‘Waxy cotton’ on plants may indicate presence of pest

Tuesday, July 25, 2017

Homeowners and gardening enthusiasts who have noticed white, cottony masses covering stems of ornamental and vegetable plants should not be hitting the panic button.

“These white masses are most likely waxy secretions of flatid planthoppers, insects that are not typically abundant enough to cause direct damage to plants, although oviposition – egg laying – punctures may kill small stems and branches,” said Eric Rebek, Oklahoma State University Cooperative Extension horticultural entomologist.

The most common and widespread flatid species in the southern United States is Metcalfa pruinosa. It occurs throughout America and Canada but is most abundant in the southern U.S. states. This species feeds on a wide variety of trees, shrubs, vines and occasionally on vegetables and weeds.

“Adults of Metcalfa pruinosa are whitish gray and measure about a quarter-inch in length,” Rebek said. “They are somewhat wedge-shaped, becoming narrower toward the rear end, and are covered with a white, powdery substance. A characteristic pair of dark spots is located in the basal half of each forewing.”

Rebek added these insects may be mistaken for moths at first glance.

Another flatid species, Flatormenis proxima or the northern flatid planthopper, is more common in northern U.S. states and Canada. Adults of Flatormenis proxima are similar in shape to Metcalfa pruinosa but are pale green.

“Nymphs of both species are white, laterally flattened and covered with long filaments of the flocculent, waxy substance they secrete on host plants,” Rebek said. “Nymphs are most common in June and July in southern states. Most are mature by mid-July and adults are present into early October. There is one generation per year in Oklahoma.”

“In Oklahoma, we’re pretty much past the nymph stage,” Rebek said. “Damage from flatid planthoppers is rare but heavily infested plants may become wilted, and leaves and stems may be covered with honeydew produced by these insects.”

Honeydew serves as a growing medium for black sooty mold. The fungi’s dark, threadlike growth gives plants or other substrates the appearance of being covered with a layer of soot.

Rebek explains honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. The insect cannot completely utilize all the nutrients in this large volume of fluid, so it assimilates what it needs and excretes the rest.

“Sooty molds can become established wherever honeydew lands, be it on leaves, twigs, fruit, yard furniture, concrete, sidewalks, statuary and the like,” Rebek said. “Control is not usually warranted unless damage or black sooty mold is observed and nymphs are still present.”

If control is necessary, Rebek recommends using products containing horticultural oil, sometimes referred to as stylet oil, or insecticidal soap.

“These products are available at most garden centers and do not cause harm to beneficial insects when applied according to label directions, which should always be followed,” he said.

Anyone seeking additional information or who has questions regarding horticultural or agricultural pests should contact their OSU Cooperative Extension county office, typically listed under “County Government” in local directories.

The Oklahoma Cooperative Extension Service is a state agency administered by OSU’s Division of Agricultural Sciences and Natural Resources, and one of three equal parts comprising the university’s state and federally mandated teaching, research and Extension land-grant mission.