles are very abundant and account for 10% of insect species and 45% of the overall number of beetles here. The characteristic of attacking healthy, native trees is much the same for many of these other invasive ambrosia beetles that we know of and this has damaging impacts to native biodiversity. The Palamedes swallowtail butterfly (Papilio palamedes) is common in the South and highly dependent on redbay, swampbay and sassafras as host plants (all three of which are attacked by the beetles). While no comprehensive studies have yet been undertaken to determine the effects of beetle damage on their populations, redbay has completely disappeared from areas infested by Xyleborus glabratus leading to speculation that numbers of the Palamedes swallowtail in these areas will soon follow.

Invasive ambrosia beetles are damaging to economies as well, although (and coincidentally so) it is via commerce and international trade that most of them have been able to move around and become established in North America. For example, a large portion of the railroad ties used in the western United States once came from oak trees in Georgia and the Carolinas. However with the detection of a new ambrosia beetle species in Oregon, (Xylosandrus crassiusculus), orders for this Southern forestry product has completely stopped. On top of this, hundreds of thousands of dollars and tens of thousands of gallons of pesticides have been spent on eradicating this single species of beetle from the area around Portland, OR. Yet another species, the walnut twig beetle (Pityophthorus juglandis) vectors the ominous-sounding “thousand cankers disease” in native black walnut (Juglands nigra) and cultivated walnut species. This disease is often deadly to adult trees and impedes the growth of saplings.

The three beetles mentioned in this post are just a few of the invasive ambrosia beetles present in the U.S. Estimates are that on top of the species confirmed to be within our borders, more than 2,000 more exist in the Asian tropics and could pose just as great a risk. To use some gardening metaphors that I came up with while digging and thinking about all of this, these examples have been cherry-picked but represent an issue that is no small potatoes. In preventing further invasions we have a tough row to hoe and if we don’t nip future incursions in the bud we’re certain to face the grapes of wrath.