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The Torah prohibits eating a variety of animals called sheratzim (singular: sheretz), as we read, “You shall not degrade yourselves through anything that swarms; you shall not make yourselves impure therewith and thus become impure. For I the Lord am your God; you shall sanctify yourselves and be holy, for I am holy. You shall not make yourselves impure through any swarming thing that moves upon the earth” (Leviticus 11:43-44).
There are three types of sheratzim: those found in water, such as frogs, shrimp, lobsters, and small worms; those which crawl on land, such as turtles, lizards, centipedes, worms, and some insects; and those which fly in the air, such as flies, mosquitoes, wasps, and bees.
Eating even a tiny sheretz is prohibited by the Torah. Therefore, if a fruit is beginning to rot, one should check it for worms before eating it. In the past, insects and worms were often found in flour, grains, and legumes. Because of this, they had to be checked or sifted before use. Nowadays though, this is not required as long as they are supplied by reliable companies and sold in clean stores. It is still a good idea to store them in dry, cool areas so they do not become infested.
Small insects are often found in leafy vegetables and herbs, such as lettuce, cauliflower, broccoli, parsley, and dill. They are often a similar color to the leaves. Therefore, to remove any insects, one must wash these vegetables. It is proper to soak them for a few minutes in a liquid that can loosen the adhesive on the sheratzim's feet (such as salt water, soapy water, or vinegar). Rinsing them off afterward will easily remove the insects, as well as any residue of sprays or poisons used on the leaves. This is the primary halakhic position. Some are concerned that even soaking and washing vegetables might not get rid of all the insects. Therefore, they buy vegetables grown under conditions that do not allow sheratzim to develop. Standard kosher certifications generally rely on the first position, while mehadrin certifications follow the more stringent opinion. (See section 34 below.)
Certain types of fish are commonly infested with worms. Since eating worms is prohibited, kosher certification on these types of fish should include the removal of the worms. If it does not, the innards of the fish should be thrown out and the fish should be skinned. The head should either be thrown out or cleaned well, in which case special attention should be paid to the mouth and gills (where worms are often found).
