General decline of forages in pastures or hayfields can be attributed to many things. Fertility is generally our first thought. Secondary factors are then assessed to see how they might have played a leading role in the reduction of productivity. These secondary factors include drought, diseases, weed pressure, herbicide injury, soil compaction, or insect damage. Producers that have areas declining in the late summer into fall can suspect insect damage from a soil borne insect complex we call grubs.
Miller County Agent Brock Ward wrote a good description of these beetles, and how to deal with them:
Grubs are the larval stage of beetles that feed on decaying organic matter and some of those species also feed on roots as well. Pastures and hayfields that have had chicken litter or other manure applied as a fertilizer are typically the areas where these grubs are the most severe. It is important to also identify the grubs that are causing the damage as control and management of them can be different. Perhaps the most troublesome is the May/June beetle (MJB). The grubs of the MJB have a characteristic “zipper” pattern in the hair on the underside of their “tail-end”. This helps you to distinguish it from other grubs like the Green June Beetle (GJB) or chafer beetles. When faced with the MJB complex of beetles, the larvae burrows deep into the soil for much of its yearlong lifecycle (as many as three years in Northern states), so treatment isn’t an option, because we can’t expose the grubs to an insecticide treatment with any repetitive certainty. In the case that you have grubs from the MJB complex, renovation or replanting after the emergence of the beetles is likely the best management strategy. Chafer beetles are much like the MJB complex of beetles but require many more of them to reduce a stand of pasture grasses. Typically the source of the forage decline in a pasture is linked to other causal agents when chafer beetles are found in heavy numbers.
If the Green June Beetle (GJB) is the source of your damaged forage, treatment can be managed with insecticide use. The GJB has a one year lifecycle and feeds up through the soil surface before burrowing down again. This feeding habit makes it easier to target the pest. Many pyrethroids are labeled for use on the GJB adults, however the larvae have fewer insecticides labeled for their control. It is unlikely that GJB alone is reducing the stand of forage. As with most secondary factors, it is likely a combination of stressors that begin to reduce the stand. An example, I have seen recently is where drought stress, soil compaction, and grubs were leading to stunted areas in a pasture.