The grapevine beetle, commonly known as the spotted pelidnota (Pelidnota punctata), is a large scarab beetle that can cause concern for grape growers across North America. While infestations are often minor, a significant population can damage the foliage and fruit of grapevines. Management requires targeting the beetle in both its adult and larval stages using non-chemical and chemical controls.
Identifying the Grapevine Beetle and Damage
The adult grapevine beetle is a distinct insect, measuring about one inch (2.5 cm) in length, with a body color ranging from off-yellow to a rich auburn red. Its wing covers, or elytra, are marked by a pattern of three black spots on each side, which helps distinguish it from other scarab species. These beetles are nocturnal, spending the day resting on the vine and becoming active at night to feed and fly, often attracted to lights after dusk.
The life cycle typically spans two years, with two distinct stages causing damage. Adults emerge in the summer (late June through September) and feed by chewing on grape leaves, sometimes leaving behind a skeletonized appearance, and occasionally consuming ripe fruit. The larval stage is a pale, C-shaped grub that lives underground, growing up to two inches long.
Adult females lay eggs near the base of host plants, often in rotting wood or decaying organic matter. The grubs feed primarily on this rotting material rather than living plant roots, meaning the pest is often not a serious threat to a healthy vine. However, high population density or grubs shifting to feeding on fine root hairs can contribute to vine weakening.
Non-Chemical and Physical Removal Strategies
Physical removal is an immediate and effective strategy for managing adult grapevine beetles, particularly because of their large size and predictable behavior. Since the beetles are most active after sunset, the best time for collection is at dusk or in the early morning before they settle in for the day. Simply handpicking the adults from the vines and dropping them into a container of soapy water ensures their disposal.
Another effective method involves placing a sheet or tarp beneath the vines and gently shaking the plant in the early morning. The beetles drop easily when disturbed in cooler temperatures and can be collected and destroyed before they become active. For small infestations, an organic insecticidal soap spray can be used to stun the beetles, making them easier to hand-collect.
Cultural practices around the vineyard can significantly reduce the potential for future infestations. Removing dead stumps, rotting logs, and excessive leaf litter near the vines eliminates the primary locations where adult females lay their eggs. Tilling the soil in the late fall or early spring can expose overwintering larvae and pupae to predators or harsh weather conditions, disrupting the two-year life cycle.
Biological controls offer a long-term, targeted solution for the larval stage in the soil. Specific beneficial nematodes, such as Heterorhabditis bacteriophora, are microscopic roundworms that actively seek out and parasitize beetle grubs. These nematodes are applied to the soil as a drench and are a safe, natural way to reduce the grub population without harming beneficial insects. For optimal results, nematodes should be applied when soil temperatures are between 60 and 80 degrees Fahrenheit, typically in late summer or early fall.
Strategic Use of Chemical Controls
When non-chemical methods are insufficient, targeted use of insecticides becomes necessary. Application must coincide with the pest’s active periods, treating nocturnal adults or subterranean grubs. Organic options such as pyrethrins or Spinosad can be used as a contact spray to control adult beetles.
Sprays must be applied in the late afternoon or evening, just before the beetles begin their nightly feeding activity, for maximum effect. Applying a repellent such as Kaolin clay, which leaves a fine white barrier on the foliage, can discourage adult feeding and egg-laying. This physical barrier must be reapplied after rain to maintain efficacy.
For the larval stage, a soil drench using a systemic insecticide can be effective, though this is generally considered a last resort due to potential environmental impact. These products are absorbed by the plant and move through the tissues, making the roots toxic to feeding grubs. However, since grapevine beetle larvae primarily feed on decaying wood, soil treatments should be carefully considered based on the severity of the root damage observed.
Regardless of the product chosen, always read and follow the label directions precisely concerning application rates and safety precautions. Avoid spraying any insecticide during the vine’s bloom period to protect pollinators, such as bees. Using the least toxic product that achieves control and applying it at the correct time minimizes risk to non-target organisms.