Ecological Studies on the Eggs of the Northern Corn Rootworm1

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Abstract
The northern corn rootworm, Diabrotica longicornis (Say), adult was found to oviposit in the soil between plants within a row of maturing corn fields. Eggs were found in decreasing numbers at 4, 8, and 10 inches from the row but none were found in the middle between 40-inch rows. The depth of egg deposition is probably governed by the degree of soil cracking, but an average of 4 samples in 1963 and 1964 showed 92.8% of the eggs in the top 4 inches of soil. In a separate study, 88% of the eggs were noted in the upper 3 inches of soil, but after plowing to a depth of 9 inches, all eggs were recovered from the 3- to 9-inch layer. Sampling between plants in several fields before plowing showed 19-558 eggs per 100 cubic inches of soil 4 inches deep. Subsequent larval counts indicated 0.19 larva per egg detected by the particular egg assay employed. Hatch occurs in northern corn rootworm eggs after diapause is broken by low temperatures, but hatch was found to be reduced or prevented when the microclimate was –2°C or below for 6 or more weeks. Spring-collected eggs held in a refrigerator (around 4.4°C) for 24 months displayed a normal hatching pattern when removed to ambient laboratory temperatures. Eggs taken from the field in early spring showed first hatch in 15.3 days when held at 25°C, but when spring-collected eggs were held at 4.4°C for 8 months first hatch occurred 9.7, 14, and 18 days after being placed at temperatures of 26.7, 22.8, and 18.3°C, respectively. This fact might indicate some physiological development of diapause-free eggs retained at 4.4°C. No eggs hatched when incubated at 31.7 or 33.9°C. Contact moisture was employed for all studies on embryonic development. Laboratory-produced eggs retained at ambient laboratory temperatures displayed 13.3% hatch in the absence of a chilling period. Likewise, eggs collected from the field on October 12 showed 21.4% of the eggs free of diapause.