TACHINIDAE (DIPTERA) PARASITOIDS OF BERTHA ARMYWORM (LEPIDOPTERA: NOCTUIDAE)

The bertha armyworm, Mamestra configurata Walker, causes periodic but severe damage to canola crops in North America, particularly in the Canadian provinces of Alberta, Saskatchewan, and Manitoba. This paper provides information on the tachinid flies (Tachinidae: Diptera) recorded from bertha armyworm in North America. A key to adults is provided for separation of the genera, and each species known from bertha armyworm is characterized with respect to recognition, biology (including hosts), and distribution. The species included are Athrycia cinerea (Coquillett), Blondelia sp., Chetogena claripennis (Macquart) species complex, Chetogena tachinomoides (Townsend) (new record), Eurithia consobrina (Meigen), Exorista mella (Walker), Lespesia archippivora (Riley), Panzeria ampelus (Walker), Phryxe pecosensis (Townsend), Phryxe vulgaris (Fallén), Spallanzania hebes (Fallén) (new record), Winthemia rufopicta (Bigot), and Winthemia quadripustulata (Fabricius). Eurithia consobrina is a Palearctic species that has been introduced into Canada for control of M. configurata but is probably not established; it is included here in the event it is introduced again and establishment is achieved.

Cold-hardiness of Lacanobia atlantica (Lepidoptera: Noctuidae), and a comparison with three other insect species that overwinter in the same habitat

The pupae of Lacanobia atlantica (Grt.) overwinter in the soil and are susceptible to freezing. Cold injury and death occur at temperatures above the supercooling point (SCP) of −26.4 °C, and below ca. −12.6 °C. Mortality increases as temperature decreases and as exposure lengthens. These results are compared with those for Mamestra configurata Wlk., Delia radicum (L.), and Athrycia cinerea Coq., which have similar overwintering habits. Their mean SCPs (−27.3 to −20.3 °C) differ less than do the values representing the highest temperature at which cold injury occurs (≈−12.6 to ≈4 °C). Response curves of three of the species were similar for changes in temperature and duration of exposure, and similarly in a plot of the time to 50% survival (LT50) against temperature. They differed only in their relative cold-hardiness. In contrast, A. cinerea was less sensitive to changes in temperature and duration of exposure. A single parameter (e.g., the mean SCP) is not adequate to describe the cold-hardiness of these species because they vary not only in the upper and lower bounds of their cold-injury zones but also in their response to temperature and duration of exposure within this zone.

DESCRIPTIONS OF THE IMMATURE STAGES AND ADULT REPRODUCTIVE SYSTEMS OF ATHRYCIA CINEREA (COQ.) (DIPTERA: TACHINIDAE), A NATIVE PARASITOID OF MAMESTRA CONFIGURATA (WALK.) (LEPIDOPTERA: NOCTUIDAE)

The immature stages of Athrycia cinerea (Coq.) including the egg, three larval instars, and the puparium are described and illustrated. The male and female reproductive systems are also illustrated and described and notes on oogenesis are provided.

SURVIVAL OF PUPAE OF MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE) AND TWO OF ITS PARASITES IN UNTILLED AND TILLED SOIL

The survival of Mamestra configurata Wlk. was determined from autumn and spring samples in 1973/74, 1980/81, and 1981/82 taken from unfilled and tilled portions of 12 fields of canola (Brassica campestris L. and B. napus L.). Survival from the pre-pupal stage in the autumn to post-diapause pupae in the spring varied from 2.9 to 56.5% in unfilled soil and from 0 to 25.4% in tilled soil. In untilled soil, the variations among fields in total mortality of M. configurata were related to parasitism by Banchus flavescens and to overwintering mortality. In tilled soil, overwintering mortality accounted for most of the variability in total mortality among fields. The percentage of mortality from injury to pupae, and their disappearance following tillage, greatly increased but this type of mortality did not account for much of the variability in total mortality. The disappearance of pupae following tillage was attributed to predation. In both untilled and tilled soils, the amount of overwintering mortality was related to the depth of accumulated snow, and mortality was lower in untilled soil because the stubble trapped more snow. The survival of M. configurata was not related to soil type, the type of tillage equipment, or the frequency of tillage. The number of adults emerging from untilled and tilled soil in one field were 2.58 and 0.27/m2 for M. configurata, 5.22 and 2.85/m2 for the parasite B. flavescens, and 4.46 and 3.60/m2 for the parasite Athrycia cinerea, indicating that the survival of the parasites was less affected by tillage than that of their host.Tillage may reduce the frequency and severity of outbreaks of M. configurata not only by increasing mortality but also by differentially affecting the survival of its main parasites.

HOSTS OF BANCHUS FLAVESCENS (HYMENOPTERA: ICHNEUMONIDAE) AND ATHRYCIA CINEREA (DIPTERA: TACHINIDAE) IN MANITOBA

Banchus flavescens Cress. and Athrycia cinerea (Coq.) were reared each year (1972–1975) from larvae of bertha army worm, Mamestra configurata Walk. (Lepidoptera: Noctuidae), collected during an outbreak of this species on rapeseed in Manitoba. Neither parasite was reared from other Lepidoptera collected at that time from rapeseed; two separate attempts to propagate the parasites on native Lepidoptera other than M. configurata were unsuccessful (Wylie and Bucher 1977; Ewen and Arthur 1976). Since 1975 M. configurata has been rare in Manitoba. In 1976 both B. flavescens and A. cinerea were reared from bertha armyworm larvae from temporary artificial populations on rapeseed at Glenlea, south of Winnipeg. The presence of the two parasites in this locality, where M. configurata was not found during the outbreak, suggested that both B. flavescens and A. cinerea parasitize other lepidopterous species in Manitoba, especially when bertha armyworms are not present.

PREVENTING AND TERMINATING PUPAL DIAPAUSE IN ATHRYCIA CINEREA (DIPTERA: TACHINIDAE)

Pupal diapause in Athrycia cinerea (Coq.), a larval parasite of bertha armyworm, Mamestra configurata Walk., was usually prevented if the parasite larvae were reared at 25°C, but not if they were reared at 20°C. Mature larvae of the parasite were more sensitive than younger larvae to this temperature effect. Diapause incidence in A. cinerea at 20°C was not reduced by using only young parent females for oviposition; by rearing the parent females at a high temperature (25°C) and long day (16 h) prior to and during oviposition; by using host larvae that had been light-programmed for non-diapause; or by increasing the photoperiod, to which the parasite larvae were exposed, from 12 to 16 h. Termination of pupal diapause was accelerated by low temperature; at least 24 weeks at 2 °C were required to synchronize adult emergence.

THE BERTHA ARMYWORM, MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE). MORTALITY OF IMMATURE STAGES ON THE RAPE CROP, 1972–1975

Larvae of bertha armyworm, Mamestra configurata Walk., collected in 1972–75 on rapeseed in the prairie provinces of Canada (predominantly in Manitoba), were killed by one virus, one fungus, one microsporidium, seven insect parasites, and by other factors. Mortality from disease was about 24% in 1972 when bertha armyworm was abundant, but much lower in succeeding years after the host’s population density declined. The most important hymenopterous parasite, Banchus fiavescens Cress., was relatively rare in 1972 but subsequently killed over 40% of the host larvae each year. A tachinid, Athrycia cinerea (Coq.), killed over 20% of the mature bertha armyworms in each generation. Each of the other five parasite species was associated infrequently with M. configurata. Details of the biology of each pathogen and parasite are presented. About 12% of the mature host larvae died because they failed to empty the gut, with subsequent penetration of the gut wall by bacteria, or because they did not form a morphologically-sound pupa. Pupae of M. configurata died in the soil following penetration of the gut wall by bacteria, as a result of soil cultivation, and from low temperatures in winter. No pathogens or parasites were recorded in field-collected pupae, though an ichneumonid, Ichneumon canadensis Cress., parasitized pupae in a field cage and may occasionally do so in natural conditions. No pathogens or parasites of bertha armyworm eggs were recorded. A preliminary survival model for bertha armyworm, based on estimates of mortality from various causes identified in this study, is presented. The possibility of preventing or reducing future oubreaks of M. configurata by colonizing additional parasite or pathogen species in the prairie provinces is discussed.

OBSERVATIONS ON ATHRYCIA CINEREA (DIPTERA: TACHINIDAE), A PARASITE OF MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE)

Athrycia cinerea (Coq.) is a univoltine parasite of bertha armyworm, Mamestra configurata Wlk., on rapeseed in the Prairie Provinces of Canada. Details of the parasite’s seasonal life history are presented. Females oviposit on third, fourth, fifth, and sixth instars of bertha armyworm, and the parasite larvae usually develop gregariously and mature on the fifth and sixth instars. Supernumerary parasite larvae on superparasitized hosts starve; surviving larvae develop more rapidly, form smaller pupae with a lower capacity for survival, and eventually produce a consistently though not significantly smaller percentage of female adults than parasites that develop on non-superparasitized hosts. Larvae of A. cinerea are often killed if they compete with larvae of Banchus flavescens Cress., the only other common parasite of bertha armyworm, and a small percentage of the pupae of A. cinerea are killed by a hyperparasitic ichneumonid, Phygadeuon subfuscus Cress.