A non-plague of non-locusts

The locusts are, it must be said, a bit of a plague on the locusts. The males call “Pha-roah! Pha-roah!” in a slow southern drawl while the females plunge their scimitar-shaped ovipositors into the thorny twigs, sometimes so deep that they break. All along the top edge of the field, the black locusts are acquiring a pruned and chastened look, as if they were the victims of some very localized storm.

Of course, 17-year “locusts” are really cicadas (see Brood XIV). According to some sources, when early European settlers first encountered periodical cicadas in eastern North America, the only parallel they could think of for such an extreme insect outbreak was the plague of locusts in the Bible, so “locusts” they became — despite the fact that, as members of the order Homoptera, they could scarcely be more distantly related to true locusts, which are grasshoppers, Orthoptera. According to the Locust entry in the Wikipedia, however, the conflation goes much further back, to “the Vulgar Latin locusta, which was originally used to refer to various types of crustaceans and insects.” The English essayist Thomas Browne was bemoaning the confusion between locusts and old-world cicadas as early as the 17th century. And obviously the popular description of the call of Magicicada septendecim as “Pharoah” shows the unabated influence of Exodus 10:4-15.

However, it would be a great exaggeration to say of these “locusts” that “they covered the face of the whole earth, so that the land was darkened; and they did eat every herb of the land, and all the fruit of the trees which the hail had left: and there remained not any green thing in the trees, or in the herbs of the field, through all the land.” Here’s what the folks at Cicada Mania say:

Question: Will the cicadas kill my trees, shrubs and flowers?
Answer: Possibly. Cicadas don’t kill flowers and shouldn’t damage shrubs, but they can do damage to young, wimpy trees like ornamentals. If you have wimpy little trees, you can net them to keep the cicadas off. Tree species that aren’t native to North America won’t fare as well as native species. Trees that lose a lot of branches typically revive after a year or two, but they will be ugly in the mean time. Cicadas actually are a benefit to trees, as they destroy the weaker branches. Please don’t use pesticides — you’ll destroy the good bugs as well as the “nuisance” bugs, and ultimately do your garden and the environment a huge disfavor.

Of course, as larvae, cicadas do subsist on tree sap, but given that cicadas and deciduous trees have evolved together, I doubt that they do much to weaken the trees. Though the science is inconclusive, it’s likely that their extensive tunneling benefits the trees by aerating the soil, and possibly by creating macropores for new roots and fungal mycelia. An AP article quotes a Dr. Frank Hale from the University of Tennessee:

The holes from which they emerge aerate the soil around the tree roots, Hale said. The millions of decaying cicada bodies supply nitrogen and other nutrients, which rain washes down the holes to the tree roots.

However, a Penn State Ag School publication for orchardists assumes the worst, and considering the unique stresses on heavily pruned and sprayed trees in monocultural plantations, it may well be on-target with its recommendation to prevent egg-laying at all costs. It also emphasizes netting if feasible, and mentions the harmful effects of pesticides on beneficial insects.

Do the various Magicicada species vary in their arboreal preferences? I’ve mentioned black locusts as a favorite for swarming and egg-laying activity (see my video at the Plummer’s Hollow blog), but I suspect that’s largely because of their location in the field-forest ecotone. The cicadas here do show an affinity for forest edges and openings, regardless of where they originally emerged. I don’t know if the fact that locust trees are nitrogen-fixing legumes makes them any more attractive as adoptive mothers to cicada larvae. We have noticed relatively few cicadas in the black walnuts that surround the main house, which might seem logical given the toxicity of walnut roots to many other plants, but it seems that if we had the rarer third species, Magicicada septendecula, that might not be the case:

Within the same brood, the three species are always perfectly synchronized, but they are separated microspatially by having different habitats within the same woodland. Magicicada septendecula prefers ovipositing in hickories and walnuts, and emerges in higher proportions under those trees than under comparable oaks. Both M. septendecim [the “Pharoah” species] and M. septendecula occur together in upland woods, but septendecim exhibits much less host specificity than septendecula. The latter species is much rarer than septendecim; it can usually be heard chorusing in local patches within a woods occupied by septendecim. Magicicada cassini is a species of floodplain woods, and characteristically can be seen to replace septendecim and septendecula as one moves down a wooded slope leading to a stream. Over much of the eastern United States, however, the original forest has been extensively disturbed. Periodical cicadas survive and reproduce surprisingly well in cutover, scrubby second growth. Tree species characteristic of floodplains, like American elm, are often a component of upland second growth, and, especially in such situations, cassini, septendecim, and septendecula become intermixed though they remain reproductively isolated. The present lack of microspatial separation in many situations, then, is an artifact of human disturbance.
—Henry S. Dybas and Monte Lloyd, “The Habitats of 17-Year Periodical Cicadas (Homoptera: Cicadidae: Magicicada Spp.),” Ecological Monographs, Vol. 44, No. 3 (Summer, 1974)

That might explain why the M. cassini cicadas are so much louder and more numerous around Canoe Creek State Park, much of which is floodplain or early-succession second growth. At our annual Audubon Society picnic last week, they were nearly deafening. In fact, a friend just told me that the dog-day cicadas are usually very numerous there, too — and regardless of the species, they make excellent fish bait, he said. Hardly the stuff of plagues.
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Today is the deadline to submit links to the next Festival of the Trees. Details are here.