Problem type: Insect
Name of problem: Nightcrawlers or dewworms
Plant name(s): Turfgrass
Symptoms / Characteristics:
Small lumps appear in affected turf due to the accumulation of worm castings or excrement. The lumps are not actually harmful to turfgrass plants; they are mainly an aesthetic issue. However, mowing over these lumps may lead to scalping, which can be detrimental to the health of the grass. Damage typically appears during the spring and fall. During the summer, warm and dry soil conditions cause the worms to retreat deeper into the soil.
Nightcrawlers or dewworms are actually a type of earthworm that migrates to the soil surface during the night or during periods of high moisture. They feed on thatch at the soil surface and proceed to deposit large amounts of waste, creating lumps in the lawn. If a few lumps can be tolerated, the worms can be extremely beneficial. Nightcrawler activity improves aeration, water percolation, soil fertility and soil structure. Nightcrawlers also reduce thatch levels that otherwise inhibit the penetration of adequate oxygen, water and fertilizer.
Control / Preventions:
Nightcrawlers surface during the night and after rainfall, allowing for easy collection. Incorporating birdfeeders and waterers into the garden will attract robins and other natural nightcrawler predators. Avoid excessive irrigation and avoid frequent, light waterings in order to reduce nightcrawler activity. A good soaking once a week should suffice, depending on the local weather conditions. In Manitoba, the recommended irrigation rate is one inch of water per week, including nature's contributions.
Power raking (vertical mowing) is perhaps the most effective way to break up nightcrawler lumps and reduce excess thatch. Power raking can be done in early spring, prior to bud break, when the plants are still dormant. Early fall is also acceptable. Power raking during the summer months should be avoided as it can cause extensive injury to actively growing plant tissue. It is important to realize, however, that power raking is only a temporary solution and will not eliminate the problem as long as the nightcrawlers inhabit the soil. Regular aeration will improve soil structure and drainage thereby reducing the occurrence of nightcrawler lumps.
Lawn rollers (though often recommended for smoothing out the lumps) cause soil compaction and ultimately create far worse problems for any plant that inhabits the soil, including trees, shrubs and turf.
Chemicals are available for controlling nightcrawlers but are not recommended as they eradicate other soil inhabitants and non-target organisms, including those that prey on chinch bugs, white grubs and other destructive turf insects. Tolerating the nightcrawlers will preserve the natural lawn ecosystem and improve the overall health of the lawn.