Immature development, fecundity, longevity, and egg diapause of Diabrotica longicornis (Coleoptera: Chrysomelidae).

Laboratory studies were conducted to determine if Diabrotica longicornis (Say) has the potential to enter prolonged egg diapause and to quantify D. longicornis fecundity and longevity. The immature developmental periods of the sibling species, Di abrotica longicornis and D. barberi Smith and Lawrence were also compared. When fed a diet of zucchini squash, the mean fecundity of D. longicornis was 185.6 eggs per female and the mean longevity was 54.2 days. The immature developmental period of D. longi cornis (mean 43 days) was significantly shorter than that of D. barberi (mean 47 days) when both species were reared on corn. A small percentage of D. longicornis eggs hatched 15 days after oviposition, 44.2% hatched after one chill period, and additional eggs hatched after a second chill period. This suggests that nondiapause and prolonged diapause traits occur within the D. longicornis population studied. The northern corn rootworm, Diabrotica barberi Smith and Lawrence has re cently been separated from D. longicornis (Say) and elevated to species status based on morphometric and color variation, and differences in mating behavior, habitat preference, and sex pheromone response (Krysan et al., 1983). Other biological characteristics have been reported that separate the two species. Quan titative differences in cuticular hydrocarbon profiles exist between the species (Golden, 1990), and a specific esterase occurs in males of D. longicornis but not in D. barberi males (McDonald et al., 1982). Also, compared with D. barberi, eggs of D. longicornis are at a more advanced embryological stage when they enter diapause (Krysan, 1982; Krysan et al., 1983). The two species do share enough taxonomic characteristics to have been de scribed as sibling species (Krysan et al., 1983). The male genitalia and sperma thecae, which are generally useful in separating Diabrotica species, are so similar that they are not diagnostic for these species. There are also no visible differences in the egg chorion sculpturing (Krysan et al., 1983; Krysan, 1987), and there is an apparent allelism of allozymes in the two species (McDonald et al., 1982). D. barberi is an economic pest of corn and various aspects of its biology have been thoroughly studied. These include the effect of temperature on egg hatch (Apple et al., 1971; Chiang and Sisson, 1968; Patel and Apple, 1967), egg diapause (Chiang, 1965; Cunningham and Peters, 1964; Krysan et al., 1986), larval host specificity (Branson and Ortman, 1967, 1971), immature development (Domi nique and Yule, 1983), and reproductive development and longevity of females (Naranjo and Sawyer, 1987). To increase our understanding of the relationships of D. longicornis and D. barberi, D. longicornis biology and ecology are being studied. Because the two species appear to be closely related, learning more about D. longicornis may enhance our understanding of D. barberi and may help determine if D. longicornis has the potential to become an economic pest. This paper reports the results of Accepted for publication 29 October 1990. This content downloaded from 157.55.39.219 on Tue, 19 Jul 2016 06:04:38 UTC All use subject to http://about.jstor.org/terms 252 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY laboratory experiments that were designed to determine if D. longicornis has the potential to enter prolonged diapause, to compare the D. longicornis immature developmental period to that of D. barberi, and to determine the fecundity and longevity of D. longicornis females. Materials and Methods All D. longicornis originated from populations located in Dundy Co., Nebraska and all D. barberi originated from populations located in Cottonwood Co., Min nesota. Eggs were collected and F{ individuals from the field collected beetles were used in the laboratory experiments. Although the native larval host range is unknown for D. longicornis, the species has been successfully reared on seedling corn (McDonald et al., 1982). To deter mine and compare the developmental periods of D. longicornis and D. barberi from neonate larva to emerged teneral adult on field corn, larvae of both species were reared on seedling corn (Pioneer 3377). Corn seedlings were placed on moist peat (72.7% moisture by weight) in one-ounce covered plastic containers. A single neonate larva was placed in each plastic container and containers were held at 23 ? 0.5?C in a 15:9 (L:D) photoperiod. New corn seedlings were added as needed. The sex and developmental period of each individual were recorded as beetles emerged. The mean developmental periods (emergence recorded daily) were cal culated for both sexes of each species. T-tests were used to compare the devel opmental periods of the two species and the developmental periods of the sexes within each species. To determine the fecundity of D. longicornis females, ?x individuals from field collected beetles were reared as larvae on corn seedlings (Pioneer 3377). Twenty pairs were caged in individual polystyrene petri dishes (100 x 15 mm). Soil (<60 mesh size particles) was placed in one-half of each dish as oviposition medium. The soil was kept moist (34.8% moisture by weight) throughout the ovipositional period. The beetles were fed zucchini squash which was replaced every 2-3 days. Each beetle pair was held at 23 ? 0.5?C in a 15:9 (L:D) photoperiod until the female died. Results of preliminary experiments indicated that D. longicornis females would readily oviposit in the soil under these conditions (Golden and Meinke, unpubl.). Any eggs found on the food were removed with a fine sable brush and placed on the soil. Eggs were collected by washing the soil from each petri dish through a combination of Number 20 and 60 U.S. Standard sieves. The mean longevity period (in days) per female and the mean number of eggs oviposited per female were calculated. To determine if D. longicornis has the potential to enter extended egg diapause, 3350 eggs were obtained from D. longicornis females collected in Dundy Co., Nebraska. The eggs were initially held at 25 ? 1?C in a 16:8 (L:D) photoperiod for 12 weeks and the number of eggs that hatched during that period was recorded. The eggs were then held at 5 ? 1?C in complete darkness for 15 weeks. At the end of that period a subsample of 500 eggs was removed from the cold room and held at 23 ? 0.5?C in a 15:9 (L:D) photoperiod for 32 weeks. The number of eggs that hatched during this period was recorded. The unhatched eggs from the sub sample were again held at 5 ? 1?C and in complete darkness for another nine weeks. At the end of that period, the eggs were held at 23 ? 0.5?C in a 15:9 (L:D) photoperiod to determine if additional eggs would hatch. The temperature regimes This content downloaded from 157.55.39.219 on Tue, 19 Jul 2016 06:04:38 UTC All use subject to http://about.jstor.org/terms VOLUME 64, NUMBER 3 253 Table 1. The mean developmental periods from neonate larva to teneral adult of D. longicornis and D. barberi. Species N Sex Mean (days) ? SEM D. longicornis 29 99 43.72 ? 1.44a 20 66 41.50 ? 1.69 D. barberi 32 99 47.00 ? 0.89a 22 66 46.68 ? 0.87 D. longicornis 49 Pooledb 42.81 ? 1.09c D. barberi 54 Pooled 46.87 ? 0.63 a Within species, no significant differences between means (P < 0.05, Mest). b Sexes pooled prior to analysis. c Means between species significantly different at P < 0.01 (Mest). were selected based on previous studies of D. barberi diapause (Chiang et al., 1972; Krysan, 1982) in which 5-7?C chill (shown to be thermal optimum for maximum egg hatch) and 20-25?C warm periods were used.