Variation in number of instars of Monochamus carolinensis (Coleoptera: Cerambycidae). (1988)

Head capsule widths were measured on 1239 log-reared larvae o? Monocha mus carolinensis (Olivier) and plotted as a frequency distribution. The number of instars could not be determined by this method. The number of instars for larvae reared on artificial diet was determined, through direct observation, to be variable. These larvae followed one of several instar pathways, i.e., individual larvae developed through three, four, five, or six stadia before pupating. Twenty-seven percent of these larvae developed through three instars while 63% went through four. A chi-square test for independence indicated that instar pathway frequencies were independent of sex but dependent upon temperature. Mean head capsule widths among diet-reared larvae were influenced by temperature but not the sex of the larvae. Monochamus carolinensis (Olivier) is a principal vector of the pinewood nema tode in the central United States (Wingfield and Blanchette, 1983; Linit, 1987). Detailed understanding of its life history and biology is necessary to determine the beetle's role in the epidemiology of pine wilt disease. This wood borer develops in the cambium and xylem of weakened, dying and recently felled trees (United States Department of Agriculture, 1985). Studying the larval development of wood boring cerambycids in nature is often difficult because of the insect's cryptic habit, the height of host trees, and the presence of several species of cerambycid beetles in a single tree. Consequently, little information on larval development in the genus Monochamus is available. Life history studies have been completed on M. titillator (F.) (Webb, 1909), M. notatus (Dury) (Morgan, 1947), M. scutellatus (Say) (Rose, 1957), M. alternatus Hope (Yamane, 1975), and M. carolinensis (Pershing and Linit, 1986a). The number of instars have been reported for M. scutellatus (Rose, 1957) and M. alternatus (Yamane, 1975). Two commonly used methods for determining the number of instars are direct observation of larval molts and plotting a frequency distribution of measurements of a sclerotized body part, e.g., head capsule width. Gaines and Campbell (1935) discussed the use of frequency distributions for lepidopterous larvae. This method has been applied recently for lepidopterous and coleopterous larvae (Kishi, 1971; Schmidt et al., 1977; Yamane, 1975; Frick and Wilson, 1981; Loerch and Cam eron, 1983). Using head capsule width measurements to determine the number of larval instars for M. carolinensis has practical application because larval development cannot be readily observed in its natural habit. Ideally the head capsule width measurements should form a multi-model distribution with each peak repre senting the mode among individuals of an instar. We report here the use of frequency distributions for determining the number 1 Current address: Monsanto Company, 700 Chesterfield Village Parkway, St. Louis, Missouri 63198. Accepted for publication 13 April 1988. This content downloaded from 207.46.13.71 on Sat, 22 Oct 2016 06:18:57 UTC All use subject to http://about.jstor.org/terms VOLUME 61, NUMBER 4 371 Fig. 1. Ventral surface of M. carolinensis third-instar head capsule. Arrows indicate points of measurement. of instars for both log-reared and diet-reared larvae of M carolinensis. Addition ally, the influence of temperature, instar pathway, and sex on the number of instars for diet-reared larvae is reported. Materials and Methods Freshly-cut Scotch pine (Pinus sylvestris) logs 30 cm in length and ca. 10 cm in diameter were placed in oviposition cages (Linit, 1985) for a period of 24 hr to obtain eggs of M. carolinensis. Oviposition on an initial 10-log set was timed to coincide with adult emergence in the field (ca. 15 May) (Pershing and Linit, 1986b). The logs were transferred to a screenhouse where they were shaded con tinuously yet subject to ambient temperatures, humidities, and photoperiods. This procedure was repeated each month until October. To obtain larvae of all instars, one log from each 10-log set was dissected at 2, 3, 4, 5, and 6 weeks following oviposition. The remaining five logs from each 10 This content downloaded from 207.46.13.71 on Sat, 22 Oct 2016 06:18:57 UTC All use subject to http://about.jstor.org/terms 372 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Table 1. Composition of pine sawyer diet.3 1. Agar 15.0 g 2. Sterilized distilled water 350.0 ml 3. Cellulose 5.0 g 4. Wheat germ 7.5 g 5. Granulated sucrose 9.0 g 6. Casein (vitamin free) 9.0 g 7. Wesson's salt mixture 2.5 g 8. Ascorbic acid 1.0 g 9. Choline chloride (50%) 5.0 g 10. Methyl paraben 0.5 g 11. Sorbic acid 0.5 g 12. Vitamin mixture, Vanderzant's 2.5 g 13. Formaldehyde (37% solution)5 1.5 ml 14. Wheat germ oil 3.5 ml 15. Dried yeast 3.5 g 16. Pine bark powder (Scotch pine)c 70.0 g a Nutritional Biochemical Corp., Cleveland, Ohio, unless otherwise indicated. b Fisher Scientific, St. Louis, Missouri. c Pine bark ground in commercial blender. log set were dissected 12, 16, 20, 24, and 28 weeks following oviposition. In addition, larvae obtained immediately after hatching provided a known popu lation of first instars. Early instar larvae in the phloem were collected by peeling the bark from logs. The remaining portion of each log was systematically cut using a band saw to locate mature larvae and pupae within the xylem. Larvae were placed in Quinter's solution (10 parts EtOH: 1 part glacial acetic acid: 1 part water) for 5 minutes then stored in 70% ethanol until head capsule measurements were made. Head capsules were measured with an ocular micrometer mounted on a stereo microscope. Measurements were made across the ventral surface of the head capsule on a transverse line projected through the hypostomal sclerites (Fig. 1) and were recorded to the nearest 0.007 mm at 500 x, to the nearest 0.015 mm at 250 x, and to the nearest 0.032 mm at 120 x. Voucher specimens were deposited in the Wilbur R. Enns Entomological Museum at the University of Missouri Columbia. Larvae reared on artificial diet were obtained from eggs oviposited on pine logs. First instars were removed from logs immediately after hatching, placed into cups containing diet, and held under constant temperatures of 20? and 25?C, with LD 11:13 photoperiod or 30?C with LD 14:10. The latter photoperiod was necessary because of the simultaneous use of that environmental chamber for another ex periment. The diet used was similar to that of Wollerman et al. (1969) (Table 1). Diet was replaced in each rearing cup every 2 weeks. Larvae reared on artificial diet were observed through pupation. Larvae that did not pupate within 250 days after hatching were placed in Quinter's solution and stored in 70% ethanol. Each diet cup was observed at 24 hr intervals for evidence of larval molting, as indicated by the presence of an exuvium and/or head capsule. The head capsule, if found, was measured as above. Head capsule measurements of log-reared and diet-reared larvae were plotted This content downloaded from 207.46.13.71 on Sat, 22 Oct 2016 06:18:57 UTC All use subject to http://about.jstor.org/terms VOLUME 61, NUMBER 4 373