A Method for Rearing Wireworms (Elateridae)
by Harry R. Bryson
Journal of the Kansas Entomological Society, Jan. 1929
The problem of rearing wireworms successfully has been a very difficult one for investigators interested in ecological studies of subterranean insects. Since these insects spend a greater part of their life cycles in the soil, it is a laborious task to observe their responses to changes in food, temperature, soil moisture, and other factors which modify their environment and in turn affect the lengths of their life cycles.
When wireworm larvae are collected and confined where they have no opportunity to select their environmental conditions, the mortality, no doubt, is greatly increased over that occurring in nature. It is difficult to determine whether the new environment will increase, decrease, or have no effect upon the lengths of the life cycles of the larvae that survive. Hence, it is almost impossible to ascertain how accurately the results of such rearings check with the length of time required to complete the cycles under natural conditions. It would be desirable, therefore, to employ a method of rearing which would approximate natural conditions.
The writer has had excellent results with a method which apparently approaches very closely natural conditions, and which should prove a valuable aid in solving many of the wireworm rearing problems.
Lane (1924) had very good results rearing wireworms in galvanized cylinders. Davis (1915) found that large flower pots could be used successfully in tearing white grubs. Graf (1914) tried several types of rearing cages for wireworms and found large flower pots to be the most satisfactory. Hyslop (1915) used a root cage in rearing wireworms, made by sinking a molasses barrel to the level of the surface of the earth. The barrel was filled with earth and the top closed with a short cylinder of sheet iron covered with gauze. McColloch (1917) described a method for rearing subterranean insects. This method proved more successful in white grub studies than in the case of rearing wireworms.
The method which had been used for rearing wireworms at this station prior to 1928 was the salve box method. Larvae collected from the field were placed into individual one-ounce salve boxes filled with sifted soil which contained enough moisture to insure the germination of the three grains of wheat placed therein to furnish food for the larvae. These boxes were then numbered to coincide with a serial number on the record sheet.
The moisture content of the soil and the food supply were maintained by changing the soil and replenishing the food every week or ten days throughout the summer. The soil alone was changed at monthly intervals during the winter when the larvae were inactive. These boxes were then placed into a rearing cave, described by McColloch (1917).
The foregoing method was supplanted by the tile method during 1928 with much better results. Ordinary unglazed drain tiles six inches in diameter and one foot in length, were used. In setting each tile, a hole was dug 22 inches deep. A smooth, flat rock was use in the bottom of the hole to furnish a base for the lower tile to prevent the escape of the larvae. Two of the tiles were matched together so as to leave no opening through which the larvae might escape. The bottom tile was placed on the rock and the dirt tamped solidly around it to hold it in position. The second tile was then placed on top of the lower one and the soil was tamped solidly around the column until level with the surrounding ground.
The remainder of the soil taken from the hole was sifted, examined for the presence of insects, and then tamped lightly into the cylinder, filling it to the top. After the soil within the cylinder had settled; it approached the level of the soil surrounding the tile. Wheat and oats planted thickly in each tile furnished an abundance of roots and sprouting grain for the larvae. A cone-shaped screen wire top, six inches high, was constructed in such a manner that it fitted snugly over the outside of the tile, and was held in place by means of a wire hoop.
It was necessary to replant the grain from time to time as the larvae killed the previous planting. During the summer, when the plants grew too rank, the screen wire cone was removed and the growth trimmed back. The soil in the tiles was not watered or treated differently from the surrounding soil, except as required to insure the germination of the cereals planted for food.
Twenty-five larvae were placed in each tile cage in the spring of 1928. Those placed in any one cage were collected at the same time, in the same locality and habitat. Whenever possible the larvae were sorted so that those in each tile cage were approximately the same size.
During the months of September and October, 1928, twenty one of the tile cages were dug up and carefully examined to determine the location of the insects at this time of the year. The top tile was removed, taken into the insectary, and placed in a horizontal position on the table. The soil was scraped out in one-fourth inch layers. This made it possible to determine the exact depth at which the insects could be found. The lower tile was removed and examined in much the same manner as the upper one.
After the lower tile was removed and examined. it was an easy matter to clean out the hole and reset the empty tiles. All of the beetles found were mounted for identification purposes, while the remaining wireworms that had not pupated were grouped by habitats and placed in a single tile cage. This left the remaining cages free to be restocked. An attempt was made to keep the pupal and larval skins and mount them with the beetles. The number of insects found in each cage was recorded, as was the depth at which each was found.
In this study 21 tile cages, each containing 25 wireworms, or a total of 525 larvae, were under observation. One hundred ninety nine adult click beetles were recovered in their pupal cells. One pupa, 24 larvae, and seven parasite cocoons were also found, making a total of 231 wireworms accounted for out of the original 525 larvae placed into the cages.
Hence, it will be seen that 38.9 percent of the wireworms collected in the field and placed in the tile cages emerged as adults the first year, while it was possible to account for 44 percent of the original number of larvae. No explanation relative to the fate of the remainder can be advanced at this time. It is interesting to note that only one adult emerged from 525 wireworms collected at the same time from the same locality confined in salve boxes and handled as described under "Methods" in this paper.
The following table shows that no beetles were taken closer than four inches to the surface, while 18 inches was the maximum depth at which they were found. One hundred seventy nine, or 90 percent of the beetles were taken in their pupal cells at depth from 5 to 12 inches inclusive. The average depth for all of the beetles reared was 9.01 inches. This is a significant fact because one of the suggested methods for the control of wireworms is fall plowing in order to destroy the pupal cell. It is interesting to note that approximately 92 percent of the 199 adults were below the six-inch plow line.
The results of the findings are recorded in the following table
Table Showing the Number of Insects Accounted For at Various Depths