Found throughout North America, There are over 20 species of root weevils that attack strawberry in North America.
Strawberry root weevils (Otiorhynchus ovatus) attack strawberries as well as raspberries, mint, fruit trees, and some ornamental plants such as rhododendrons.
They are nocturnal and rarely seen during daylight hours, they do most of their damage under the cover of darkness. They are a brownish-black beetle like insect approximately 1/4 inch long.
Like all weevils, they gnaw ragged edges into young leaves. The leaf damage is primarily cosmetic and generally doesn’t seriously effect the plants output.
The larvae stage is the most damaging, the larvae are white, legless grubs with light brown heads that feed on the plant roots and crowns, tunneling into them as they go, inhibiting growth, decreasing production, and sometime killing off plants and entire fields.
An evening stroll in the vegetable garden with a flashlight may help to discover this pest in it’s adult stage, when it’s easier to control the infestation before they reproduce.
Whether you find a few scouts or an invading hoard, their presence indicates more are on the way, either via egg laying or drawing their friends and family from the neighboring landscape for a romp in the hay.
Weevils can’t fly, at night, when they are active, it’s easy shake the adults off the plants and onto a white sheet or paper then destroy them.
Peak egg laying generally occurs from late July through August, lighter egg laying does occur in late may or early June depending on the region. Eggs are laid in the soil near the plant bases, they are white when laid, but soon change to an amber color.
One solution for controlling serious infestations is to uproot and destroy strawberry plants in the infested areas, till the soil to destroy overwintering larvae, and plant anew with better controls in place.
Autumn is a good time to lash out against these weevils by mowing the strawberry plants like you would mow a lawn, the plants will come back in the spring.
In the spring or late summer, apply parasitic nematodes which will help curtail their population expansion. For successful use of nematodes you should allow sufficient time for multiplication of the nematodes in hosts (weevil larvae) and dispersal of nematodes throughout the soil. Early- to mid-May application is optimal.
The larvae are also attacked by some general predator insects including Rove and ground beetles such as carabid beetles. Some strawberry plants have a natural or bred resistance
Milky Spore is another weapon in the war against weevils , it is not harmful to beneficial insects, birds, bees, pets or man. The product is approved and registered with the EPA, Milky Spore will not affect wells, ponds or streams. As larvae and grubs burrow through the soil, feeding -they ingest the spores.
The spores germinate inside the grubs, and multiply rapidly in their blood. When the bacteria become very numerous in the blood they again form spores, completing the bacterial cycle.
Infected grubs survive for 7-21 after their blood has become loaded with spores, particularly in low temperatures. As the grub carcass decomposes, it releases billions of new spores. It is only effective against newly hatched larvae within the soil, not once they enter the plant.
Bacillus thuringiensis (Bt), a spore forming bacteria that produces substances which are toxic to many insect species, but virtually harmless to humans and mammals is somewhat effective against the larvae stages it should be applied as soon as the presence of adults is evident, to head off the imminent egg laying and hatched larvae.
Neem based Products, such as Azatin or Azadirachtin [extract of the neem seed-] prevents the larvae from developing normally and is also a good alternative for later larval stages. These products degenerate rapidly in nature and need to be reapplied frequently.
For effective use mix Neem oil with equal parts of water and saturate the ground around the plants, allowing it to soak in thoroughly.
In severe infestations, there are a number of broad spectrum pesticides registered for use against weevil, sold under varying trade names.
Acephate is one I would recommend. Bonide 941 and 951, Orthene 97.4% Acephate and Agrisel Acephate are a few brands readily available to home gardeners.