Grubs In Pasture and Hay Fields

Over the last three or four years, more reports are coming in of stand loss due to grubs in pasture and hay fields (Figure 1). During dry years their impact can be exaggerated as the crop struggles to deal with two major stresses at once. Stand loss can also result in poor overwintering of the crop, and an increase in weed establishment. Although there are limited options for management of grubs, knowing if grubs are present and which ones are feeding on your crop can help manage their impact. 

Figure 1. Extreme grub damage in hay field in Eastern Ontario. Photo credit: Chad Garrod, O'Neill's Farm Supply. 

Which Type of Grub?

Knowing which grub is feeding on your crop can sometimes help determine when they will be causing the most injury and whether they are going to be a problem for several years to come. Fall is a great time to sample for grubs, as long as you do it before the ground starts to freeze. After that, the grubs move deeper in the ground and wait until spring to feed again. 

Take a shovel and pail or plastic container to the problem areas. Grub activity is usually higher in sandy areas and on knolls, particularly on the south facing slopes. Dig a soil sample 1 foot by 1 foot by 6 inches (30 cm X 30 cm X 15 cm) and place it in the container. Sift through the soil with your hands, breaking up any clumps. Count how many grubs are found in each sample. Take a look at their raster patterns to determine what type of grub they are.

European Chafer - 1 Year Life Cycle

European chafer have a one-year life cycle. Adults lay their eggs in moist soil during June and July. The larvae hatch from these eggs and begin feeding late-July or early-August until the ground freezes. The larvae then move deeper into the ground, coming back up in the spring to feed again, until they start to pupate in May. Like all grubs, they are white and C-shaped, with an orange-brown head. To distinguish them from other grubs, you will need to look at their posterior end and determine the pattern of rasters (anal hairs). European chafer have a "Y"- shaped pattern compared to other grubs (Figure 2).

Figure 2. European chafer Y shaped raster pattern on posterior end of the larva.

June Beetles - 3 Year Life Cycle

June Beetles have a three-year life cycle, so are more damaging and more difficult to manage. The start of year 1 is spent as an adult, which does not feed on the crop. The adult emerges from the soil in May to mate and lay eggs in the soil in June. These eggs hatch and the small larvae begin to feed until late-fall. June Beetle grubs do tend to be less tolerate to cold soil temperatures than European chafer, and may move down deeper in the soil earlier than chafers. These June beetle larvae then continue as larvae throughout the entire second year of their cycle, capable of feeding throughout the year, and increasing in size to reach their final instar before overwintering. The third and final year of their cycle starts as a full grown 3^rd instar larvae, which feed until about mid-June or early-July, when they finally pupate and become adult. Adults remain in the soil to overwinter and begin the life cycle again the following spring. Their rasters are in an oval shaped pattern (Figure 3).

Figure 3. June beetle oval shaped raster pattern on posterior end of the larva.

No Chemical Control Options

Although there is no economic threshold for grubs established yet for pasture, 1 to 2 grubs per square foot most likely indicate a concern. Unfortunately, no chemical control options are registered. Although some products have been proven to work in turf, most potential foliar options have been shown not to be effective in field situations. The best method of managing a grub problem is through cultural methods, as well as good pasture and hay management.

Control Options

Well managed pasture with a good mixture of legume and grass species may help to reduce stand loss, as grubs tend to feed more on the grass species. Overseeding or reseeding may be required for a few years to compensate for what the grubs have taken out.

Plowing may be necessary (where possible) to try to reduce heavier populations of grubs. It may be necessary to rotate out of the susceptible forage crop into another field crop (such as corn or soybeans) where insecticide seed treatments are available to help reduce the grub population. There are no seed treatments registered for grub control in cereals.

Although there are some natural enemies of grubs, they tend not to dramatically reduce populations.. A bacterial disease (Bacillus popilliae) known as the "milky disease" can infect grubs, particularly in lawns during wet years. There are also some predators, including ground beetle larvae, that can help but have not shown reduction of these pests in field crops.

For more information on grubs, refer to the OMAFRA Publication 811, Agronomy Guide for Field Crops, and the CropPest newsletter on the OMAFRA website.

Related Links

• OMAFRA Agronomy Guide for Field Crops
• OMAFRA CropPest Newsletter

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