By John Tooker
Perennial-grass forage crops and fall-seeded small grains are easy to ignore this time of year, but I encourage folks to walk these crops periodically to check them for insect infestations, particularly aphid populations. There is a suite of species commonly referred to as grain aphids that can infest small grains and forage grasses, including timothy (Figure 1). Small populations of aphids, no matter the time of year, are usually not a problem, because these small, sedentary, soft-bodied insects are sitting ducks for many common predators like lady beetles and damsel bugs. But in fall, natural-enemy populations reduce their activity because of colder temperatures, and even a few frosts (depending on where you are in Pennsylvania), while aphid populations are more resilient and able to withstand the cold temperature longer, providing them a window of predator-free time.
If conditions are right, aphid population growth can be strong and economically damaging populations may develop. In some cases, these large populations can induce a color change in the crop, causing small grains to turn yellow, brown (Figure 2), or even purple at the tips of leaves and then this color will slowly spread down the plant. This color change is often the clue that aphids are in the field, revealing populations that should have been detected earlier with scouting. Beyond the threat of small aphid populations becoming large, aphids can also be a risk for spread of barley yellow dwarf virus (BYDV). This pathogen may not be evident until spring, but may be acquired by small aphid populations feeding in fall.
Figure 2. Timothy field in Montgomery County browning from aphid feeding. Photo by Andrew Frankenfield, Penn State Extension
I recommend walking small grain and grass hayfields in autumn to check for aphid populations. If populations exceed 25 aphids per foot of row, it would be wise to treat that portion of the field with an appropriate insecticide. See the Agronomy Guide for treatment options.Source : psu.edu