Using Boric Acid On Trees to Kill Fruit Flies

Aug 19, 2011
Rena Jean Sherwood

Drosophila repleta, a fruit fly - Bbski/Wikimedia Commons

Although a boric acid sugar water spray will kill fruit flies in trees, it needs to be handled with caution.

Boric acid powder, also known as orthoboric acid or hydrogen borate, found often in powders to kill cockroaches, does kill fruit flies when mixed with sugar water, notes Dead Snails Leave No Trails: Natural Pest Control for Home and Garden (Random House Digital, Inc.; 1996.) The same solution also kills ants on trees. Unlike other pesticides, boric acid will not repel insects like fruit flies.

Unfortunately, boric acid also kills people, so it needs to be handled with caution. Boric acid may also chemically burn leaves, so always spray a few leaves leaves a day before spraying an entire tree to test its effect.

How Boric Acid Kills Fruit Flies

Boric acid kills fruit flies in two ways. First, boric acid corrodes the protective waxes on insect exoskeletons. This causes bodily moisture to escape and the insect to dehydrate. When swallowed, boric acid poisons the stomach by interfering with the body’s ability to absorb electrolytes. The body becomes too dry and stops functioning. Boric acid, touted as a “natural” alternative to conventional pesticides, it is still a poison.

Method of Use

Boric acid solution spray is best used on trees instead of indoor plants. Combine 1 tablespoon of boric acid powder; 1 tablespoon of sugar and 1/3 cup of water. Fill a garden misting bottle with the liquid. In the morning when dew is still on tree leaves, spray 1 or 2 leaves to test if the spray is too harsh for the tree. Wait 24 hours to see if the leaves change color or rot. If the leaves look the same, spray plants. The sugar will attract hungry fruit flies and the boric acid will kill them.

Warning

Never spray boric acid solution on or near fruit, vegetables or herbs destined to be eaten. Boric acid may accidentally be swallowed since it is odorless. Symptoms include red skin rash; vomiting blue-green liquid; diarrhea; fever; drowsiness; blisters; skin peeling; sudden decrease in urination and loss of coordination, even to the point of going into a coma. The US National Library of Medicine notes that small children can die from boric acid poisoning, also known as Borax poisoning.

Alternatives

Boric acid solution is not entirely safe to use indoors, but fruit flies can still be controlled without using poisons. For very tiny fruit flies above fruit or in a vegetable garden, use a hand-held vacuum cleaner to suck the flies out of the air Dispose of the bag or the bag’s contents in an outside trash can immediately, so flies do not have time to crawl back out of the vacuum.

Another alternative recommended by University of Kentucky entomologist Mike T. Potter is to use traps baited with cider vinegar. These can be used indoors or outdoors. Place a tablespoon of cider vinegar into a glass or plastic jar. Roll up a sheet of paper to make a funnel and place in the jar. Make sure the bottom of the paper funnel does not touch the cider vinegar. Fruit flies crawl in but the funnel prevents them from escaping.