Utah State University DigitalCommons@USU All PIRU Publications Pollinating Insects Research Unit 1960 The Biology of Heterostylum rubustum (Diptera: Bombyliidae), a Parasite of the Alkali Bee George E. Bohart Utah State University W. P. Stephen R. K. Eppley Follow this and additional works at: https://digitalcommons.usu.edu/piru_pubs Part of the Entomology Commons Recommended Citation Bohart, G. E., W. P. Stephen, and R. K. Eppley. 1960. The Biology of Heterostylum rubustum (Diptera: Bombyliidae), a Parasite of the Alkali Bee. Ann. Ent. Soc. Amer. 53(3): 425-435. This Article is brought to you for free and open access by the Pollinating Insects Research Unit at DigitalCommons@USU. It has been accepted for inclusion in All PIRU Publications by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. ( Reprinted from fu'<NALS OF THE ENTOMOLOGICAL SOCIETY OF rumRJCA Vol. 53, No. 3, May, 1960 THE BIOLOGY OF HETEROSTYLUM ROBUSTUM (DIPTERA: BOMBYLIIDAE), A PARASITE OF THE ALKALI BEE1 G . E. BOHART,' W. P. STEPHEN, Ai\ID R. K. EPPLEY3 ABSTRACT H eterostylum robustu m. (Osten Sacken) is the principal very brief second ins ta r, and a soft, helpless third ins tar , parasite of the a lkali bee (Nomia mela11deri Ckll.) in the to a tough, more active fourth instar. Some lat vae Northwestern States. It also parasitizes other species apparently mature on a single host, but others pa rt ially of Nomia and at least one species of both Nomadopsis and drain the fluids from a second as well. In the late Halictus. It eject-s eggs into and near the nest mounds summer or fall the mature larva makes an overwin tet ing of its host, but does not readily discr iminate between nest cell in the upper few inches of soil. It pupates in the late burrows and other depressions in the ground. The first­ spring, and shortly before ecdysis bores to the ground instar larva finds its way to a host lan ·al cell, where it surface. The adult emerges as soon as the puiJa t hrusts waits w1til the host lan' a is fu ll grown before feeding oo its head and t horax clear of the surface. Mating usually it. The parasite larva passes t hrough four instars, takes place on \·egetation surrounding t he host nesting progressing from a slender, active first instar t hrough a s ite. As the principal parasite of the alkali bee excellent alfalfa pollinator, cruefly responsible for (1Yomia melauderi Ckll.) over much of its range, t he lllgh yields in the major seed-growing areas H eterostylum robuslmn (Osten Sacken) is an im­ of Waslllngton, Oregon, Idaho, and Wyoming. portant economic insect. The alkali bee is an In some areas where alkali bees were formerly abundant, robustum now holds their population 'Tech nical paper No. 000, Oregon Agricultural Experi­ t o an insignificant level. The percentages of ment Station. Accepted for publication November 24, host prepupae destroyed vary from over 90 in 1959. T hese studies represent a cooperative project by t he U. S. Depar tment of Agriculture, the Utah Agr icul­ nesting sites of Cache Valley, Utah, to 5 or less t ural E:>;periment Station, and Oregon State College. in some sites near Wapato, ·washington. In The research in Oregon was supported in part by the many areas 20 to 40 percent of the host larvae Malheur County Seed Growers Association. E. A. Cross are destroyed. In Oregon a nesting aggregat ion and W. P . Nye assisted in the observations in Ut ah. •Entomology Resear ch Division, Agricultural R e­ of over half a million bees in 1955 had an incidence search Ser vice, U.S.D .A. of 91 percent parasitism in 1956. I n 1957 the 30regon State College, Cor vallis. aggregation was only half as large, but parasitism ( 425 I J -126 Annals of the Entomological Society of A rnerica [Vol. 53 fell Lo 50 percent. In 195 parasitism was 30 more gregarious individuals. Furthermore, some percent and the bee population appeared to be of the late-emerging bees continue to lay eggs nearly stabilized. after most of the parasites have disappeared and a partial second generation appears in late August after all the flies have disappeared. The first alkali bees to emerge each summer concentrate their nests in the most nearly ideal portions of the nest­ ing sites. Consequently, several hundred to a thousand or more nests appear in these favored areas in spite of an annual parasitism of well over 00 percent. DISTRIBUTION A.m HO T R.\ rcE The recorded geographic range of Ilete:·osfylum robusfttm includes the Pacific and R ocky Moun­ tain States as well as the southern Great Plains, the lower Mississippi Valley, and Florida (Painter 1930, 195 ) . In the Western States it is found primarily in the larger valleys. Since its range greatly exceeds that of .Yomia melanderi. it must successfully parasitize other species of bees. Frick (195 ) stated that. he has seen a specimen of robusfmn from Colorado labeled as a parasite of .Y omia bakeri Ckll. In the vicinity of Logan it has been seen to deposit eggs in the nests of several species of solitary ground-nesting bees in and near the nesting sites of alkali bees. T he larvae have been observed developing normally in the cells of .Yomia triangttlifera Vachal a nd .\"omadopsis anlhidius Fowler. Early stages of ( the larvae have also been found feeding on pre­ pupae of ~\"om ado ps is scttfellaris Fowler, a small species that nests in alkali bee sites, but full­ grown larvae and pupae have not been fo:.tnd in strict association with this species. The senior FIG. 1.-Adult female Helerostylum robusl~ttn ovipositing author once saw adults laying eggs in a small over nest entrance of N omia melanderi. nesting site of H alicltts rubimndus Christ. Sub­ sequently first- and third-instar larvae, which Before 19-!6 there were several alkali bee nest­ appeared normal, were seen on the host prepupae. ing sites west of Logan, Utah, each containing Although robustmn seems to be an ever-present over 100,000 nests. Bees from these sites were parasite in alkali bee nesting sites in Washington, responsible for some excellent seed yields in Oregon, Idaho, northern Nevada, and northern neighboring fields. T odd (1946) described the Utah, it has not been found in Wyoming or ground on these sites as covered by thousands of central and southern Utah, although a single empty bombylliid pupal skins. For the next 10 specimen of Heferostylmn englelrardti Painter was years parasitism in the Logan area was extremely seen laying eggs in an alkali bee nesting site in high and alkali bee populations were too small to central Utah (near Delta). In the Uintah Basin benefit the farmers appreciably. Since 1956 a of eastern Utah a small number of larvae were slight reversal of this downward trend has been taken which closely resembled ro b1~stmn , but cer­ observed. It may have been the result of para­ tain details observed in their life history were site control measures attempted on an experi­ different from those observed for robustum in mental basis. However, another important dip­ other areas. terous parasite in this area, Euphytomima nomi­ In northern Utah and in Oregon the adults of avora] ames, was controlled by the same measures, robuslmn were observed taking nectar from so that the benefit cannot be ascribed solely to bee flowers of alfalfa and sweetclover. H owever, fly control. its host-plant range probably includes many other Apparently two factors have been responsible fl owers with moderately long corolla tubes. In for the failure of the bombyliid parasite to wipe 1947 it was conspicuously abundant in many out its host completely in the Logan area. Each alfalfa fields in northern Utah, but was never year a few of the bees nest in a solitary fashion observed to trip the flowers. I t rarely alighted { and thus avoid the heavy attack suffered by the on the flowers, perferring to sip nectar while \ 1 \) (j()] Bohart el a!.: Biofol!,y of Ilelcrostylum rolms/11111 -127 ( ( FIG. 2.- T eneral male (left) and female (r ight) of lfelcroslylum tobus/um. hovering directly in front of them. T he habit .\Ot; I.TS of ho\-cri n~ \\"hile taking nectar has als 1 been seen T he adult (fig. I) is conspicuous and fasc i nat.in~ in Bo mbylius but not in . lnllirax. Exoprosopa, as it pcrsistemly hovers and dips 0\·er alkali be and other short-tongued genera of Bomhyliidae nests ''ilh its \\ings producing a continuous high (Clausen 19-W). pitched whine. It is a large, robust species (fig. 2) 4. 40 \ Fires ~....25 '<l~ 2o # I' \ \ ' ' ' ,r--;--,<'-.1 _ :,_ I ' '------------- IS 20 25 .JO I 10 ~----------------------,June ----------------------~-~-------,Ju~ --------~ Em11rgMce S60son F IG. 3.-Emcrgence of Heterostylum rabtts/um and .Vomw mcla11dcri in Oregon. -!2 Annals of the Eutomological Society of America l \·ol. 53 ( with unclouded wings, tan thora", a gray, buff, On warm days emergence begins about a.m. and black pattern on t he abdomen, and a long and continues until about. l p.m., with the peak proboscis. Wit h age it loses much of its ab­ occurring bet'l\·een 9 and 10. After hardening, dominal hair and becomes nearh· uniformh· dark the still feeble adul ts flv to a nectar source and do gray. The sexes are similar in appearance -except not begin ovipositing -over the nest for at least. that t he males arc holoptic and the females 2 da,·s.