Life history and development of Scymnus frontalis (Fabricius) (Coleoptera: Coccinellidae) on four species of aphid

The ladybird beetle, Scymnus frontalis (Fabricius), was imported from Tur key as a potential predator of the Russian wheat aphid, Diuraphis noxia (Mordviklo). Larval developmental time and adult fecundity were examined for S. frontalis fed four aphid species: the Russian wheat aphid; the greenbug, Schizaphis graminum (Rondani); the English grain aphid, Macrosiphum avenae (L.); and the pea aphid, Acyrthosiphon pisum (Harris). Larvae reared on Russian wheat aphids completed development 0.7 to 1.4 days sooner and weighed 0.5 to 0.3 mg less than those reared on English grain aphids or pea aphids, respectively. No differences were found in adult survival or fecundity for adults reared on Russian wheat aphids, greenbugs, or pea aphids. The intrinsic rate of increase (0.73) was significantly greater for S. frontalis fed Russian wheat aphids compared with those fed pea aphids or greenbugs (0.60 and 0.63, respectively). In a prey preference test, S. frontalis larvae ate 323/600 firstor second-instar Russian wheat aphids and 274/600 English grain aphids indicating a slight preference for Russian wheat aphids as prey. The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), has become a threat to wheat production in the United States since its introduction in 1986 (Thompson, 1987; Stoetzal, 1987). The RWA is believed to be indigenous to the southern Soviet Union, Iran, Afghanistan and countries which border on the Mediterranean, where it is not considered a serious pest. Introduction of natural enemies from these areas to the United States may facilitate biological control of the RWA. One potential natural enemy of the RWA which was imported from Turkey into the United States is the ladybird beetle, Scymnus frontalis (Fabricius). Two species of Scymnus have been observed in association with RWA in small grain fields in the past; Scymnus morelleti was found within rolled, RWA-infested wheat leaves in South Africa (Aalbersberg et al., 1984), and Berest (1980) reported finding S. frontalis in RWA-infested wheat fields of the right bank steppe zone of Ukrain ian SSR. Naranjo et al. (1990) studied the development of S. frontalis in the laboratory. Little else is known about the biology of S. frontalis and more infor mation is needed in order to evaluate its potential as a biological control agent for the RWA. The purpose of our study was twofold. The first was to compare the quality of several aphid species as prey for S. frontalis by measuring immature development time and survival, and adult weight, survival and fecundity of individuals fed 1 Present address: 5902 West Gary Drive, Tempe, Arizona 85282. 2 USDA, ARS, Plant Science Research Laboratory, 1301 N. Western St., Stillwater, Oklahoma 74075. 3 USDA, ARS, Beneficial Insects Research Laboratory, USDA, 501 South Chapel Street, Newark, Delaware 19713. 4 Mention of a commercial or proprietary product does not constitute an endorsement by the USDA. Accepted for publication 1 November 1990. VOLUME 65, NUMBER 4 411 exclusively on each species as immatures. The second was to establish whether S. frontalis would preferentially eat RWA when allowed to choose between RWA and another cereal aphid species. Materials and Methods Adult S. frontalis were obtained from the USDA, ARS Beneficial Insets Re search Laboratory, Newark, Delaware, descended from beetles originally collected in Beypazari, Turkey, on 14-19 June 1988. For the first of two trials (23 November 1988), 100 eggs were collected during a 24-hour period. The adult beetles laid eggs readily on a cloth substrate which was then cut into squares, each containing a single egg. Eggs were placed in individual 4 5-ml capacity polystyrene vials (Daigger Scientific), and capped with white snap-caps with a 17-mm hole in the top covered with fine mesh screen. Eggs were incubated at 22? C ? 2? C and a 15:9 L:D cycle. Upon eclosi? n, first-instar S. frontalis larvae were separated into three, equal-sized groups and fed one of the following species of aphid: the greenbug, Schizaphis graminum (Rondani), RWA, or English grain aphid, Macrosiphum avenae (L.). Aphids were from colonies maintained in the greenhouse on wheat or barley seedlings. Larvae were maintained individually and fed an excess of aphids. Duration in days of larval and pupal stages and weights of newly emerged adults were recorded. For the second trial (5 January 1989), 126 eggs were collected as previously described. Conditions were the same for this trial except that larvae were fed either RWA, greenbug, or pea aphid, Acyrthosiphon pisum (Harris). The temperature was slightly higher, 24? C ? 2? C, but the photoperiod was the same as for the previous trial. Larval and pupal developmental times and weights of newly emerged adults were recorded. To determine fecundity and survival, individual newly emerged female beetles were randomly paired with a male in a 9-cm diameter, 7-cm tall, 400-ml capacity plastic container with a screen lid. Pairs were fed the same aphid species as immatures until 13 March 1989, when colonies of RWA and greenbug were depleted. After that date, all pairs were fed pea aphids. Each pair was given a 1-cm piece of moistened cotton wick daily as a water source and a 1-cm by 5-cm section of cloth as an oviposition substrate. Analysis of variance (PROC GLM, SAS Institute, Inc., 1982) was used to test for treatment differences. Each trial was analyzed separately to determine differ ences among S. frontalis fed different diets in mean fecundity, length of the larval and pupal stages, length of the pre-oviposition period, and adult weight. The following life table statistics, described by Birch (1948), were estimated: intrinsic rate of increase (rm), gross reproductive rate (GRR), net reproductive rate (R0), and mean generation time (MGT). A jackknifing algorithm (Meyer et al., 1986) was used to estimate rm. Use of this method facilitated calculation of standard errors of rm estimates and statistical comparison of estimates using ? -tests. In the field, S. frontalis would be exposed to a mixture of aphid species whereas in the above tests only one species was offered. It is possible that S. frontalis avoids feeding on RWA if other species are available. To test this possibility, a feeding choice test was conducted using newly hatched S. frontalis larvae. First and second-instar RWA or English grain aphid nymphs were obtained by gently sifting mixed-aged, laboratory-reared aphids through a fine mesh screen. Aphids were counted and placed in a 45-ml capacity plastic vial with a 4-cm piece of 412 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Table 1. Mean developmental times of Scymnus frontalis immatures fed four different species of aphids. Each trial was analyzed separately, and letters were assigned to statistically significant pair