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The overwintering mature larva of the parasite Anastatus disparis Ruschka within the intact gypsy moth, Lymantria (= Porthetria) dispar L., egg chorion is susceptible to freezing and has a mean supercooling point of −28.8±0.14°C. This level of cold hardiness is not increased by exposure to sublethal low temperatures. Over 85% of larvae survived 10 months at 0°C and 4 days at −12° or −18°C, both followed by an additional 2–3 months at 0°C, but over half of the survivors went into prolonged diapause which was not fulfilled by subsequent rearing at room temperature. The parasite is apparently capable of surviving winter conditions over the same geographic area in Canada as the gypsy moth, and the introduction of A. disparts into locations in Canada where gypsy moth is now present is recommended.

Females ofPleolophus basizonus (Gray.) show a very strong avoidance of cocoonedNeodiprion sertifer (Geoff.) prepupae in which there are eggs, larvae or prepupae ofLophyroplectus luteator (Thunb.). When multiparasitism does occurP. basizonus is the successful competitor but adults are much smaller than those from hosts that were not multiparasitized.

The overwintering adult of Ooencyrtus kuwanai (How.) is susceptible to freezing. Male adults from New Jersey have a mean supercooling point of −9.6 ± 0.37°C, significantly higher than those from Maine (−12.3 ± 1.08°). Neither group had its cold hardiness increased by exposure to sublethal temperatures. Female adults from New Jersey had a mean supercooling point of −10.8 ± 0.69° when tested after rearing at 22°C, again significantly higher than that of similarly treated female adults from Maine (−15.4 ± 0.90°). Exposure of New Jersey females to sublethal temperatures decreased their supercooling point significantly, to −14.5 ± 1.59°. Maine female adults were not tested after that treatment. No adults survived continuous exposure to 0°C for 30 days. Hence this parasite is apparently incapable of surviving in the current range of the gypsy moth in Canada.

Adult femaleLophyroplectus luteator (Thunb.) oviposit in all feeding stages ofNeodiprion sertifer (Geoff.), although they prefer the smallest of any larvae present at a given time. Females tend to distribute eggs contagiously both within and between host feeding colonies. Eclosion of eggs does not occur until after the host has spun its cocoon. Larval development then requires approximately 1 month in the laboratory. There is an obligatory eonymphal diapause which requires a minimum of 60–80 days at 2°C to fulfil, and post-diapause development requires 3–4 weeks.

Lophyroplectus luteator (Thunb.), a European ichneumonid parasite previously established on Neodiprion sertifer (Geoff.) in southwestern Ontario, was released near Hepworth, Ontario in 1970, on Manitoulin Island and in Sault Ste. Marie, Ontario in 1972. All three releases resulted in establishment. Data on attack levels and host densities are presented for up to 4 years after release. Dispersal within release areas was rapid. In the first generation after release on Manitoulin Island parasites were obtained up to 900 m from the release site.

The first North American collection of the European pine sawfly, Neodiprion sertifer (Geoff.), was made in Somerset County, New Jersey, in 1925 (Hamilton, 1943). It was first recorded in Canada near Windsor, Ontario, in 1939 (Raizenne, 1957), and by 1946 it had spread as far east as London (Watson, 1946). This serious pest of pines has continued to spread, and is now found throughout most of southwestern Ontario (Fig. 1) (Sippell and MacDonald, 1958). This is the only infestation of this species in Canada.

The upper limit for continuous development ofPleolophus basizonuslies between 25° and 30 °C since all stages develop at the former temperature and larval mortality is complete at the latter temperature. The lower limit lies between 5.5° and 10 °C, since an eonymphal diapause occurs at 5.5 °C. Between these extremes, diapause incidence varies inversely with temperature. The photoperiod experience of the parent has no effect on diapause in the progeny when the latter are reared at 15° and 18 °C, but there is an inverse relation between parental photoperiod experience and progeny diapause incidence at 10 °C. Although larvae and pupae also survive 3 months at 2 °C, most individuals probably overwinter in eonymphal diapause and their diapause requirements are fulfilled in 3 weeks at 2 °C. The incidence and rate of post-diapause development is directly related to temperature.

In Neodiprion sawflies the cocoon stage, which extends from the completion of larval feeding until the appearance of the adult, is of considerable ecological interest. On the basis of voltinism and the seasonal occurrence of the cocoon stage, the genus as a whole may be divided into two distinct groups, which, however, do not correspond to the two major taxonomic categories of Ross (1955). On the one hand are species that overwinter only within the cocoon (e.g. swainei, lecontei, virginianus, excitans, and others); these may or may not have more than one generation per year. On the other hand are species that overwinter primarily in the egg stage (e.g. sertifer, pratti, abietis, taedae, and others); all of these are univoltine and may remain within the cocoon for an extended period in the summer, when conditions are still favourable for further generations. The European pine sawfly, Neodiprion sertifer (Geoff.), is typical in this respect. Despite its wide distribution in Eurasia and North America, and the variety of climates that it encounters, the length of the cocoon stage is so adjusted that adults do not emerge until late summer or early autumn, regardless of when larval feeding and cocoon spinning occurred.

Aptesis biasizona (Grav.) is a common and widely distributed parasite of diprionid sawflies in central, western, and northern Europe, with parasitism varying from three to 90 per cent on different hosts and in different areas. One of these sawflies, Neodiprion sertifer (Geoff.), is very commonly attacked, with parasitism of 30 to 80 per cent in Czechoslovakia, 10 to 52 per cent in Hungary, and 17.5 to 40 per cent in Sweden (Morris et al., 1937).

The possibility of imperfect coincidence between the appropriate stages of Neodiprion sertifer (Geoff.) and two of its important parasites was demonstrated. One of the parasites, the indigenous ichneumonid Exenterus canadensis Prov., which attacks late-stage larvae, has good spatial coincidence; but some members of each generation suffer from imperfect temporal coincidence, or asynchrony, caused by the interaction of temperature influence on parasite development rate and temperature variability between development sites in the litter. The second parasite, Pleolophus basizonus (Grav.), is an introduced, multivoltine ichneumonid cocoon parasite. It may be imperfectly synchronized in its first generation each year and may show imperfect spatial coincidence in all generations through its inability to attack host cocoons beneath approximately 1 in. or more of litter.The intricate relations between parasite and host density, time, attack, and coincidence were investigated using the basic functional response submodel developed by Holling, a submodel that describes changes in oviposition behaviour with time, and a submodel that predicts the number of hosts attacked, given the number of eggs laid and data on the distribution of eggs among hosts. In the two species studied, the effect of asynchrony in one generation cannot be considered without considering the influence of superparasitism. At low host densities, superparasitism largely buffers the effects of decreased synchrony. This buffering effect decreases as host density increases until when each parasite is attacking all the hosts it can, it is almost eliminated. Imperfect spatial coincidence in one generation merely lowers the usable host density. Thus its effect can be seen in the functional response of the parasite to host density. When host–parasite interactions over 25 to 35 host generations were simulated, using initial conditions resembling those ensuing when small numbers of both host and parasite invade a previously unattacked stand, populations became stable after passing through one or more oscillations. Decreasing temporal or spatial coincidence increased host and parasite densities at the peaks of oscillations and increased the ultimate steady density of host and parasite, until coincidence was reduced to nearly half. At this level, the host escaped the regulating ability of both species of parasites.

The oviposition behaviour of Neodiprion sawflies has been the subject of much investigation in recent years. The impetus for this interest seems to lie in the paper by Atwood and Peck (1943), in which it was suggested that the number and spacing of eggs on needles were a useful tool for the identification of memhers of this important group of conifer defoliators. Ghent (1955) has analysed the egg clusters of N. pratti banksianae Roh., and Ghent and Wallace (1958) have investigated the behaviour responsible for the pairing of eggs on adjacent jack pine (Pinus banksiana Lamb. ) needles by N. swainei Midd. More recently, Ghent (1959) has presented a study of the factors determining the spacing of eggs by the European pine sawfly, N. sertifer (Geoff.). Of these, the 1955 and 1959 papers, in addition to making valuable contributions to our understanding of the behaviour of adults, cast doubt on the usefulness of the number and spacing of eggs as identifying characters in these species.

A generalized competition model for predators or parasites was developed from data obtained from a specific parasite–host system. It was structured in three parts. The first simulates the effects of exploitation, where the number of attacks and their distribution among prey or hosts determine how many prey or hosts survive. Since the negative binomial distribution described these distributions consistently, the exploitation submodel was developed from it. The second portion of the competition model concerned interference between searching predators and parasites. Although interference is a universal phenomenon, we were able to show that its effects become important only at predator densities much higher than those that occur in nature. Thus the interference component can be essentially ignored. The third and final component concerned the outcome of competition between parasite progeny within their host. It was developed from Fujii’s competition model which allows for the simulation of both scramble and contest types of competition.These three submodels of competition were combined and coupled with a previously published model of the effects of prey density on attack. In this way the full consequences of different prey and predator densities could be simulated using a model whose constituent parts had been carefully tested for descriptive adequacy. The simulations showed the way individual predator attack, per cent predation, and progeny production were affected by different degrees of contagion in the distribution of attacks, by scramble vs. contest competition, and by the degree to which parasites could avoid hosts already attacked.

Ourbreaks of the jack-pine sawfly, Neodiprion pratti banksianae Rohwer, occur in northern Ontario at irregular intervals. Two independent outbreaks within 50 miles of each other – one on Great Cloche Island (McGugan et al., 1952) which lies in the North Channel of Lake Huron between Manitoulin Island and the mainland, and the other near Nairn Centre, approximately 30 miles west of Sudbury, Ontario – offered an opportunity to study the parasitism of this important defoliator. In 1954, when the study was started, heavy defoliation had been reported for several years in the Great Cloche outbreak, and light defoliation had occurred for one year in the Nairn Centre outbreak. Populations decreased in the Great Cloche area but remained relatively constant at a low level in the Nairn Centre area during 1955 and 1956, when the work was completed.

Lophyroplectus luteator (Thunb.), a European ichneumonid, has been established as a parasite of Neodiprion sertifer (Geoff.) from a release made in 1962, and has dispersed approximately 12 miles (19.3 km) in the 7 years following introduction. A second, smaller release in the same year failed. In a third release, made in 1964, recoveries were made every year until sampling was stopped in 1971, but dispersal has only been approximately 2 miles (3.2 km).