Partnerships
Together for good
I acquired my current Found Objects because the landowner of one of my research sites decided he loved pines but hated hardwoods, so he girdled and poisoned all the water oaks and cherries on his land. I salvaged some of the cherry for lumber, but that is another story. By the next summer the dead oaks were dropping limbs. After a summer rain added more weight to the limbs, I began to be afraid to walk among these disintegrating oaks as branches cracked and thumped on the ground with scary frequency. Many of these branches broke into pieces on impact, exposing the blackened galleries made by ambrosia beetles.
Being an ambrosia beetle is a life-history partnership (symbiosis) between a beetle and a fungus, rather than a taxon of related beetles. The driving force behind the evolution of the ambrosia symbiosis has to do with wood as a foodstuff. The only part of the dead or dying tree that is a decent diet is the phloem just under the bark, and this is eaten by an array of bark beetles and their larvae, who carry a fungus that helps deliver the coup de grace to the tree. The wood that makes up most of the tree, in addition to being hard to digest because lignin-bound cellulose is resistant to chemical attack, is also very low in the nutrients, especially the nitrogen that animals need for their own tissue. In other words, wood is a lousy diet.
The ambrosia syndrome allows insects to exploit this very large and indigestible resource through a partnership with one or more species of fungus. These partnerships are a kind of “deal” between the beetle and the fungus--- you give me a ride and I will feed you. In contrast to bark beetles, ambrosia beetles don’t eat wood even though they use their mandibles to bore neat galleries through it. Instead, they carry a fungal inoculum in special structures called mycangia in which they sequester fungus or fungal spores before they leave their natal tree to hunt for the dying tree in which they will breed. The fungus infects the walls of the galleries and proliferates into the xylem of the wood, breaking down the wood and assimilating the products into its own tissues. The beetles cruise up and down their galleries, collecting the fungal mycelia that grow into their galleries as the sole food for themselves and their larvae, a state of complete dependence on their fungus. They are fungus farmers, tending their fluffy crops in the darkness of their wooden tunnels. The opportunity to make such deals must be dirt common, for they have evolved independently several times among the wood-boring insects.
The ambrosia beetles that made my Found Object are an unidentified species of scolytine beetle, a subfamily of the huge weevil family with thousands of ambrosia beetle species (more on that below). The beetles were no longer present after breakage exposed their galleries and rendered them useless, but these galleries tell an interesting, although somewhat conjectural story. It is likely that each gallery was initiated by a male who bored through the bark of the dying tree, excavated a wide chamber (the nuptial chamber) and then attracted a number of females (up to six in my Found Objects) to share his home and to mate with him in his nuptial chamber. Each of the females then made her own gallery to serve as a creche for her own reproductive efforts. Each female bored short cavities (“cradles”) into the floor and ceiling of her gallery, oriented along the grain of the wood. Adult beetle galleries are always at right angles to the grain of the wood. This orientation is clearly visible below in the blocks cut from the broken branches. Orientation is unlikely to be in response to gravity because the branches grew at various angles to the vertical trunk of the tree.
The female in her gallery laid an egg into each cradle and, in a fine example of maternal care, then cruised up and down collecting fungal mycelium to feed to her developing larvae. This task was made easier because larval heads all faced the gallery. The fully-grown larvae then pupated in their cradles before emerging as adult beetles. Before they departed their natal nest, they collected fungal spores into their mycangia, ready to inoculate the dying tree in which they hoped to breed.
Being cooped up in a closed space with their offspring, cultivating a crop, and continual care for their young is conducive to evolving sociality, and some species of ambrosia beetles are among the few insects that have done so. A peculiar mode of sex determination in which females are much more related to their sisters than to their brothers or mothers is another factor that selects strongly for sociality, as it does in the ants, wasps, and bees. But perhaps that is a subject for another essay.
An important down-range effect of ambrosia beetles is that they carry the agents of wood rot deep into the wood. Were it not for these and other wood boring insects and their passenger/partner fungi, wood would rot only from the surface inward, and decay would be very slow. Forests would be a pile of pick-up sticks of dead logs. As it is, rot begins soon after a tree dies and continues until its remains are indistinguishable from soil, all its substance having been turned into carbon dioxide, soluble minerals, or the substance of other living things (such as fungi). The early growth of the ambrosia fungus in the xylem of the sample of pine below is visible as the dark staining. Loss of value in sawn logs creates significant economic losses, especially in the coniferous logging industry of the Pacific Northwest.
Partnership with fungi is an enormously flexible world of opportunity. In the scolytine beetles alone, there are over 3,000 ambrosia beetle species, many of whose life history has evolved independently. Their mycangia are especially complex and provide the symbiotic fungus with great pampering because, being the beetles’ sole food, failure is not an option. The mycangia are located on different parts of the body in different ambrosia beetle species and may even supply protection and nutrition for the fungal spores within them. The few examples below show the diversity of scolytine ambrosia beetles.
All these independent evolutionary origins should make it no surprise that the life habits and host range of ambrosia beetles vary extremely widely. What’s more, the particular species of fungus associated with each beetle species may also vary a great deal. Some beetles transmit multiple species of fungus, while others are more faithful to one or a few, or may harbor a different species at different times. An interesting question is how these associations arise and evolve. When an adult beetle departs from its natal gallery, does it have a preference as it packs the spores or mycelia into its mycangia?. Do their choices have consequences down the line? It’s not hard to see that the fluidity of this system is high.
Clearly, My Found Object is only one of many possible examples. Many ambrosia beetles, especially exotic, invasive ones attack living trees, and some kill them (or rather, their fungus does), threatening several native tree species. Many ambrosia beetles bore into twigs rather than trunks to create small galleries in which they raise their fungus and their offspring. One such twig-borer annually prunes hundreds of twigs on our magnificent sycamore tree. I am not sure if they invade shaded, dying twigs or their invasion kills the twig, but small bunches of dead leaves appear during the summer, and most of them have a tiny hole just tree-ward of the dead leaves (below). During the summer, these are often occupied by adults, larvae, and the fluffy fungus they eat. I don’t know the family structure with respect to males and females, nor which sex makes the initial entry into the twig, but it is certainly a family affair. After the beetles finish, the twigs snap easily at the hole, revealing a blackened cavity at the break.
Many ambrosia beetle species are quite catholic in their tastes. The species that killed the magnolia twig below may or may not have been the same as the ones that invaded the sycamore twigs above. The tiny entrance hole was located a few centimeters treeward of the dead leaves.
Ambrosia beetles remind us that no creatures go through life on earth alone, and that most animals that look to us like “complete individuals” are actually teeming farms of microorganisms, many of which fulfill functions that are essential to the animal’s life, others detrimental. For us humans, this has been obvious but failed to rise to full consciousness for decades, but in recent times the light has gone on with respect to the essential nature of these partnerships. We can better appreciate how ambrosia beetles rank among animals that practice agriculture, including fungus gardening ants and termites. When it comes to growing mushrooms, humans are latecomers by about 100 million years.
I enjoyed the "feel" of this essay. Your vivid writing coaxed me to imagine mother beetles shuttling along in moist narrow spaces, tending to their brood in arrays of chambers in total darkness. There's may be a very tactile, acoustic and aromatic world.
Now if I could just learn a bit more about my own biome.....
A lot here to process, and really learn from. I am often struck by the power of motherhood and how extremely protective Mums are to their young. Today the article in Guardian about an Orca which had apparently adopted a baby pilot whale warmed my heart.