The Secret Life of Morel Bugs

© Peter Werner
Original publication: Mycena News, September 2002

Over the years, spring mushroom hunters have noted the presence of a unique, odd-looking type of "beetle" found on morels, often in abundance. As luck would have it, there was a good morel season this last spring, which coincided with my taking a class in insect taxonomy at San Francisco State. I have always been interested in insect-fungus relationships, and therefore set out to identify and find out as much as I could about this "beetle" and its relationship with morels.

Aradus debilis Uhler, female, California

As it turned out, this "beetle" wasn't a beetle at all, but rather a true bug (order Hemiptera), specifically an aradid or flat bug (family Aradidae). Aradids are easily recognizable with a hand lens, having a flattened body, a small pair of crossed half-sclerotized wings (the hemelytra - which distinguish them as true bugs) surrounded by a segmented lateral margin (the connexivum), and a head with mouthparts in the form of a long beak (the rostrum) rather than mandibulate mouthparts. However, the hemelytra are only fully developed in the adult stage, and the wingless nymphs, which are very common on morels, might be very easily mistaken for beetles.

I have identified all of the adults I collected or was given as Aradus funestus, a common species in the montane and boreal regions of western North America. However, because most of the aradids I observed were nymphs rather than adults (and hence, not keyable to species), I am unable to say with any certainty that only one species of Aradus was present on morels. I encountered quite a few nymphs with bicolored antennae, which is not reported as characteristic of this species. This may be simply an unreported phenotypic variant, or it may mean that more than one species of Aradus is present on morels.

So, what are these bugs doing hanging out in morels? Like most aradids, they're fungus feeders. Aradids are not ordinary fungivorous insects, however - instead of chewing up the mushroom with their mandibles, they have highly specialized mouthparts for penetrating the mycelium and sucking out the contents, much the way an aphid would feed directly on a plant's phloem sap. Coiled inside of the rostrum, an aradid has a long thin tubelike stylet for penetrating hyphae; this stylet is quite long and evidently can follow a hyphal strand for a distance surpassing its body length. As Debbie Viess first pointed out to me, this attachment is quite strong, and aradids can actually hang from their stylets if pushed away from the surface of the morel.

On one of my forays to the Star Fire site, I was able to make some close observations of this feeding behavior. Interestingly, once a morel is picked, a aradid will continue its feeding for only a minute or two. It then withdraws its stylet from the hypha, and begins "probing" around the surface of the morel with its stylet, apparently not finding a new point to feed from. While I cannot say for sure what is happening here, my guess is that the flow of materials through the hyphae stops soon after the morel is cut from its base. This stops the flow of food to the hypha on which the aradid is feeding, making it search for a new point to insert their stylet. The probing behavior may be due to the fact that the aradid's stylet could be sensitive enough to detect active translocation simply by feeling around the surface of the morel.

After conducting a thorough search of the scientific literature and the Internet, I've found that there have been no detailed studies done on the feeding behavior or ecology of these interesting creatures, just incidental observations by insect taxonomists. Aradids are usually associated with wood-rotting fungi, and typically feed directly on mycelium growing in rotting logs, or directly on the fruiting bodies of polypores and resupinate fungi. (However, on burn sites where morels were present, I have never observed them underneath bark or on polypores.) I have also not found any source mentioning any association of aradids with morels - mycologists so far haven't delved into the identity of insects encountered on morels, and entomologists haven't looked at morels as a site for collecting or studying insects.

I did encounter some intriguing references to some Scandinavian literature on aradids that are adapted to post-forest fire succession, and that deliberately seek out burnt wood. My guess as to the ecology of the Aradus species we encounter is that they are possibly adapted to post-forest fire succession, either surviving the fire within logs that aren't entirely consumed by the fire, or they are attracted to the scent of burnt wood. In spring, they produce a new generation, hence, the abundance of nymphs. These newly emergent aradids then take advantage of the abundance of morels fruiting after the fire. After the morel flush finishes, they probably move on to the abundance of wood-rotting fungi present on the burn site.

While I've observed an abundance of aradids on fire morels, I have never seen these bugs on "natural" morels - that is, morels not associated with fires. Unfortunately, my observations of "naturals" are very limited, so the above-mentioned hypothesis lacks confirmation. Aradid ecology is a wide-open area that hopefully will get the attention of some enterprising entomologist interested in insect-fungus relationships.

Aradus sp. Longitudinal section through the head showing coiled setae (StB), labrum (OL), labium (Lab), æsophagus (Os), salivary gland (SpDr), and duct (SpG), and dilator and retractor muscles (m. dil., m. retr.)

I should also note that aradids are not the only arthropod fauna associated with morels. Wireworms, which are the larvae of click beetles (family Elateridae), are often encountered in morels when gathering or cleaning them. Mark Lockaby presented me with several rove beetles (family Staphylinidae) that came scuttling out of some morels that he was cooking. Millipedes are sometimes found living in the hollows of morels, and, being detritivores, probably feed on morels as they decay. In short, there are many other organisms that share our love of morels, hence the importance of cleaning them and checking their hollows as soon as possible after finding them.

Further reading:

(Thanks to: Dennis Desjardin, Mark Locakaby, Margareta Luff, Kevin Sadlier, and others who collected insect specimens for me; John Hafernik and Paul Marek for their excellent instruction in insect identification; Cheryl Barr of the Essig Museum of Entomology (UC Berkeley) for use of the museum's collection of aradid specimens; and Steve Taylor for his expertise on aradids and valuable feedback.)