I live in a condo Toronto with a south facing balcony garden that has very large potted plants.
My garden has a problem with Black Vine Weevils who show up late in spring and completely destroy all of my plants. How can I eradicate this problem and what products and methods are available? In addition, my plants are severely depleted of nutrients. What can I do to solve both problems this spring?
Sorry to hear you have this annual challenge with this destructive pest.
In order to successfully tackle a pest problem it is necessary to understand the life cycle of the pest. The black vine weevil spends winter usually as a larva, in the soil around the root zone of the plants in which it feeds. Occasionally some adults may survive winters if they find suitable shelter, including areas in and around homes (so keep a eye out for them!). Larvae resume feeding in the spring and after becoming full grown they pupate in the soil, and adult weevils emerge in mid-June. They continue to feed on the plants during the night and cause characteristic notching wounds. After about 2 weeks, the females begin to lay eggs around the base of plants. Eggs hatch in mid summer, and the legless larvae feed on plant roots until cold weather temporarily stops their development. The good news is only one generation are produced per year so the key is to interrupt this cycle.
Methods of control to try first are: shaking the weevils off the plants at night onto to dropcloth and destroy them. You could consider repotting the plants to change the soil, and you can recruit beneficial micro organisms . The most widely used microbial biological control method is the application of Bacillus thuringiensis, or Bt and B.t. var. san diego (Bt-sd) controls black vine weevils (among others). There is speculation that use of this microbial control could develop resistance so it is best to use Bt with care, and only after trying cultural or physical control methods for which resistance is not a factor. You can purchase Bt in many forms, available at most garden centres (they would recognize it if you mentioned you wanted Bt-sd insecticide). Enquire at your nearest garden centre. Follow label directions carefully.
You can also apply parasitic nematodes to the soil (again enquire at your garden centre) with the aim of control of the insect pests during their larvae stage. Timing in application is key so follow the directions carefully. As much as we all wish for a one time elimination solution you may have to re introduce the nematodes each year and/or continue with the other control strategies year on year.
With regard to adding nutrients to the soil you can do this by adding a liquid fertiliser when you water your plants (again follow the instructions on the bottle) and you also have an opportunity if and when you repot or refresh the soil in the pots. Be sure to use a soil specifically for containers and you can mix in some slow release fertilizer pellets. Applying mulch on top of the soil will also help with moisture retention to the benefit of the plants especially given their sunny southern location. Good luck with overcoming this years infestation and I hope your plants look their best yet on your balcony this year.
Cornell University also has some information on this pest that you might find useful including a life cycle chart: