Bessa Harveyi Research Articles

Life Table of Bessa harveyi (Diptera: Tachinidae) Parasitizing Pikonema alaskensis (Hymenoptera: Tenthredinidae)

A life table of Bessa harveyi (Townsend), a parasite of yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer), was constructed from field samples of eggs, firstinstar maggots, and adults. Major mortality factors of the parasite, in order of importance, were overwintering maggot mortality, superparasitism, egg infertility and sloughing, and fall emergence. Overwintering mortality included flooding, desiccation, freezing, predation, encapsulation by the host, and failure of adult flies to exit host cocoons. No obligatory diapause was detected when B. harveyi maggots were reared in the host at room temperature (21–22°C) and under constant light. The mean time for maggot development was 8.4 days (SE = 0.33). The sex percentage of 151 laboratory-reared puparia was 51.7% females. Total B. harveyi parasitism on P. alaskensis larvae was 16.7% in 1978 and 21.2% in 1979. B. harveyi is not well adapted to P. alaskensis . Therefore, B. harveyi is a polyphagous parasite that concentrates on the most predominant host in its range. We conclude that no practical method of improving the effectiveness of B. harveyi on P. alaskensis currently exists.

Egg Production, Oviposition Sites, and Adult Longevity of Bessa harveyi (Diptera: Tachinidae), Parasitizing Yellowheaded Spruce Sawfly, Pikonema alaskensis (Hymenoptera: Tenthredinidae) 1

Female Bessa harveyi (Townsend) caged with males laid a mean of 27.7 eggs per female on prepupal Pikonema alaskensis (Rohwer) larvae, with 37% not laying eggs. Virgin females caged without males laid a mean of 4.7 eggs per female, with 64% not laying eggs. The pre-oviposition period was 6.3 days in the greenhouse and 9.1 days in the laboratory. There was a highly significant difference in the mean number of mature follicles per female between egg-laying females (5.3) and those not laying eggs (1.1). The difference in mean longevity between the laboratory (20.9 days) and greenhouse (11.3 days) was highly significant. There was no significant difference in longevity between males and females in either environment. Female B. harveyi survived longest at a low temperature of 20°C and a high relative humidity (RH) of 90%. With increasing temperature within the range 20 to 28.5°C, egg production per unit time increased, but adult longevity decreased, so that total eggs produced remained about the same. There was a highly significant difference in the length of puparial stage between females (14.3 days) and males (13.5 days) at 20°C and 80% RH. Female B. harveyi preferred the dorsal thorax of P. alaskensis for oviposition. The origin of scars on host larvae is uncertain. Some scars are unsuccessful B. harveyi attacks, and others may be the result of attacks by other parasites and predators.

PARASITES OF THE YELLOWHEAD SPRUCE SAWFLY, PIKONEMA ALASKENSIS (HYMENOPTERA: TENTHREDINIDAE), IN MAINE AND NOVA SCOTIA

AbstractThirteen species of parasites were reared from Pikonema alaskensis (Rohwer) larvae and cocoons collected in Maine and 12 species from larvae collected in Nova Scotia. Of the larval parasites, the most common species in Maine were the ichneumons Rhorus sp. 1 and Syndipnus rubiginosus Walley, and in Nova Scotia, the ichneumons Aderaeon bedardi (Provancher), Mesoleius sp., Rhorus sp. 2, and S. rubiginosus. Of the tachina flies, Palexorista bohemica (Mesnil) was more common in Maine, while it and Bessa harveyi (Townsend) were equally common in Nova Scotia. Only two cocoon parasites were reared in Maine, the ichneumon Endasys pubescens (Provancher) and the pteromalid Tritneptis diprionis Gahan. No egg parasites were reared. Parasitism totaled 12.3% in Maine larvae, 16.1% in Maine cocoons, and 46.7% in Nova Scotia larvae.

Parasitism of the Larch Sawfly by Bessa harveyi (Diptera: Tachinidae)1,2

Parasitism of the larch sawfly, Pristiphora erichsonii (Hartig) (Hymenoptera: Tenthredinidae) by Bessa harveyi (Townsend) (Diptera: Tachinidae) was measured in 1973 and 1974 in Minnesota. Most overwintering live maggots were covered with a brown encapsulation sheath from which they escaped in spring. Averages of 26–52% of the maggots died from undetermined causes. Dead maggots were covered with a black encapsulation sheath. Undetermined mortality probably is an important factor influencing population dynamics. Superparasitism (ca. 23%) was not as important as speculated from field observations. Hosts “unsuccessfully” parasitized by B. harveyi showed reduced survival. The high incidence of dead maggots and encapsulated eggs of Mesoleius tenthredinis Morley (Hymenoptera: Ichneumonidae) (ca. 33%) considerably reduced opportunities for interspecific competition.

EGG PRODUCTION, OVIPOSITION BEHAVIOR, AND SEX RATIOS OF BESSA HARVEYI (DIPTERA: TACHINIDAE) PARASITIZING THE LARCH SAWFLY, PRISTIPHORA ERICHSONII (HYMENOPTERA: TENTHREDINIDAE)

AbstractIn Manitoba, the emergence of Bessa harveyi (Townsend) adults is well synchronized with the availability of larvae of its major host, Pristiphora erichsonii (Hartig). Mating occurs within 3 days of emergence and the females begin egg-laying 1–2 days later. Mean longevity was about 7 days for males and 12 days for females. Total egg production per female averaged 40 and 30 in 2 years with a maximum of 169 eggs from a single female. The total number of eggs laid per female was positively correlated with temperature.

Parasite Complex of the Larch Sawfly 1 in Minnesota 2

Only 2 parasites of the larch sawfly, Pristiphora erichsonii (Hartig), were found in all 16 study plots in northern Minnesota from 1970–72: the tachina fly Bessa harveyi (Townsend) averaged 30% parasitism over all plots; and the ichneumon Mesoleius tenthredinis Morley seldom exceeded 10% parasitism. Approximately 26% of the B. harveyi adults emerged in late-summer rather than the more beneficial spring emergence. Olesicampe benefactor Hinz, an ichneumon introduced into Minnesota in 1971 and 1972, was recovered from all 8 release plots. Parasitism reached 48% in one release plot, but was ca. 2% in other release plots by 1972. Dispersal into Minnesota, from releases in 1961 in Manitoba, was conspicuous by 1972 when O. benefactor was recovered from 5 of 8 check plots. Mesochorus dimidiatus Holmgren, an ichneumon hyperparasite of O. benefactor , was found in Minnesota for the 1st time.

Mesoleius tenthredinis 1 and Other Parasites of the Larch Sawfly in the Eastern United States

The ability of the larch sawfly, Pristiphora erichsonii (Hartig), to resist an introduced hymenopterous parasite, Mesoleius tenthredinis Morley, by encapsulating its eggs was noted in the 1950's in central Canada and the Great Lake states. Examination of host prepupae from several eastern states indicated that populations of immune larch sawflies built up in this region in the early 1960's. The pteromalid Tritneptis klugii (Ratzeburg) and the tachinid Bessa harveyi (Townsend) were also found parasitizing the sawfly in portions of this region. B. harveyi is reported for the first time from New York and tentatively for the first time in Illinois. Eclytus n. sp. was identified from the larch sawfly in New York. Because the larch sawfly has a prolonged emergence period, it can only be economically controlled in forests and plantations through biological methods rather than insecticides. In view of the widespread distribution of sawfly strains resistant to M. tenthredinis in the eastern states, the release there of the European ichneumon Olesicampe benefactor Hinz, which was successfully introduced into Canada recently as a sawfly parasite, is recommended.

DETERMINATION OF PREMATURE LARVAL DROP AND OTHER CAUSES OF LARCH SAWFLY MORTALITY

AbstractFifth-instar larvae of the larch sawfly, Pristiphora erichsonii (Hartig), of different weights, were deprived of food and placed in containers with moist moss. A low percentage of the early fifth-instar larvae were able to spin cocoons, but the percentage rapidly increased to a plateau as the larval weight increased. All but the heaviest larvae suffered appreciable mortality before first-year adult emergence was complete. A curvilinear relationship between larval weight and survival to the adult stage or to larvae in prolonged diapause is evident. The percentage of mature ova in adult females was reduced at the lower larval weights, with corresponding increases in the percentages of near-mature and large immature ova. There were linear relationships between larval weights, adult weights, and numbers of oöcytes. A method for assessing premature larval drop and other causes of mortality under field conditions is described and data for a number of years and plots are presented. Variations in the percentages of unparasitized normal late fifth-instar larvae were large. Data indicate that parasitism by Olesicampe (Holocremnus) sp. nr. nematorum (Tschek) and Bessa harveyi (Townsend) is the most important factor, probably a key factor, contributing to this variation.

Influence of the Health of the Food Plant of the Host on Host-finding by Tachinid Parasites

AbstractIn the laboratory, the parasitic tachinids Drino bohemica Mesn. and Bessa harveyi (Tns.) made two to four times as many attacks on tenthredinid sawfly larvae exposed on unhealthy as on those on healthy food trees. Olfactory stimuli from the unhealthy trees were preferred by the parasites to those from healthy trees. The possibility of greater masking of stimuli from hosts on healthy foliage than from those on unhealthy foliage was apparently of little importance. In the field a moderate to severe attack by sawfly larvae on a tree will cause the tree to become unhealthy and this increases its attractiveness to the parasites. Thus, the host larvae indirectly assist the parasites to find them. Stimuli that are directly influenced by the health of the food plant of the host may interact with those from other environmental factors in determining the apparent degree of preference by tachinid parasites for different hosts.

The Status of Bessa harveyi (Tnsd.) (Diptera: Tachinidae)

AbstractThe taxonomic status, life history, distribution and hosts of Bessa harveyi (Tnsd.) are discussed. In its development on the larch sawfly, Pristiphora erichsonii (Htg.), a partial second generation of parasite adults emerges after the host larvae have completed feeding. Alternate hosts are scarce and rarely parasitized so that there is little or no successful reproduction by these adults.

Loss of Attractiveness to Bessa harveyi (Tns.) (Diptera: Tachinidae) of Host Larvae During Cold Storage

A study of factors that influence host references of the parasitic tachinids Drino bohemica Mesn. and Bessa harveyi (Tns.) (Monteith, 1955, 1958a, 1958b, 1962) included tests with successive generations of the parasites. Large numbers of the sawfly larvae used as hosts were collected each summer and, when they had spun their cocoons, were put into cold storage until required. In some seasons sufficient larvae of a particular species were not available to conduct experiments for a twelve-month period though a supply of larvae of the same species collected in the previous year was in storage. If larvae stored for one year or longer did not change in their attractiveness to the parasites they could be used in tests together with larvae stored for less than one year. This is a report of an investigation to determine if changes occur during cold storage in the attractiveness of sawfly larvae to B. harveyi.

A Note on Temperature and Postdiapause Development of the Larch Sawfly and Its Parasite Bessa harveyi (Tnsd.)

Some information of interest on the postdiapause development of the larch sawfly, Pristiphora erichsonii (Htg.), and its tachinid parasite Bessa harveyi (Tnsd.) has been obtained in the course of laboratory rearings. Sawfly cocoons were placed in cold storage at 2 to 3°C. in September or October and held until the following year, usually late March, when they were incubated at 10 or 15°C. They were then held at constant temperatures until emergence was complete. Sawfly and parasite emergence was recorded daily.

Influence of Plants other than the Food Plants of their Host on Host-Finding by Tachinid Parasites

Earlier work indicated that food plants of the tenthredinid sawflies attacked by Drino bohemica Mesn. and Bessa harveyi Tns. influence host-finding by these tachinid parasites (Monteith, 1955, 1958a). There is a high degree of interaction between stimuli produced by the host larvae and by their food plants (Monteith, 1955, 1958b).It was observed during field studies that the percentage parastism by B. harveyi of the larch sawfly, Pristiphora erichsonii (Htg.), varied with increasing height above the ground and in different sections of tamarack trees, Larix larician (DuRoi) K. Koch, on which the sawfly larvae were feeding. It appeared that shrubs and trees other than L. laricina influenced host-finding by B. harveyi.

The Relative Effects of Cocoon Submergence on the Mortality of the Larch Sawfly, Pristiphora Erichsonii (Hymenoptera: Tenthredinidae), and Its Parasite Bessa Harveyi (Diptera: Tachinidae)1

Laboratory experiments on effects of submergence of larch sawfly cocoons, immediately after the overwintering period, confirmed earlier reports on the resistance of the eonymphal stage to flooding. An ichneumonid parasite, Mesoleius tenthredinis Morley, appeared able to survive flooding as well as the host, but the tachinid Bessa harveyi (Tnsd.) was found susceptible even when the flooding was of short duration. The early inception of growth and the rapid development of B. harveyi maggots, and the resultant active metabolism, seemed to account for its susceptibility to submersion. Sawfly larvae parasitized by this fly were found to have a higher rate of oxygen consumption than unparasitized larvae. Susceptibility to submersion seems to be a factor limiting the effectiveness of B. harveyi , and may account for its reported sporadic distribution in localized areas.

Estimation of Parasitism of Larch Sawfly Cocoons by Bessa harveyi Tnsd. in Survey Collections

The Forest Insect Survey at the Forest Biology Laboratory, Winnipeg, has been compiling records for a number of years on the percentage of cocoons of the larch sawfly, Pristiphora erichsonii (Htg.), parasitized by the tachinid Bessa harveyi Tnsd. Sawfly cocoons were collected each fall from the soil in infested stands, and those containing living larvae were dissected to determine the percentage of parasitism by B. harveyi. These estimates have been used to provide an index of parasitism (Lejeune and Hildahl, 1954), but are of limited value for a number of reasons: (1) estimates can be expressed only as percentage of sound cocoons parasitized; (2) total parasitism cannot be estimated because a portion of the parasites emerge from the cocoons before collection; and (3) estimates of parasitism may not be representative of the stand because there is a tendency to collect cocoons where they are easiest to find; hence all the cocoons in a collection may be from one or two small areas. If the proportion of cocoons containing B. harveyi varies within a stand such collections may give unreliable estimates of parasitism.