South Carolina Studies:
Insecticide efficacy in suppressing barley yellow dwarf (BYD) is affected by local epidemiology. Field experiments were conducted in the South Carolina coastal plain over a nine-yr period to determine the relationship between seasonal vector activity and BYD suppression with selected insecticide treatments. Four potential cereal aphid vectors of barley yellow dwarf virus (BYDV) colonized wheat in a consistent seasonal sequence. The greenbug, Schizaphis graminum (Rondani), and rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), which is a cryptic subterranean species; typically reached peak population levels in early winter (Dec. and Jan.). Bird cherry-oat aphid (BCOA) Rhopalosiphum padi (L.) was a later colonizer with populations increasing in Jan. and peaking in late winter to early spring (Feb. and Mar.). The final aphid colonizer was the English grain aphid, Sitobion avenae (Fabricius), which peaked in late April. When wheat was planted within the optimal agronomic seeding interval, timed insecticide applications indicated that most BYDV transmission occurred in Feb. and Mar., implicating BCOA as the major vector. BYD incidence and wheat yield loss were also highly correlated (P= 0.004 and P = 0.007, respectively) with BCOA peak population density in wheat over the nine growing seasons. BYD incidence and yield loss were not correlated with population levels of the other three potential vectors. Foliar applications of Karate/Warrior (lambda-cyhalothrin 0.02 - 0.03 lb ai/ac) in early Feb. were more effective in suppressing BYD and protecting wheat yield than imidacloprid seed treatments (Gaucho 0.75 oz ai/100 lb seed) or Feb. foliar applications of disulfoton (0.75 lb ai/ac) or dimethoate (0.5 lb ai/ac) because Warrior provided better residual control of BCOA during Feb. and Mar. Properly timed, reduced rates of Warrior (0.02 lb ai/ac) were as effective as imidacloprid seed treatment plus foliar Warrior (0.03 lb ai/ac) or multiple Warrior applications. This information is useful in both prophylactic and threshold-based management programs. Growers who choose to treat high management wheat preventively can effectively control BYD by applying Warrior with topdress nitrogen in early February. For threshold-based programs, scouting efforts should prioritize checking for BCOA in late January and February. Species identification is important but it should not be necessary to scout for subterranean aphids. These conclusions are inappropriate for other areas of the southeastern United States with different epidemiological conditions.
The management and use of selected insecticide treatments for control of aphids and suppression of BYD were studied during four seasons in winter wheat in Georgia. Aphids were mostly the bird cherry-oat aphid and greenbug during vegetative growth stages and the English grain aphid during grain filling. Early plantings had much greater aphid numbers and incidence of BYD symptoms than later plantings. Planting at the end of the recommended planting period reduced aphid numbers and BYD incidence and usually avoided yield losses. Treatment of seed with imidacloprid (Gaucho) at 0.5 to 1.0 oz ai/100 lb of seed reduced aphid numbers by >90% for 45 to 60 days after planting. Yield responses to Gaucho use varied with aphid number and BYD incidence but averaged 6.4 bu per acre at the 0.5 oz rate over all trials. A foliar application of lambda-cyhalothrin (Karate/Warrior) on seedling or tillering wheat also controlled aphids, suppressed BYD incidence and prevented yield losses of winter wheat. Foliar applications of dimethoate, disulfoton and methyl parathion reduced aphid numbers but did not reliably reduce BYD incidence and prevent yield losses. In contrast to the South Carolina results, BYD incidence and yield response to treatment were highly correlated with aphid numbers during the first 60 days after planting. Under Georgia conditions, preventative BYD treatments must be applied at planting or within 30 to 60 days after planting. Scouting programs should also emphasize detecting aphids within 30 to 60 days after planting. These results show that insecticides can be used effectively to control aphid vectors, suppress BYD incidence, and minimize grain yield losses on winter wheat in the southeastern United States.