THE BIOLOGY OF OPIUS MELLEUS GAHAN, A PARASITE OF THE BLUEBERRY MAGGOT ^ By F. H. LATHROP, Senior Entomologist^ Division of Deciduous Fruit Insects, and R. C. NEWTON, Junior Entomologist, Division of Cereal and Forage Insects, Bureau of Entomology, United States Department of Agriculture INTRODUCTION One of the interesting phases of the investigation of the blueberry maggot, Rhagoletis pomonella Walsh,^ in eastern Maine (7) ^ proved to be a study of the biology of the parasite Opius melleus Gahan.^- ^ The study reported in this paper was conducted at Cherryfield, Me., during the years 1925 to 1929. The most intensive work was performed during the summer of 1929. HISTORY Opius melleus was originally described in 1915 by Gahan (2j p, 73) from one female specimen found on Mount Washington, N. H., and was almost simultaneously described by Kichmond (reported by Woods (10)) as Biosteres rhagoletis from 21 specimens reared from puparia of Rhagoletis pomonella from blueberries collected at Cherry- field, Me. In 1919 Gahan (4) cited B. rhagoletis as a synonym of 0. melleus, and in the same year (3) he described two additional species, 0. richmondi and 0. lectus, both of which, he states, were swept from blueberry barrens in association with 0. melleus and may also parasitize B. pomonella. Woods (10) was the first to rear 0. melleus from puparia of the blueberry maggot, and he states that Severin reared it from the apple maggot. Good {5) described the method of oviposition into the apple maggot. Lathrop and Nickels (7) summarized the life history of 0. melleus, gave tables indicating the percentage of parasitism of the blueberry maggot, and showed that this parasite could remain in the soil as long as four years and then successfully emerge as an adult. During the present study Opius melleus proved to be the dominant parasite of the blueberry maggot in Washington County, Me., and was found in sufficient numbers to indicate that it is an important factor in the economy of its host. 0, richmondi, reared in small numbers, was insignificant in comparison with 0. melleus. No specimen of 0. ledus was observed in the cages. If 0. ledus para- sitizes the blueberry maggot at all, it must be a minor species in eastern Maine. SOME HOST RELATIONSHIPS Rhagoletis pomonella infests blueberries {Vaccinium spp.) and huckleberries (Oaylussacia spp.), and also apples (Malus spp.) and haws (Viburnum spp.). The insect is commonly termed ^^blueberry 1 Received for publication Apr. 21,1932; issued February, 1933. 2 Order Diptera, family Trypetidae. 3 Reference is made by number (italic) to Literature Cited, p. 159. 4 Order Hymenoptera, family Braconidae. 5 The writers are indebted to A. B. Gahan for determinations of the species Opius melleus and 0. richmondi. Journal of Agricultural Research, Vol. 46, No. 2 Washington, D. C. Jan. 15,1933 Key_No. K-233 158010—33 4 (143) 144 Journal oj Agricultural Research Vol. 46, No. 2 maggot'' or ''apple maggot'' according to the fruit with which it is associated. The two forms of the species appear to be ecologically distinct (^, 8)y but no morphological basis for separation has been discovered,^ although the apple maggot is distinctly larger than the blueberry maggot in every stage. Oyius melleus parasitizes both the apple maggot and the blueberry maggot. It is a significant fact that specimens of the parasite from the apple maggot are distinctly larger than specimens reared from the blueberry maggot. (Fig. 1.) Other than size, no structural différ- ence affords a basis for separation of the species from the different hosts. Apparently 0. melleus exhibits a tendency toward the develop- ment of two divergent strains parallel to the ecological forms of the host. FIGURE l.—Opius melleus; dorsal view of female with wings spread: A, From the apple maggot; B, from the blueberry maggot. X 8. Note the similarity of structure and the difference in size LIFE HISTORY OF THE PARASITE IN RELATION TO THAT OF THE HOST Opius melleus is well fitted to parasitize the blueberry maggot. The ovipositor of the female parasite is long enough to reach the host within the berry, and the life cycles of the two species coincide, to the advantage of the parasite. (Fig. 2.) The flow of emergence of the adult parasites approximately parallels that of the flies, following it at an interval of 10 to 15 days. (Fig. 3.) Emergence of the females follows that of the males by 4 or 5 days. The earliest of the female parasites begin oviposition in 12 to 14 days after emergence, just as the maggots of the later instars appear in the berries in increasing numbers. The period of oviposition by the para- sites coincides mth the period of maximum abundance of maggots of the late second and third instars. A portion of the host puparia remains in the soil for two years or more, thereby establishing a cycle of two 3^ears or longer. Opius melleus parallels this behavior, a portion of the population exhibiting a cycle of two or more years. 6 Since this paper was written, C. H. Curran has described the blueberry maggot as a distinct species, Rhagoletis mendaz, in American Museum Novitates No. 526, 1932. The validity of the species awaits confirmation by other dipterists, Jan. 15, 1933 Biology of Opius melleus Gahan 145 The relationship existing between the blueberry maggot and its parasite, Opius melleus^ is such as would be expected to occur between species in their native habitat, which has been occupied for an indefi- nitely long period under stable environmental conditions. Such con- ditions would permit the mutual adjustments which are exhibited by the maggot-parasite association. LIFE HISTORY OF OPIUS MELLEUS The investigation of the life history of Opius melleus was conducted largely as a series of careful field studies, but in certain phases a com- bination of field and laboratory methods was employed. OVERWINTERED PUPARIA y MAGGOTS -JUNE- -JULY- -AUGU5T- -StPTCIiBE.« ^ FiGUEE 2.—Summary of life history of Opius melleus in relation to the blueberry maggot during an approximately normal season, Cherryfield, Me. EMERGENCE OF ADULTS Opius melleus spends the winter as a full-grown larva within the puparium of its host just beneath the surface layers of the soil. (Fig. 2.) Pupation takes place within the enveloping puparium about 30 days before the adult parasite emerges. When Opius melleus emerges from the puparium, it is apparently fully pigmented and the wings are fully expanded and hardened. The insect works its way upward through the soil by movements of the head, body, and appendages. Parasites that were observed in a glass- front observation box had much more difl&culty in emerging than did the flies. The parasites showed distinct signs of fatigue after climbing upward through 2 inches of dry sand, and no parasites were observed to emerge from a depth of 4 inches. The flies emerged from this depth with comparative ease. 146 Journal of Agricultural Research Vol. 46, No. 2 The difficulty experienced by the emerging parasites suggested that parasitized maggots might pupate at a more shallow depth in the soil than do nonparasitized individuals. A study of 1,694 puparia indi- cated that there is no significant correlation between the percentage of parasitism and the depth at which the puparia are normally found. EMERGENCE — OF FLIES OF PARASITES 1927 -^.-jr -Ou. -..o._.^ .9. 92 6 -^'^ .t- -t...-..«..-..±... 1929 -^ V ^ -■*— I I I I I I I I I I I I I I I I I I I M I I I I I I I I M I I I I I I I I I I I I I I I I I I I I I I I I [ I I I JULY *- -Í AUGUST FIGURE 3.—Cumulative curves showing the emergence of adults of Opim melleus and its host during the seasons of 1927,1928, and 1929 The fact that on an average 99 per cent or more of the puparia are found within 1 Yi inches of the surface of the soil makes it apparent that under field conditions the parasites have no serious difficulty in emerging from the soil. The seasonal emergence of adults was studied by observations of the cages used in the studies of Rhagoletis pomonella by Lathrop and Jan. 15, 1933 Biology oj Opius melleus Gahan 147 Nickels (7). Blueberries heavily infested with maggots were placed on wire-screen trays over a series of plots during the late summer and autumn of each season. Each plot was 4 by 5 feet in size. The mag- gots dropped from the berries and entered the soil, where they pu- pated. During the following season each plot was covered by an emergence cage, and a careful record was made of the flies and para- sites that emerged. The emergence of flies and of parasites during the three seasons in which observations were made is best comprehended by a study of the accumulated emergence for each season. The records for the parasites are summarized in Table 1, and are illustrated graphically in Figure 3. It will be noted that the cumulative emergence curves have a characteristic form, depicting a slow rise early in the season, followed by a period of maximum emergence during which most of the adults appear, and a gradual cessation of emergence toward the end of the season. TABLE 1.—Summary of emergence records of adults of Opius melleus for three yearSf Cherryfieldj Me. Date of accumulated emergence in percentages indicated Date of first Date of last Year Sex emergence 10 25 50 [Male July 3 July 21-22 July 27 July 30-Aug. 1. - Sept. 17. 1927 Female July 10 July 28-29 Aug.