The Pesky Problems of Boat Pests

Most insects we find on our boat are harmless, and proper identification is a good start.


Ever since I fell under the spell of E.B. White’s classic tale “Charlotte’s Web,” I’ve been more than reluctant to stomp on every bug we see on deck—especially spiders. I know my arachnid empathy might come back to bite me, literally. Nevertheless, when someone on board shouts, “Spider!” I still reach for a clear plastic container (to trap the uninvited guest) and sheet of notebook paper (to slip under the plastic dome, for extraction).

Each time I do, I imagine prominent American philosopher and famous bug lover E.O. Wilson nodding with approval.

I know that my love of bugs is not universal—especially for those bugs regarded as pests, so for those who’ve not the time or inclination to trap and release tiny creatures who bite, sting, or devour property, here are some other ways to deal with pests on board.

Grain Weevils. We described in detail the treatments (microwaving, baking, CO2, etc.)  for grain weevil in PS November 2018, so we won’t repeat those tips here.

Spiders. Fundamentally, we’re both on the same side. They eat insects. However, at some point they become annoying. The best way to remove them from the cabin is to eliminate their natural food source by eliminating insect pests. Because they are at the top of the food chain, they are more susceptible to pesticides that most insects, so any pesticides applied for insects will probably kill the spiders.

At certain times of year and in certain areas, spiders can descend on the wind, each predator dangling from a single thread of silk. The lifelines create a natural framework, and the breeze delivers a steady supply of mosquitoes and small flying insects. The dark corners of the cockpit are naturally attractive to no-web spinning spiders.

There seems little point in trying to poison them on deck; fresh ranks arrive daily. But repellents have proven reasonably effective, at least in the cockpit and around the companionway. We’ve used Star Brite Spider Away with considerable success, focusing on dark corners and the edge of the companionway. They are less effective on the lifelines, but if you go sailing, the wind and spray will generally take care of that.

Ants. Because they need a fresh water source, these are typically only an issue on the hard or when there is fresh water in the bilge. Eliminate the water source. If they persist, we found Terro Liquid Ant Bait trays to be quite efficient. Some of the ants will die in the tray or in the immediate vicinity, but others will carry the poison back to the nest, eliminating the nest. They should be placed as close to the nest as practical and where the ants run. If there are small children or pets on the boat, locate them so that they are inaccessible.

Fruit Flies. The simplest cure is prevention. Keep fruit either in a basket draped with netting or in a net laundry bag. Monitor your store rigorously for signs that something is getting overripe and eat it first.

In a house, vinegar traps and sticky paper work. But a brisk sail with the hatches open usually blows them out, if there is no over ripe fruit to hold them. At night, a vacuum cleaner is quite effective at collecting the flies and mosquitoes that gather around cabin lights (after you put the netting up).

Bees and Wasps. Prevention is best. Seal up holes, particularly in spars. Try to remove the nest at the very beginning, when they are relatively harmless.

The best answer for a bees nest is a bee keeper. Most of the bees in the Caribbean and Mexico are Africanized; they get mad, swarm, and don’t stop stinging. The keeper will move the queen to a box, along with some comb, wait for the workers to return in the evening, and remove the box after dark. Try moving the boat mid-day; the workers that are out will be unable to find the boat-repeat as needed.

Wood-boring beetles leave pinholes and worm-like trails where they’ve been at work.

Roaches. We’re a fan of boric acid and borate-based cleaners such as Formula B (Homemade Mildew Preventers That Really Work, PS Blog May, 2018), in part because they are effective biocides, but also because bugs hate them. Thus a little boric acid in the pantry makes sense to us. The roaches do not eat it directly, but it does get on their legs, and when they clean themselves they die.

When borax-based cleaners have not worked, we’ve found that either Terro Liquid Ant and Roach Bait trays are effective, sometimes using both.

Termites. Cabinet damage is bad, but if they get into the glassed-in wood, the boat may become a total loss is short order. Bug bombs don’t work-the galleries are too deep. DIY injections are sometimes effective against minor infestations, but often as not they fail to reach the whole colony. Our advice is act immediately; while you fiddle around with do-it-yourself solutions, the bugs are chewing wood and devaluing your investment. Better to admit that its time to call in the professionals.

For more on dealing with pests, including some eco-friendly preventions that won’t put the environment or your crew at risk, see PS August 2019, “Giving Bugs the Big Goodbye.” If you do get bit by a spider the NIH offers an identification guide, and advice on treatment.

Finally, if you have a favorite insect preventative or entertaining insect story to share, I’d love to hear about it in the comments section, or you can send it direct by emailing me at

Darrell Nicholson
Darrell Nicholson is Director of Belvoir Media Group's marine division and the editor of Practical Sailor. A lifelong thalassophile, he grew up sailing everything from El Toro dinghies to classic Morgans on Miami's Biscayne Bay. In the early 90s, he left a newspaper job to sail an old gaff-rigged ketch across the Pacific and has been writing about boats and the sea ever since. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at


  1. Living in a coastal Oregon logging area, we have an annual termite swarming event. At times it’s impossible to walk the beach without stepping on termites with each step. Understandably, these hungry insects end up on many boats, our sailboat included. Although we checked far and wide in search of an exterminator who could treat our boat, they all declined. Our solution was to sail to Mexico and leave the boat on the hard for a summer in the hot sun. When we returned the next fall all termites were naturally exterminated from the high heat.

  2. When i bought an old steel 9 metre sailing yacht in Brasil, it was quite literally crawling with cockroaches, some as big as 5cm long. Sprays had no effect, and the cardboard ‘roach hotels’ were little better. Then we scattered piles of Boric acid powder in every dark corner. Magic ; after just one week, we never saw another cockroach. By the way, please note that ‘Borax’ is a trade name of a cleaning soap. It is not the same as boric acid, although there may be chemical similarities. I have been told that powder trademarked ‘Borax’ will not have the same effect. I believe that boric acid may also be called boracic. But check for yourself on the internet, the info is all there. You should be able to buy boric acid powder in any chemist/pharmacy, anywhere in the world ( ? ).

  3. Mosquitoes: you may be breeding mosquitoes on board. Many mosquitoes breed in standing water. Maybe not salt water but even boats in salt water get rained on or bilge freshwater. Any standing water that doesn’t dry out, like anchor lockers or bilge or around a drain or inside a drain hose. Dry them out. To check the water for larvae scoop some up and look closely for wiggly larvae 1/8th to 3/16th inch. 4 to 6 days to mature into flying hungry adulthood. Larvae die if dry. Example:My boat’s fuel locker for the outboard drains rainwater but leaves a quarter inch and that’s enough to breed. There is a product that you leave in water that kills larvae but is claimed to be harmless to pets. I don’t know but i don’t think it’s harmful to rope. It’s sold in little donuts in hardware stores, easy to break a donut into chunk, leave in water. “Mosquito Dunks” is trademark brand, a bacterium bacillus thuringiensis israelensus bti supposed to last a month.
    (btw on land, rain gutters and flower pots breed zillions. Fish eat the larvae so many ponds with enough fish don’t breed many. Empty out pots, trash, anything, clean rain gutters and flush, then get in boat and sail away)

  4. A solution of rosemary oil and vinegar can be applied to dock lines and other surfaces to discourage spiders. Obviously does not harm them, but can keep them off. In the alternative, just teach the larger onesd to roll tack.

  5. We lived in Floridafor 35 years in 3 different houses that we had built on land we purchased. Through out the construction phase, I would sprinkle Roachproof (boric acid powder) in all the nooks and crannies I could find. We never had roaches in the house. We also did everything to give the yard lizards a good home!