Control of the chalcid wasp Pteromalus apum Retzius, a parasite of Megachile rotundata (Fabr.) (Hymenoptera, Pteromalidae & Apidae)

SVEND N0RGAARD HOLM & JENO FARKAS

Svend N0rgaard Holm & Jeno Farkas: Control of the chalcid wasp Pteromalus apum Retzius, a parasite of Megachile rotundata (Fabr.) (Hymenoptera, Pteromalidae & Apidae) Ent. Meddr 53: 59-64. Copenhagen, Denmark, 1986, ISSN 001-8851.

The alfalfa leafcutting bee Megachile rotundata (Fabr.) is parasitized by the chalcid wasp Pteromalus apum (Retzius) in Europe. A brief description of the wasp is given, and the emergence period of adults, the number of parasites/host and sex ratios are treated. The effect of the insecticide dichlorvos on bees and wasps has been studied during the incubation period of the bees. A procedure for an effective control of para­ sitic wasps and predators with dichlorvos without harm to the bees is proposed.

Svend N0rgaard Holm, Department of Crop Husbandry and Plant Breeding of the Royal Veterinary and Agricultural University of Copenhagen, H0jbakkegard, Agrovej 10, DK-2630 Tastrup, Denmark.

Jeno Farkas, Research Center for Animal Production and Nutrition, Pf. 57, Godollo, H-2101, Hungary.

The alfalfa leafcutting bee (Megachile rotun­ Thomson, 1878). According to Boucek & data (Fabr.)) is believed to have been acci­ Graham (1978), P. apum is the only species dentally introduced into North America of the genus Pteromalus which is parasitic in from Eurasia in the first half of this century the nests of megachiline bees. The wasp was (Stephen, 1962), but is now the most com­ accidentally introduced from Europe and monly used pollinator of alfalfa seed fields probably arrived in Canada with the bee in North America (Parker, 1980), and alfalfa (Richards, 1984). Because of its high repro­ seed yields have been greatly increased after ductive capacity and short emergence time colonization of this bee in artificial hives in (Eves et al., 1980), this parasite has the po­ Canada (Hobbs, 1968; Peck, 1969). During tential to cause severe economic losses. recent years these bees have been propagated The wasp is associated with solitary bees and exported from Canada to countries in and seems a very common parasite of indi­ other parts of the world. As several Chalci­ genous leafcutting bees nesting in buildings doid wasps are known to parasitize the bees in Denmark. Nests with alfalfa leafcutting under Canadian conditions (Eves et al., bees placed on or close to sheds or houses 1980; Hobbs & Krunic, 1971), these ship­ are therefore always heavily infected with ments of leafcutting bees could result in the this parasite, and without control the whole introduction of some of the parasites into population of leafcutting bees is destroyed. new areas (Parker, 1982). The aim of the present experiments was The most common chalcidoid wasp para­ to investigate the possibility of destroying site associated with the alfalfa leafcutting the emerging wasps during incubation of the bee in the USA and western Canada is bees before the bees were released in the Pteromalus apum Retzius (1783) (synonyms field. The expefiments were conducted in P. venustus Walker, 1835; P. planiscuta 1983 and 1984. ·

59 Material samples for rearing in incubation trays, and The bees were originally imported from 400 cocoons from each sample were ran­ Montana, USA, and Beaverlodge, Alberta, domly selected and examined by x-radio­ Canada, in 1979-1981. Some of the wasps graphy using the method of Stephen & Un­ may have been present in the imported co­ durragd (1976). coons. In order to determine how far the wasps were able to crawl before they were killed by Methods the insecticide, a polyethylene tray 300 cm. The fully grown larvae of Megachile rotun­ long and 50 cm wide was placed in the in­ data spend the winter in their cocoons. The cubating room in 1983. Daily records were cocoons used in the experiment were re­ made of the number of emerged wasps and moved from wooden grooved nesting boards their position on the tray (Parkas & Szalay, 3 during the winter and kept at 6-8"C until 1982). A plexiglass box (0.9 m ) in which June 1st. A room was then equipped for in­ temperature, humidity and light intensity cubation of the bees at 3o·c. The appro­ could be controlled was used to determine priate humidity was ensured by sprinkling the death rate of bees and wasps at three 3 with water. concentrations of dichlorvos: 3.2 g per m 3 On the ninth day of incubation a Shell (prescribed dose), 6.4 g per m and 12.8 g per m3• Vapona Strip was hung up in the incubation room to kill the adult Pteromalus apum as Description of Pteromalus apum Ret­ soon as they emerged from the bee-cocoons. zius The active ingredient in the strip is 18.6% The wasp differens in colour and size from dichlorvos (0.0 - dimethyl - 2.2 - dichlor­ the description by Eves et al. 1980 as fol­ vinylphosphate). Normal concentration is lows. Females are black with a dark brown 3.5 g Vapona Strip per m3• In this experi­ to metallic blue-green colouration. Males ment only 2.1 g per m3 was used unless are similarly coloured except on the head otherwise stated. 343,000 cocoons were in­ and thorax where they are more metallic cubated and exposed to Vapona Strips in green. Both sexes have dark brown legs. 1983 and approximately 650,000 in 1984. Length, of 25 adults of each sex and 25 mature larvae, means and rages A. UNIKUM insect-attracting device with a Females 3.14 mm (2.6-3.7 mm) Phillips HPW (E/70/2) 125 watt bolb was Males 2.9 mm (2.6-3.9 mm) used to attract the wasp away from the co­ Mature larvae 2.4 mm (1.6-2. 7 mm) coons in 1984. Before the cocoons were exposed to the effect of dichlorvos from a Vapona Strip, Results 100 cocoons were selected at random and Emergence exposed to dichlorvos for 7 days and exam­ The adult parasites started to emerge on the ined for mortality. All lOO bees emerged 9th and 1Oth days of incubation at 3o·c except one which was found dead in an im­ and continued to emerge over a period of 5 complete cocoon. Several similar tests were to 6 days. At 25"C the emergence period was made, and in no cases the bees were harmed 11 days. Peak emergence occurred on the or killed by the insecticide as long as they 3rd or 4 th day and after the 6th. day more were inside the cocoons. than 92% of all the parasites had emerged The Vapona Strip hung on the 9th day of (Fig. 1). incubation was left until a few small males of M. rotundata (the first sex to emerge) Generations were killed. A few parasites and predators The overwintered wasps were reared at dif­ other than P. apum were also destroyed at ferent temperatures for successive genera­ the same time. In order to ensure adequate tions (Table 1). at 30"C the wasps went exposure of the emerging wasps to the insec­ through three successive generations before ticide, the incubation trays were left open. entering diapause while at 20· and 2YC only Each year all cocoons were divided into two generations could be reared before dia-

60 PER CENT OF TOTAL 30

20

10

1 2 3 4 5 6 7 8 9 10 11 DAY OF EMERGENCE Fig. l. Percentage of Pteromalus apum emerging on successive days following the emergence of the first wasp. Based on 339 males and females incubated at 25°C.

Fig. 1 Den procentvise fordeling af klakkede Pteromalus apum i forhold tit det forst klakkede individ. Baseret pii 339 hanner og hunner ved inkubation pii 25°C. pause occurred. Further generations of the (Richards, 1984). On contacting the host she wasps could not be obtained until the dia­ drums it with the antennae and selects an pause was broken by subjecting the mature oviposition site on the cell and inserts the larvae to low temperatures (3-SOC) for three ovipositor (Fig. 2). months. The emergence period was about 15 days for the first two generations reared at 30T. In the third generation at this temperature only very few wasps emerged, and they emerged on average after 31 days. At lower temperatures the incubation period was cor­ respondingly longer and took 36 days at 20°C compared with 20 and 26 days for first and second generations reared at 2YC (Table 1).

Number of Number of days to emergence at: generation 20T 25T 30°C

I. 36 20 15 2. 26 16 3. 31 Fig. 2. Egglaying Pteromalus apum.

Table I. Number of days to emergence of succes­ Fig. 2. .!Eglaggende Pteromalus apum. sive generations of Pteromalus apum at different temperatures before entering diapause.

Tabell. Antal dage inden klakningfor tre succes­ sive generationer afPteromalus apum vedforskel­ Number ofparasites/host and sex ratio lige temperaturer inden diapausens indtraden. All adult wasps emerged through a single 1 mm hole chewed in the side of the bee cell. The males emerged first and mated with the Oviposition females as soon as they emerged from the Shortly after emergence from the cocoon fe­ cocoons. Estimates of the number of wasps males start to search for a new host per host and sex retio, based on 61 bee cells,

61 are shown in Table 2. A total of 1,100 The UV-light used in 1984 appeared to be wasps were reared, giving an average of 18 more attractive and effectively caused the per cell. Most of the host cells contained wasps to leave the bee cells very soon after both male and female wasps, although some emergence. The method removed the wasps cells contained only one sex. In general a effectively and they were killed by the insec­ greater number of females than males emer­ ticide before the females could lay eggs in ged, and an overall sex ratio of I : 1.26 males the inparasitized cocoons in the incubation to females was found. trays. Sex Number of wasps per cocoon Concentration normal 2x 4x Average Range No of wasps 10 10 10 d' 485 8 0-35 First wasps dead after 38 min. 18 min. 10 min. 11 615 10 0-32 All dead after 41.6 m. 22.2 m. 11.2 m. Total 1.100 18 6-52 No of bees 6 5 First bees dead after 28 min.- 5 min. Table 2. Number of Pteromalus apum emerged All dead after 34min.- 13 min. from 61 Megachile rotundata cocoons.

Tabel 2. Antal Pteromalus apum kla!kket fra 61 Table 3. Survival time of Pteromalus apum and Megachile rotundata kokoner. Megachile rotundata after exposure to different concentrations ofVapona.

Effect ofdichlorvos Tabel 3. overlevelsestid for Pteromalus apum og The effect of the insecticide dichlorvos on Megachile rotundata efier pavirkning afforskellige bees and wasps was studied using three dif­ koncentrationer af Vapona. ferent dose rates (Table 3). The first wasps died after 3 8, 18 and 10 minutes exposure to normal (lx), 2x and 4x normal concentra­ tions of dichlorvos, respectively. Discussion At the normal concentration it took ap­ Various control procedures have been im­ proximately 42 minutes before all of the plemented in the USA and Canada to con­ wasps were dead, and this extinction time trol chalcidid wasps and to remove them was halved for each doubling of the concen­ from the M. rotundata cells during the in­ tration. cubation period (Hobbs, 1968; Johansen & When exposed to dichlorvos most bees Eves, 1969; Eves, 1970; Hobbs & Krunic, died more quickly than the wasps; at the 1971; Waters, 1971; Brindley, 1976; Par­ normal concentration of dichlorvos the bees ker, 1980; Hill et al., 1984). died within half an hour after emergence. Pteromalus apum was probably introdu­ There were no detrimental effects on ced into Denmark with importation of M. unemerged bees even when the cocoons were rotundata cocoons in 1979-1981 (Holm, exposed for 7 days at the normal concentra­ 1982), but it also parasitizes indigenous leaf­ tion. The intact cocoons effectively protect­ cutting bees in Denmark, and it is therefore ed the developing pupae of M. rotundata important to control this parasite in the po­ against the concentrations of dichlorvos used pulations of leafcutting bees used for polli­ in these experiments. nation of crops (Holm, 1983). The female The distance the wasps were able to crawl wasps are able to lay eggs inside a new co­ before they died was recorded using the po­ coon within four hours from emergence lyethylene tray and the results are shown in (Richards, personal communication). The Fig. 3. Over twenty percent were able to results obtained show that normal concen­ move more than 3 m, before they died of ex­ trations of dichlorvos can be used to exter­ posure to the insecticide. The adult wasps minate the emerged female wasps before are attracted to light, but not all adults were they are able to parasitize other cocoons and attracted, which indicates that white light is thereby produce the second generation not attractive to the wasps. which would normally emerge under field

62 DEATH RATE (%) 30

20

10

0 ·5 1.0 1.5 2.0 3.0 DISTANCE IN M FROM RELEASE POINT (0) TO LAMP Bu~B (3.0) Fig. 3. Effect of dichlorvos on death rate of 164 males and Ill females of Pteromalus apum at different distances from the release point (0).

Fig. 3. Den relative effokt af dichlorvos pa dodeligheden af 164 hanner og 111 hunner afPteromalus apum iforhold til klakningsstedet (0). conditions and cause great damage to the The fumes from the strips adhere readily next generation of bees. to wooden incubation thrays and other sur­ Dichlorvos is effective in controlling in­ faces, and losses of emerging bees may occur sect parasites and predators of Megachile ro­ as the vapours are released. To prevent sub­ tundata because these normally emerge be­ sequent losses after the treatment with fore the bees emerge from their cocoons. M. dichlorvos it is recommended that the bees rotundata requires a longer incubation pe­ be transferred to uncontaminated trays in riod for emergence as an adult (Holm & which they can be transported to the hives Skou, 1972) than do the indigenous Danish and released. species of parasites and predators. The in­ digenous parasitic bee Coelioxys inermis Acknowledgments Kirby, which is occasionally found in M. ro­ The authors would like to thank Dr. Karl­ tundata nests in Denmark, was effectively Johan Hedquist, Swedish Museum of Natu­ destroyed by use of dichlorvos. ral History, Stockholm, for identifying P. apum; Dr. J. P. Skou for reviewing the ma­ Conclusions nuscript, Dr. Brian Dennis for revising the The use of Vapona Strips with dichlorvos English text and Mrs. Kerstin Henriksen for together with UV -light to attract the wasps valuable technical assistance. away from the cocoons of leafcutting bees ensures an effective control of parasitic Sammendrag wasps and predators, and if used with care Kontrol af Chalcididen Pteromalus apum does not cause bee mortality. Retzius, en parasit pa bladskrerebien Me­ Proposed control procedure: Insecticide gachile rotundata (Fabr.) (Hymenoptera, vapour strips should be used with great care Pteromalidae & Apidae). because excessive concentrations and/or Lucemebladskrerebien (Megachile rotundata prolonged exposure periods may cause mor­ Fabr.) udnyttes kommercielt til best0vning af lu­ tality among the bees. The vapour strips cerne i USA og Canada, og den blev indf0rt i should be placed 15-20 cm from the ceiling Danmark i 1981 med henblik pa best0vning af on the 9th day of incubation af 30'C and lucerne og kl0ver m. v. Bieme angribes af snylte­ hvepsen Pteromalus apum Retzius, der kan de­ left in the room for 5"6 days or until all pa­ struere hele populationen af bier, hvis der ikke rasites and predators have emerged. The gennemf0res en effektiv bekrempelse af snylteren. concentration of the insecticide must be suf­ Denne atbandling omhandler en metode til at de­ ficient to destroy the wasps soon after they struere snyltehvepsene under inkubationsperioden have emerged, but should not exceed the re­ afbieme, f0r de udsrettes i fmmarkeme. commended concentration. Snyltehvepsen Pteromalus apum er 2,6 til 3,2

63 mm. Hunnerne har sortbrun til metal-gmn farve; ceedings of the First International Symposium hannerne er tilsvarende farvet undtagen pa hove­ on Alfalfa Leafcutting Bee Management, det og brystet, hvor de er mere metalgmnne. Saskatoon, Canada, 80-82. Straks efter klrekning og parring ops0ger hunnen Hill, B.D., K.W. Richards & G.B. Schaalje, 1984: en kokon med en bilarve eller puppe. Hun ind­ Use of dichlorvos resin strips to reduce parasi­ f0rer lreggebrodden igennem bladcellen og koko­ tism of alfalfa leafcutter bee (Hymenoptera: nen og lregger reggene pa bilarven (Fig. 2). Megachilidae) cocoons during incubation. - J. Snyltehvepsene klrekkes fra kokonerne fra den Econ. Ent. 77 (5): 1307-1312. 9. til den 15. dag ved en inkubationstemperatur Hobbs, G.A., 1968: Controlling insect enemies of pa 30°C. Ved lavere temperaturer foregar klrek­ the alfalfa leafcutter bee, Megachile rotundata. ningen over en lrengere periode (Fig. 1). Ved klrek­ -Can. Ent. 100: 781-784. ning af overvintrede snyltelarver udvikles ved Hobbs, G.A. & M.D. Krunic, 1971 : Comparative 3o"C tre successive generationer, f0r diapause ind­ behavior of three chalchoid (Hymenoptera) trreder (Tabel 1). Gennemsnitligt klrekkes der 18 parasites of the Alfalfa leafcutter bee, M egachi­ snyltehvepse pr. parasitiseret kokon i forholdet 1 le rotundata, in the laboratory. - Can. Ent. 103: han til 1,26 hunner (Tabel 2). 674-685. Snyltehvepsene drrebtes med Shell Vapona Holm, S.N. & J.P. Skou, 1972: Studies on trap­ Strip (18,6% dichlorvos), der opsattes pa den 9. ping, nesting, and rearing of some Megachile dag, og indtil alle snyltehvepsene var klrekket af species and on their parasites in Denmark. - kokonerne. Ved normal koncentration (I 04 g Ent. Scand. 3: 169-180. Vapona Strip pr. 30m3 rum) d0de snyltehvepsene Holm, S.N., 1982: Management of Megachile ro­ gennemsnitligt efter 41,6 minutter. Ved stigende tundata for pollination of seed crops in Den­ koncentration af dichlorvos reduceredes tiden for­ mark. - Proceedings of the First International holdsvis (Tabel 3). Distancen, snyltehvepsene be­ Symposium on Alfalfa Leafcutting Bee Manage­ vregede sig inden 3 m, f0r de d0de, fremgar af Fi­ ment, Saskatoon, Canada, 223-233. gur 3. Holm, S.N., 1983: Introduction and propagation Resultaterne viser, at det er muligt ved brug af of the leafcutting bee (Megachile rotundata) in normal koncentration af dichlorvos at drrebe alle Denmark. - Vth. International Symposium on snyltehvepsene, f0r de er i stand til at prdducere Pollination, Versailles, France, 455-460. en ny generation, og uden at det skader bierne. Johansen, C. & J.D. Eves, 1969: Control of alfalfa Ved en inkubationstemperatur pa 3ooC placeres leafcutter bee enemies. - Cooperative extension Vapona Strip 15-20 cm. fra loftet pa den 9. dag, service, Pullman, Washington State, E.M. eller nar de f0rste snyltere klrekkes. Da dichlorvos 2631, 10 pp. trrenger ind i trreet i klrekningskasserne, anbefales Parker, F.D., 1980: Alfalfa leafcutter bee- reduc­ det at overf0re kokonerne til en anden kasse straks ing parasitism of loose cells during incubation efter behandlingens afslutning, f0r bierne klrekkes. (Hymenoptera: Megachilidae). - Pan-Pancific Entomologist 55: 90-94. Parker, F.D., 1982: Introduction of alfalfa leaf­ cutting bee to areas and selection development. References - Proceedings of the First International Sympo­ sium on Alfalfa Leafcutting Bee Management, Boucek, Z. & M.W.R. de V. Graham, 1978: Saskatoon, Canada, 198-211. British check-list of Chalcidoidea (Hymenopte­ Peck, 0., 1969: Chalchoid (Hymenoptera) parasi­ ra). Taxonomic notes and additions. - Ent. tes of the alfalfa leafcutter bee, Megachile ro­ Gaz. 29, 225-235. tundata, in Canada.- Can. Ent. 101: 418-422. Brindley, W., 1976: Carboryl control of chalci­ Richards, K.V., 1984: Alfalfa leafcutter bee ma­ doid parasites from alfalfa leafcutting bees. - J. nagement in eastern Canada. - Communica­ Econ. Ent. 69 (3): 225-228. tions Brach, Agriculture Canada. Publication Eves, J.D., 1970: Biology of Monodontomerus 1495E, 53 pp. obscurus Westwood a parasite of the alfalfa Stephen, W.P., 1962: Propagation of the leaf­ leafcutting bee, Megachile rotundata (Fabricius) cutter bee for alfalfa seed production. - Agricul­ (Hymenoptera: Torymidea); (Megachilidae). - tural Experiment Station, Oregon State Univer­ Melanderia 4: 1-18. sity, Corvallis. Station bulletin 586, 16 pp. Eves, J.D., D.F. Mayer & C. Johansen, 1980: Pa­ Stephen, W.P. & J.M. Undurraga, 1976: rasites, predators, and nest destroyers of the X-Radiography, an analytical tool in popula­ alfalfa leafcutting bee, Megachile rotundata. - tion studies of the leafcutter bee Megachile Western Regional Publication No. 32, 15 pp. pacifica. - J. Apic. Res. 15: 81-87. Farkas, J. & L. Szalay, 1982: Melittobia acasta - Waters, N.D., 1971: Insect enemies of the alfalfa as the most important parasite of the alfalfa leafcutter bee and their control. - Idaho current leafcutter bee Megachile rotundata. - Pro- information series no. 16.3, 4 pp.

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