"Another approach to dealing with insects in lettuce is to grow them insect-free. This has indeed been accomplished by some companies in Israel... However... they could not prevent certain flying insects from landing on the crop after it was harvested. Fortunately, however, these flies are easily noticeable and wash off readily with a simple water rinse, and customers are therefore admonished to rinse these products before use. To ensure that this directive is heeded, the lettuce is sprayed with “clean” sand prior to packaging – making then virtually inedible without compliance!" (source)
And I was wondering why the insect-free lettuce here has so much "dirt" on it. It looked like a scam - charge exorbitant prices for a low-quality product that they don't even bother to clean properly. And in fact it was merely a clever trick to make sure that what we ate was in fact kosher. Perhaps the kashrut situation in Israel is not quite as dysfunctional (relative to the US) as it seems.