Microplitis croceipes (Hymenoptera: Braconidae) Diapause Response to Insectary Conditions

Abstract

Annual cycles of activity and development occur across the class Insecta and have evolved as a strategy to avoid or exploit environmental extremes (Saunders 1982). Parasitoids, especially host-specific species, necessarily respond to seasonal activities, or occurrence, of their host. Understanding the seasonal relationship between agricultural pests and their parasitoids is important to understand the parasitoids' potential for biological control (Powell et al. 1989). To better understand the seasonal synchrony between Heliothis spp and Microplitis croceipes (Cresson), photoperiod and temperature studies were undertaken in the laboratory and insectary during 1984 and 1985 at the University of Arkansas. Diapause is induced in H. virescens by a photoperiod of 10:14 (L:D) and inhibited by a photoperiod of 14:10 (L:D) in a Louisiana biotype (Phillips & Newsom 1966). High temperature (270C) counters the effect of short daylengths. The sensitive stage is the larva, in which M. croceipes is endoparasitic. In the laboratory, M. croceipes demonstrates 50% diapause induction in response to a critical Light:Dark ratio of 10.30:13.30 (21?C) and 11.00:13.00 (18?C) (Brown & Phillips 1990). Temperature alone (15?C) also induces 100% diapause at a photoperiod of 14:10 (L:D). The M. croceipes larva is the sensitive stage and the diapausing stage is the prepupa. There is significant interaction between photoperiod and temperature in diapause induction. Development times from cocoon formation to adult emergence is highly variable. This variability may be favorable in that it allows M. croceipes to cope with erratic and unfavorable conditions during the spring when host populations may be low or seasonal climatic changes have not stabilized. For the insectary study, 20 H. virescens were reared to third instar at 24?C and 14:10 (L:D) photoperiod (non-diapausing conditions) in 180 ml waxed paper cups (rearing chamber) containing approximately 5 ml of pinto bean medium (Burton 1969). Parasitoids were reared from field-collected Heliothis spp. and held at 26C, 50% RH, and 14:10 (L:D) photoperiod. Hosts were exposed to the parasitoid by placing two 5-7 day-old M. croceipes females in the above rearing chamber for 8 h. Parasitized hosts were then transferred to individual 30 ml rearing cups containing pinto bean diet and placed in the insectary on different dates during a two year period (Table 1). The insectary was 3.7 by 6.1 by 2.4 m with vertical walls made of standard mesh screening material. Media cups containing the host and diet were placed in the insectory and observed on a biweekly basis until 22 December, when the parasitoid cocoons were removed. Cocoons were then placed in a 14:10 (L:D) (long day) photoperiod and 24?C regimen and monitored daily for emergence. Ambient temperatures and humidities in the insectary were recorded with a Model 594 Bendix Hygrothermograph (Bendix Environmental Science Division, Baltimore, Maryland 21204). Diapause response averaged 3.4% when the parasitized hosts were placed in the insectary in late August (Table 1). Diapause increased to 98.6% when hosts and parasitoids were placed in the insectary on 25 September. This 30 day period represents