This Invisible Pest Makes Its Presence Known

September 7, 1985|By Denise Salvaggio of The Sentinel Staff

H.G. Wells' classic story, The Invisible Man, has been the basis of numerous movies throughout the years. The ''fun'' element in every cinematic version of the tale involves seeing things happen that seemingly have no cause, such as footprints suddenly appearing in snow and objects moving through mid-air.

The actions of the movie's main character (a scientist who discovered a formula for invisibility) are really the result of special effects. But in real life, similar mysterious occurrences can be traced to an actual individual -- such as the whitefly (family Aleyrodidae), the invisible insect.

Unlike the story's see-through scientist, who starts out just as opaque as everyone else, this gnatlike white 1/16-inch creature begins life in a transparent state, becoming visible only in adulthood -- but just barely. If not for the whitefly's tendency to gather in masses, it would be overlooked on its preferred habitat of the leaves of citrus and ornamentals.

Also, if you look closely enough -- say, through a microscope -- the adult whitefly's two sets of wings will come into view. A still closer inspection will reveal that its wings and body are covered with a fine white powdery wax. What can be seen is deceptive, because while the whitefly resembles a gnat or moth, it is related to unpopular scale insects like the soft brown, black, tea and Florida red scale.

Aside from being unseeable at the beginning of its life, the strange insect's career parallels that of the fictitious invisible man in its capacity to wreak havoc upon an unsuspecting populace. After hatching from small, cigar-shaped eggs attached to the underside of leaves, the flattened oval whitefly larvae immediately commence making trouble without ever leaving their ''home.''

Because their bodies (which are about the size of the head of a pin) are transparent, the whitefly larvae can do a great deal of damage before anyone notices. The larvae live on the undersides of the leaves, in which they insert their tiny beaks to feed on sap. The leaves wither and drop with seemingly no cause, as if they were the work of an invisible entity.

The larvae have a few more tricks up their transparent sleeves. They excrete large amounts of honeydew, which provides an excellent medium for the growth of sooty mold. Besides being unattractive, sooty mold interferes with photosynthesis, retarding plant growth and causing early leaf drop. Honeydew also attracts ants.

While it's difficult to fight what you can't see, there are some unexpected allies in the war against the whitefly. Some species of parasitic wasps lay eggs within a whitefly larva. The wasp egg hatches, and feeds from the whitefly larva's organs as it grows. An adult wasp eventually emerges from the shell of the dead larva's body.

The ''friendly fungus'' aschersonia is another whitefly enemy. Although the fungus manifests itself as a series of unsightly raised orange dots over the leaf, it attacks whitefly larvae.

If this help isn't enough, resort to pesticides containing Diazinon, Malathion, Acephate or Dimethoate. For best control, apply when the young have hatched, which would be about three weeks after adults are abundant.

NEXT WEEK: The tumbling flower beetle.

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