Participants in Monday's Southwest Ohio BYGLive! Diagnostic Walk-About held at Stanley Rowe Arboretum observed final instar nymphs, called "pupae," of the Mulberry Whitefly (Tetraleurodes mori) on the undersides of holly leaves. The round, shiny black pupae are ringed in a white fringe and are commonly mistaken for a scale insect. Indeed, whiteflies are not "flies" (order Diptera); they belong to the same order (Hemiptera) as scale insects and share certain traits with both armored and soft scale insects.
As with scale insects, whiteflies develop from eggs to adults through incomplete metamorphosis. Once eggs hatch, the immature whiteflies increase in size by passing through four nymph stages, called "instars." Like armored scales, only the 1st instar nymphs are mobile, so they are commonly called "crawlers." The 2nd, 3rd, and 4th instar nymphs are immobile; these "settled" nymphs remain firmly attached their host plants as they sip plant sap.
The 4th instar nymphs molt into the winged and reproductive adult stage. Somewhere along the way, 4th instar whitefly nymphs became known as "pupae" which can create confusion because only insects with complete metamorphosis (e.g. flies, butterflies, beetles, etc.) have a true pupal stage. However, entomologists refer to the unusual looking 4th instar mulberry whitefly nymphs found by the Walk-About participants as "pupae" (singular = pupa). It's important to note that mulberry whiteflies aren't the only whitefly species with unusual looking pupae. In fact, whitefly pupae are often used to identify the whitefly species owing to their settled lifestyle and unique appearances.
As with soft scales and certain aphids, whitefly nymphs and adults both feed by inserting their piercing-sucking mouthparts into phloem vessels to extract amino acids that are dissolved in the sugary plant sap flowing through the vessels. They discharge excess sap from their anus in the form of sticky, sugary "honeydew" that may become colonized by black sooty molds. For this reason, symptoms produced by whitefly infestations are sometimes mistaken for soft scale or aphid infestations.
Mulberry whiteflies have multiple, sometimes overlapping, generations. They have a very wide host range beyond their namesake host and may be found on holly, mahonia, mountain laurel, hackberry, sweetgum, maple, and dogwood. All stages, from nymphs to adults, confine their feeding to leaves of their host plant. Most references indicate this whitefly rarely causes significant injury to woody ornamentals. Indeed, Walk-About participants found no outward symptoms on infested Ilex opaca 'Canary' and other large-leaved hollies in the Arboretum. However, an infestation on the small-leaved I. crenata 'Glory' produced stippling symptoms and loss of older leaves that could be mistaken for spider mite injury.