This summer, Brent Elliott, an entomologist with Manitoba Agriculture, received many calls about spittlebugs in trefoil.
Elliot said the species in Manitoba trefoil fields is the meadow spittlebug, or Philaenus spumarius.
Spittlebugs are small, soft bodied insects closely related to leafhoppers. They feed by sucking plant juices, like leafhoppers, but seldom cause economic damage.
They are unique in that during the nymph stage they produce a nest of spittle, a foamy or bubbly looking mass of liquid. This spittle is composed of excess excreted fluids combined with mucous-like fluids into which air bubbles are introduced.
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It is thought the spittle mass is protection from predators and prevents the nymphs from drying out.
Nymphs are found in the spittle and may be green or brown. They are usually less than 10 millimetres in size. Adults are rarely seen because they can be quite active and move away easily. The spittle masses are usually in the leaf axils or on stems.
There have been no studies to determine the economic threshold for spittlebugs on trefoil. The primary reason is that the population would have to be extremely large to justify an insecticide application that would yield an economic return.
Elliot said the presence of the bugs can be obvious, which alerts growers, who become concerned. However, economic damage is unlikely and at worst the bugs will cause some stunting of affected plants.
In wet years, as 2007 started out to be, spittlebugs tend to be more numerous and more visible because they tend to form spittle masses near the top of the plants. In drier years, they form masses lower down in the canopy where humidity is higher. Elliot said when moisture is abundant, the bugs seem more abundant too, but they’re really just more visible.
He does not recommend control measures for this insect for two reasons. There are no insecticides registered on trefoil and there is likely no economic damage being done.
Even if a spray is applied, it is likely the spittle mass would protect the nymphs from the insecticide.
Timely cutting, if possible, is probably the most effective form of control for this infrequent pest.
