Galls are best collected not too long before they reach maturity. Only experience with each species, involving continued observation of specimens of the galls, and obtaining of such information as may be supplied by the size of the gall, the degree of development shown by the insect within the gall, etc., can supply information of the approach of the emergence date for a particular species. Most of the solid, hard, woody, or dry-shelled galls may be kept successfully for many months, but the adults of soft, spongy, or hollow, succulent galls cannot be raised if the galls are gathered more than a week or even a few days before the insects are due to emerge.
Sufficient care is not usually exercised to keep separate the several species collected at any one time. Confusion as to the species of the host plant from which the gall was obtained and the hopeless confusion of any adults that may emerge from the galls before they are finally sorted are the results of putting galls of several species into a single receptacle. A large part of museum material is rendered worthless by such methods of collecting. Paper bags of several sizes are readily available means of separating the galls in the field but, if extensive collecting is to be undertaken, it is preferable to make cloth bags, which are much stronger, take up less room, and may be more securely tied at the top than paper bags.
Undoubtedly the most satisfactory means of rearing the adult wasps is to place the galls on an inch or more of moist sand in a low glass jar at least four or five inches in diameter. The moisture thus supplied keeps the galls from drying out and dying; but the galls should not be piled in too deeply or those on top will not mature and those on the bottom will rot. The glass may be covered with cloth fastened on by an elastic band (which does not wear out-of-doors), or tied with twine (which is less convenient if it is desirable to open the jars a number of times). The jars must be broad and low, and should be kept out-of-doors, under some shelter but where the normal temperature and an abundance of air and wind will prevent mold from developing. Indoors, mold is almost certain to prove a problem. The use of earthern flower pots prevents too much moisture accumulating in the sand, and such receptacles for that reason are preferable to glass jars, but in the pots the emergence of the insects is not so easily observed. Galls that are to be kept over winter or for a long time before the insect matures may be kept in earthen pots until the approach of the date of emergence.
The growth of the galls under controlled conditions should be effected on plants growing in cultivation and kept from possible attack
