what products would be good to use around boat docks & piers for spiders, slugs, snakes, flies, gnats, mosquitoes, dragonflies?
Treating docks that extend out over the water is a very touchy area. There are actually many products specifically labeled for use on this site and you can view the entire list quickly on PestWeb in our Product Documents resource. Select the "Products by Approved Site" tab and then "Exterior Non-landscaped settings / Boat docks and piers". What you will find for insect control is that all of the products are pyrethrum based, so no residual insecticides made the list. The problem is that any pesticide dripping into the water below presents a problem, whether or not it really ends up there at a level high enough to kill any aquatic organisms. Just introducing toxic materials into public waterways is not allowed, so spraying the dock itself with a hand sprayer is unlikely to be allowed. The Maxforce Fly Spot Bait you see on this list, which might be an excellent product for the fly control, is labeled for "Marinas", so exactly how you interpret that site is a good question that should be discussed with your local regulatory agency. Since the fly bait can be "painted" on carefully and none of it should fall into the water, this may be a choice for the flies on your list.
Otherwise, any application would need to have the assurance that no liquid (or solid) material could fall into the water, so treating the docks themselves may not be possible other than by fogging toward pests such as spiders, or fogging onto mosquito resting sites. But, let's address the other animals on your list. First - dragonflies. In my opinion there is no reason at all to kill any dragonfly, as they are completely harmless to people and highly beneficial due to their predatory habits. If people are annoyed because dragonflies keep hanging around, oh well......... they really do need to learn to accept that not all bugs should die.
Snakes and slugs should be addressed at their origin, which is on the land that connects with the dock. For slugs the use of bait products around the areas near the entrance to the dock will help to kill many of them and prevent them from slithering out onto the dock itself. The only reason I can think of that would lead slugs to live on the dock would be the presence of algae that they might feed on, and this could be removed with a power washer. Snakes would be unlikely to live on the dock, so removing their habitat on shore, eliminating any rodents they may feed on, and perhaps using snake traps within closed stations on shore to capture them would be the answers.
Spiders are tougher, but again we have no options for applying residual products to the dock itself. You can find at least one of the web removal products on the list on PestWeb, so this could be a help in eliminating webs and preventing new webs on some surfaces. Otherwise I believe we are down to vacuuming and perhaps fogging with pyrethrum, which will have limited effect.
Mosquitoes and gnats may be controlled, in part, with source management. If they are breeding in this waterway itself then perhaps the use of labeled larvicides will help to reduce their numbers. But, since this likely is a larger lake or river that may not be an option, so that leaves us with trying to control the adult insects, always a bad option. The use of light traps will draw many of them, but these might be best placed away from the dock to draw the bugs to the traps and not to the docks. Otherwise, it is wearing repellents and long sleeves during those periods when the mosquitoes are active, and keeping unnecessary lights off to avoid attracting gnats.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.