This species overwinters as larvae in two different stages of development. The life cycle takes two years to complete (some studies indicate a three-year cycle), and almost all of this is spent as larvae feeding on grape roots. They bore into the roots and crown below the soil surface, reducing the productivity of the vine. Roots may be hollowed and sometimes packed with frass. Vines eventually die; there may also be increased susceptibility to cold injury. Young larvae are spread throughout the root zone while older larvae are found on larger roots close to the trunk. Ninety per cent of the pupae are within 35 cm of the trunk and the mean depth in the soil is 9-10 cm. A lack of plant vigor is usually the first sign of the presence of this pest. Another indication is the presence of cast pupal skins protruding from the soil near the base of the trunk in late July and August.
Full-grown larvae are about 25 mm long, white, and have brown heads. They leave the roots and pupate in cocoons near the soil surface beginning in June. Adults emerge 35-40 days later, beginning in about the first week of July, with greatest numbers present in the last two weeks of July (see flight data from a 2-year study of lure efficacy in 1986-1987). Moths are wasplike in appearance. The body is generally brown. The top of the head has orange; antennae are orange with brown-black markings; the abdomen is dark brown with reddish-brown markings, with a very narrow yellow band on posterior edge of segments two, four, and sometimes on six. Legs are orange with brown-black markings. The forewings are dark and mostly opaque. The hind wing is more transparent (see photo above, and University of Kentucky link). These moths are daytime fliers. After flying for several days, females begin ovipositing on grape foliage, canes, and weeds. Each female lays an average of 300 eggs. About two weeks after hatching, first instar larvae drop to the ground and tunnel to roots. The greatest natural mortality occurs at this point in the life cycle. Only 1.5-2.7% survive the first stage because of predation, parasitism, and desiccation; but, once established in roots, mortality is very low. Infested vines are usually encountered randomly across a vineyard. Larvae do not travel very far in the soil, usually remaining on the roots of a single vine.
Control of this pest may be difficult
(especially in established populations), and severity has been
increasing in Virginia vineyards. There
is currently a single insecticide registered, Lorsban 4E,
which has a very preharvest interval, complicating
timing. Entomopathogenic nematodes
have been effective in lab and greenhouse, but field results are
variable. Effectiveness is affected be environmental
variables such as soil moisture. Contact insecticides are
ineffective against subterranean larvae, although soil injection
of fumigants shows promise. Some soil barrier treatments have
recently been shown to be effective. An effective cultural
control method involves mounding soil beneath vines
after borers have pupated, and then leveling the ridges in the
fall or spring. When adults leave the cocoons they are unable to
dig to the surface. Timing is important because if mounding is
done too early the larvae merely tunnel up into the ridge before
pupating. This approach is labor intensive, especially
since mounds must be pulled down again in the fall. Proper
weed control appears to be important in borer management
because of increased larval mortality at the exposed soil
surface. When vines are infested, nitrogen fertilization may
help overcome effects of damage. An action threshold
suggested by University of Kentucky is 5% of the vines having
pupal skins present.
A mating disruption product, Isomate GRB, is now registered. The label is available (Great Lakes IPM), as is the Safety Data Sheet (SDS) (Pacific Biocontrol). The rope-style pheromone dispensers should be placed in the last week of June, at a rate of 100 ropes per acre.
The pheromone blend used is (E,Z)-2,13-octadecadienyl
(Z,Z)-3,13-ODDA (99:1). Traps are available from Great Lakes IPM.
Traps should be placed in the third week of June. Because
traps will attract male moths from outside the vineyard, numbers
do not necessarily correlate with root infestations. Traps
for some clearwing moths are not very specific, and are likely
to catch many nontarget related species. This is less of a
problem with grape root borer traps, however a few nontarget
species may show up. One is the squash vine borer.
It can be easily differentiated from GRB by the heavy bands of
red-orange on the abdomen, heavy red on the legs, and green
coloring on the wings; this moth is heavy bodied and quite
fuzzy. See the linked images of squash vine borer from University
of Kentucky. Any clearwing moth with broad
yellow or white bands on each segment, or a black body, or clear
forewings is not grape root borer.
Updated 28 June 2016