Identification and Control of Weevils
Carrot weevil infestations can be detected by examining plants for oviposition scars, which is much simpler than looking for adults, which are nocturnal. To check a suspected oviposition scar, rub off the dark spot and check for a puncture beneath. If a puncture is found, chances are that a weevil has laid eggs there.
The larvae will tunnel through both the stems and roots of carrots and related plants such as parsley and celery. Their activities stunt plant growth, leave plants susceptible to various pathogens and in severe case kills off plants. or killing plants. Infested plants, carrots roots, celery stalks, parsley are generally unfit for harvest.
Overwintering adults will emerge from neighboring lawns, weeds, garden debris in mid- to late spring, feed and lay eggs on plant stems. When the larvae hatch, they bore into the stems and travel downwards into roots. Then pupate in soil by late spring to early summer. Second generation adults emerge in midsummer under optimal conditions, late summer otherwise.
Most Weevils can't fly, the carrot weevil does have functional wings but rarely flies. Trap crops, susceptible plants placed in uninfected areas will help draw the weevils away. Floating row covers, which are not always feasible for home gardeners has proven effective in commercial operations - bear in mind they are labor intensive and must be removed in a timely fashion to allow pollinators access to the plants.
Drench infested soil with parasitic nematodes. Parasitic nematodes a\k\a beneficial nematodes are especially effective against the weevil larvae. The nematodes 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.
Remove mulch and other plant debris to eliminate hiding places from around plants. If they are a voracious problem cut down on watering, water only when necessary as they prefer moist damp conditions. Hand removal of weevils is somewhat effective in small plantings or container gardens. Remember they are nocturnal and won't be seen much in daylight.
Barriers such as Tanglefoot and diatomaceous earth are helpful in fighting off weevil infestations , as well as against many other garden pests. Tanglefoot, or similar products applied to trunks will prevent these flightless beetles from feasting on foliage.
[Bt] 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.
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 are a few brands readily available to home gardeners.