Several different insects including a group of beetles collectively known as root weevils can attack strawberry plantings. There are several common pests in this complex including the strawberry root weevil (Otiorhynchus ovatus), rough strawberry root weevil (O. rugosostriatus) and the black vine weevil (O. sulcatus). Any of these weevils can seriously damage strawberry plantings if left unchecked.

The main damage these insects cause is by feeding on the root system thus weakening the plant. Root feeding is done by the immature larvae (grubs) of these insects. Under heavy pest pressure, root feeding can severely stunt the plants causing yield reductions. Severely damaged plants can die during summer drought or during cold winters. Adult weevils will feed along leaf margins causing leaves to have scalloped edges. This damage is not considered serious unless there are large numbers of feeding adult weevils.  The arrow in Figure 1 points to an O. sulcatus larva (black vine weevil) on a strawberry root, while Figure 2 shows black vine weevil damage in the field.

Generally, strawberry root weevils can be managed by simply rotating plantings to new areas. This delays infestations of new plantings because adult strawberry weevils don’t fly. However, rotation is not always an option for growers with limited space. While chemical insecticides can be effective against weevil adults, there are no effective insecticides registered for controlling root-feeding grubs in established plantings. Fortunatly, good biological control of these grubs can be achieved with insecticidal nematodes.

Selection of an insecticidal nematode product
There are 2 types of insecticidal nematodes effective against grubs of weevils that attack strawberries. These are Heterorhabditis bacteriophora and H. marelatus. Both nematodes can be extremely effective if the nematodes are viable and applied correctly. Other insecticidal nematodes are available including Steinernema carpocapsae but these are ineffective against strawberry weevils. Therefore, it is important to make sure you purchase and apply the correct nematode.

Storage and handling
Store the product according to the directions supplied by the manufacturer. In general, store nematodes in a cool dry place out of direct sunlight. Refrigeration is sometimes required. Do not allow them to freeze or be exposed to extreme temperatures. Do not store diluted products.

Application
Insecticidal nematodes can be applied through conventional or backpack sprayers or irrigation systems. Peter Nitzsche (Figure 3), a Rutgers Cooperative Extension Agricultural Agent, is shown applying insecticidal nematodes to strawberry using a backpack aprayer.   Application rates vary but generally 1-2 billion nematodes are applied per acre. Insecticidal nematodes can be tanked mixed with certain products but this is not recommended because not all tank-mix combinations have been investigated for compatibility with nematodes. In addition, H. bacteriophora are more sensitive to insecticides than other nematodes thus its best not to mix these nematodes with insecticides. Instead, apply these nematodes at least 2 weeks before or after a pesticide application.

The following points should be followed when applying nematodes:

1. Make sure application equipment is clean,
2. Apply nematodes during early morning or late evening when lethal sunlight is minimal,
3. Apply nematodes to moist soil, never soil that is hot and dry. Pre-irrigate with at least 1/4 inch of water before applying nematodes if soil is dry,
4. Apply nematodes when soil temperature is between 60 and 85°F.,
5. Apply nematodes with a high volume of water to prevent nematodes from drying out before post-application watering is initiated,
6. Agitate the sprayer to ensure proper mixing and dispersion of the nematodes during application,
7. Use 50-mesh (or coarser) screens to prevent clogging or remove screens if necessary,
8. Apply at least 1/4 inch of irrigation immediately after applying nematodes to help move them into the soil.

Disposal
Spray remaining nematodes and sprayer rinse water directly on the crop or dispose of on-site or according to federal and local regulations.

Evaluating results
Control of grubs can be expected if the nematodes were in good condition, properly applied, and environmental conditions (good soil moisture and temperature) were favorable. Mortality is temperature dependent. It takes longer for the nematodes to kill grubs in cooler soil than warmer soil. However, nematode infested grubs should be visible within about a week after application. Dig up plants that you suspect are infested and look for the insects near the roots and crowns. Nematode-infected grubs and pupae should have a rusty reddish-brown color compared with the creamy white color of healthy grubs and pupae.  Figure 4 shows a H. bacteriophora infested O. sulcatus pupae on the left and a healthy pupae on the right. 

Suppliers
Refer to the Insect Parasitic Nematode Retail Suppliers page.