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.

Review of Literature

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.

Methods

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.

Presentation of Data

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

Beetles

Passalid beetle

Blister beetle

British stag beetles

Oryctes' horn

Depth Adults Larvae Pupae Parasitic Coccons*
4 4 0 0
5 12 0 0
6 32 0 0
7 14 1 0
8 26 1 0
9 32 7 0
10 21 4 1
11 25 0 0
12 17 5 0
13 3 2 0
14 3 1 0
15 5 1 0
16 2 0 0
17 0 0 0
18 3 0 0
19 0 0 0
20 0 0 0
21 0 0 0
22 0 2 0
Total 199 24 1 7
* No depth record kept.

The 24 wireworms recovered were found at depths varying from 7 to 22 inches, with an average of 10.6 inches. No record of depth was taken on the parasite cocoons found in the cages.

From the presentation of data, one can readily see that the use of the drain tile cage has proved unusually satisfactory for rearing wireworms and could be readily adapted to rearing other subterranean insects. The writer believes this method offers more nearly natural conditions than any previously described. There are a number of other advantages that should be mentioned at this time.

(1) It is a very economical method, since the six-inch drain tiles can be purchased for about ten cents each. Very little time is required to set the tile cages and they are easily removed for examination. The larvae placed in them do not require frequent handling and feeding, as in the salve box method, and this decreases the mortality due to oversight and injury.

(2) The larvae have considerable freedom of movement vertically in the soil. They have an opportunity to select the desired moisture and temperature conditions. The moisture content of the soil within the 21 tile cages was apparently the same as that of the surrounding soil. Since the tiles are porous, the circulation of soil moisture and air is inhibited very little after they become moistened. The soil within the tiles, containing the wireworms, had numerous tunnels which suggest the possibility of studying burrowing methods and responses to changes in the soil moisture or temperatures.

(3) This method offers an opportunity for the investigator to study the feeding habits of the larvae. Various foods, baits and treated seeds could be tried under outdoor conditions because the wireworms would be in confinement where they could be observed more easily than in the field.

(4) Preliminary experiments indicate that this method will prove invaluable for rearing the wireworms from the egg to the adult stage. It will be difficult to get the moulting record of the young larvae, but this is a laborious task at best. It could be done by setting a single tile in the soil to be examined at frequent intervals.

(5) A larger number of larvae can be handled in the rearing work. This will facilitate the determination of the species that exist in a given locality, since a larger number will reach maturity. Since the larval and pupal skins can be obtained from the pupal cells, a study of the larval and pupal characteristics can be made in an effort to identify the different species.

The beetles that emerge from the tile cages are in splendid condition for taxonomic studies. There were only five beetles in the group of 199 individuals that could not be mounted and used for study. Those which emerge from the salve boxes are usually distorted, abnormal in color, or with crumpled wings.

The writer believes that this method has a wide range of adaptability to many phases of soil insect rearing. Further tests are necessary to test the lasting qualities of the tiles.

Summary

1. Investigators in the life history studies of subterranean insects have been in need of a successful method for rearing wireworms.

2. Thirty eight and nine-tenths percent of the wireworms collected and placed in cages made of six-inch, unglazed drain tiles, emerged as adult click beetles, while it was possible to account for 44 percent of the 525 larvae placed in the tiles.

3. Approximately 92 percent of the 199 beetles taken from their pupal cells were below the six-inch plow line, with an average depth of a fraction over nine inches.

4. The tile method is not only more economical but, also offers a greater opportunity for observing the food habits and other activities under approximately natural conditions. Since large numbers can be handled, this method will no doubt prove invaluable for rearing the beetles from the egg stage to the adult, as well as insure a better condition of specimens to be used in taxonomic studies.

Literature Cited

Davis, J. J. 1915. Cages and Methods of Studying Underground Insects. Jour. Econ. Entom. 8: 135-139.

Graf, John E. 1914. A Preliminary Report on the Sugar-beet Wireworm. U. S. Dept. Agri., Bu. of Entom., Bull. No. 123. p. 21-24.

Hyslop, J. A. 1915. Wireworms Attacking Cereal and Forage Crops. U. S. Dept. Agri., Bull. No. 156, p. 14.

Lane, M. C. 1924. Simple Methods of Rearing Wireworms (Elateridae). Jour. Econ. Entom., 17, 578-582.

McColloch, J. W. 1917. A Method for the Study of' Underground Insects. Jour. Econ. Entom. 10:183-188.