There is no concern of insects being found in dried herbs, whether they are heat-dried (as are most herbs and spices found at home or in a restaurant) or freeze-dried. However, (a) Eretz Yisroel is a large exporter of herbs, and therefore one must always be on the lookout for Israeli herbs, which require hafrashas terumah and ma’aser, and (b) freeze dried herbs require hashgachah. The reason for the latter rule is that the process of freeze-drying uses very sophisticated and expensive equipment, and this equipment will not remain idle when they are not producing herbs. Instead, it will be used for shrimp, meat, and other sensitive foods, and since heat is needed for the process, all freeze-dried items require hashgachah.
As with dried herbs, dried spices do not need hashgachah assuming they are “pure”. If a spice blend is just made of herbs and spices, then it is also acceptable to use without hashgachah. However, blends sometimes contain other ingredients – we have seen bacon, citric acid, colors, disodium guanylate (DSG), disodium inosinate (DSI), erythorbic acid, flavor, hydrolyzed soy protein, monosodium glutamate, oils, vinegar, and other ingredients – and therefore either require hashgachah or a careful reading of the ingredient panel to be sure they are truly pure.
Some examples of spice blends which are or are not problematic are as follows: (1) Berbere, Chinese Five Spice, Mulling Spices, and Ras el Hanout have no set formula, and therefore need hashgachah, (2) Chili and Garam Masala sometimes have additives, (3) Herbs de Provence and Tandoori Masala are usually made with just pure spices, and (4) Gumbo File (pronounced “Gumbo Fee-Lay) and Pnach Phoron (Bengali Five Spice) are always just made from specific spices and do not need hashgachah.
It is possible to extract the concentrated flavor component from an herb. If this extraction is done with steam or via cold-pressing, the extract will be called an “essential oil”, such as eucalyptus oil or basil oil. These do not pose a kashrus concern. However, one should be aware that (a) European companies may refer to a flavor as an oil, and would use the name “cherry oil” to refer to “cherry flavor, and (b) oils extracted from vegetables or fruits, such as corn oil or almond oil, are not essential oils, and require hashgachah.