I recently came across clumps of intensely white cottony material at eye level on the stems of a red elm (= slippery elm, Ulmus rubra) along a forest trail in southwest Ohio. A close examination revealed the insects beneath the white fluff to be nymphs (immatures) of fulgoroid planthoppers (order Hemiptera, superfamily Fulgoroidea).
Fulgoroid nymphs of many species congregate in groups, or "colonies," and are commonly obscured by a dense cloak of tangled waxy, white, cotton-like "fluff." They are very common on a wide range of understory plants in the woods of Ohio. They are usually described as occurring on stems close to the ground. However, I commonly find the nymphs clustered on the stems and the undersides of leaves high above the ground making them more obvious.
This is significant because the fluffy-cloaked planthopper nymphs may be mistaken for more serious pests like mealybugs or woolly aphids. Indeed, I received a phone call yesterday from a home gardener who was concerned their herbaceous perennials were being overwhelmed by “mealybugs.” They sent a photo revealing the true culprit to be planthopper nymphs.
Planthoppers seldom rise above the status of nuisance pests. However, the collective sap-sucking damage produced by high populations of nymphs may stunt new growth.
If control is required, the nymphs can be washed from plant stems using a coarse stream of water from a garden hose which will also wash away the white "fluff." Insecticide applications are seldom warranted, but if needed, insecticidal soap applications are highly effective and will preserve the hopper's natural enemies.
Fulgoroid planthoppers are relatively small insects with the adults seldom measuring more than 1/4" in length. The adults of many species have broadly triangular-shaped front wings that they hold tent-like over their abdomens. The adults are commonly found resting on plant stems and are often mistaken for moths.
The nymphs are revealed by carefully removing the flocculent material. The light green nymphs vaguely resemble airplanes with wing pads angled backward like the swept wings of a jet. Late instar nymphs look like some form of Star Wars troop vehicle with tufts of white filaments streaming behind.
One of the most frequently encountered planthopper in Ohio is the citrus planthopper (Metcalfa pruinosa). It ranges throughout the eastern U.S. and feeds on citrus in Florida which accounts for its common name.
Like their aphid, mealybug, and soft-scale cousins, planthopper adults and nymphs use their piercing-sucking mouthparts into phloem vessels to tap plant sap. They discharge the excess sugar-rich liquid from their anus in the form of a sticky, sugar fluid called "honeydew" which can become colonized by black sooty molds.
For the Defense
It’s well documented that honeydew may serve as a bribe to entice ants and other insects to serve as bodyguards for sap-sucking insects. The mutualistic relationship between ants and planthoppers, aphids, soft scales, etc. as well as other organisms is called myrmecophily which means “ant-love.” The ant-love going on with the planthoppers I found on red elm morphed into ant-hate as the myrmeco-troopers delivered some painful bites while I was holding stems to take pictures!
The exact role of the white fluff covering planthopper nymphs has long been debated. It is speculated that waxy fluff repeals water. It is also thought that the cotton-like filaments may protect the nymphs from predators. Imagine a lady beetle intent on a planthopper meal coming away with nothing but a mouthful of waxy fluff.
If the fluff doesn't work to defend against predators, the nymphs can hop impressive distances to live another day; thus the "hopper" in the common name. Exactly how they hop was revealed in a fascinating paper published in 2013 in the journal Science (see Reference below).
The authors provide a rich description of both the jumping mechanism and the mechanics: The nymphs, but not adults, have a row of cuticular gear (cog) teeth around the curved medial surfaces of their two hindleg trochantera. The gear teeth on one trochanter engaged with and sequentially moved past those on the other trochanter during the preparatory cocking and the propulsive phases of jumping. Close registration between the gears ensured that both hindlegs moved at the same angular velocities to propel the body without yaw rotation. At the final molt to adulthood, this synchronization mechanism is jettisoned.
So, planthopper adults can't jump. Their defense appears to be mimicking leaves and creeping along so as not to attract attention.
Burrows, M. and G. Sutton, "Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect," Science, Vol. 341, Issue 6151, pp. 1254-1256