PS Advisor November 2018 Issue

Combatting Weevils

Most rice and grain sold in western countries is fumigated, generally with phosphine. However, this often kills only the live insects, leaving the eggs able to germinate. Surely there are other ways available to the cruisers to extend the life of rice and grains.

Using a 10-pound bag of brown rice, we tested what seemed like the most practical options. We also dug into the extensive research on this topic.

Photos by Drew Frye

Freezing temperatures knocked the weevils out cold—so to speak.


Freezing is fatal to all larvae and adult insects, but unless you have a deep freezer that lowers temperatures to at least 13 F, there’s no guarantee you are bug free. Just barely freezing (as in the case of a typical boat freezer) won’t work. At 20F there is a 30 percent survival rate for rice that is stored for long periods. This is because the eggs of some species remain intact through the winter before hatching in spring. If you keep the rice at below freezing temperature for 72 hours, or can keep it at sub-zero temperatures for a day, your chances of weevil-free flour are much greater.


Any approach using heat must be thorough. Microwaves penetrate several inches, so up to five pounds can be treated without opening the bag, although it should be turned and shaken half way through the process. We tried 90 seconds in a 1500 watt oven and got excellent results with rice, flour, and pasta.


If baking in a conventional oven, it is recommended that you stir the rice half way through the process to insure even treatment. We tried the recommended process—an hour at 250F. Like the microwave, it worked, but it damaged baking mixes.

Vacuum packing

This method does not work. Atmospheres containing less than one percent oxygen are known to be lethal to all insects. No home vacuum sealing equipment can reduce the 21-percent oxygen in ordinary air to that level. Also bags can easily develop tiny leaks, re-admitting air.


Atmosphere’s containing more than 35 percent CO2 are known to be lethal to all insects. Eggs are slightly more vulnerable, with 100 percent mortality when exposed air that is 20 percent CO2. The lack of oxygen also slows the process of turning stale. If dry ice is available, sealing a few ounces of dry ice in a five gallon bucket along with your grain until the ice evaporates should wipe out the insects. You may have to release pressure by lifting the op occasionally


These and other pre-packeged oxygen absorbers ( can kill living insects, but some larvae still hatch.


Baking, microwaving, and freezing will kill weevils. If you are based on shore, these are simple processes to carry out. Carbon dioxide, also, is almost guaranteed to get rid of weevils. Oxygen absorbers are very handy and helpful, though not a certain solution. Vacuum sealing is not effective.

Once you’ve taken care of the bugs, remember that treated products only stayed that way when they are well-sealed. Bugs can crawl though tiny spaces and will chew through plastic bags within hours.

Drew Frye is a technical editor for Practical Sailor. He also blogs at his website

Comments (4)

I'm not clear if this article is saying all brown rice and grains have eggs or only outside developed countries? It would also be helpful to know if there is any health risk to eating the eggs or larvae?

Posted by: Elzaar | October 29, 2018 6:20 PM    Report this comment

All grains can contain eggs, even in developed countries; fumigation is quite effective, but often less than 100%. Additionally, the infestation can come from outside, after packaging but before purchase; grain weevils are present world-wide as an introduced species and can easily chew through paper or plastic bags. We've seen the holes!

We did not investigate the health risk. Certainly, an infested container can be made non-wholesome within a few weeks.

Yes, bay leaves are helpful as a repellent, but not if already infested. We tested that too.

Posted by: Drew Frye | October 28, 2018 11:06 PM    Report this comment

Add a few bay leaves to the containers. Adding salt is also effective

Posted by: Celenoglu | October 27, 2018 1:22 PM    Report this comment

Remember If you find 2 weevils take the smaller, as you must always choose the lesser of 2 weevils!

Posted by: tomdaisho | October 27, 2018 1:00 PM    Report this comment

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