Proc. Nati. Acad. Sci. USA Vol. 88, pp. 8194-8197, September 1991 Ecology Chemical attraction of kleptoparasitic flies to heteropteran insects caught by orb-weaving spiders* (miichild flies/Hemiptera/defensive secretion/rans-2-hexenal/kairomone) THOMAS EISNERtt, MARIA EISNERt, AND MARK DEYRUP§ tSection of Neurobiology and Behavior, Mudd Hall, Cornell University, Ithaca, NY 14853-2702; and §Archbold Biological Station, P.O. Box 2057, Lake Placid, FL 33852 Contributed by Thomas Eisner, June 24, 1991 ABSTRACT Insects of the heteropteran families Pen- MATERIALS AND METHODS tatomidae (stink bugs) and Coreidae (squash bugs), when being was Laboratory observa- eaten by the orb-weaving spider Nephila attract flies The study done in January 1989. clavipes, were at Archbold Biological Station, Lake the Milichiidae. The flies on the bugs and, tions made the of family aggregate Florida. Tests with the spiders were at Highlands in the spider's meal. Stink bugs and Placid, as kleptoparasites, share Hammock State Park, Sebring, Florida, a preserved site squash bugs typically eject defensive sprays when attacked; live oak and cabbage palm, where Nephila only densely shaded by they do so when caught by Nephila, but the spray webs were abundant. m mally affects the spider. Evidence is presented indictiug Feeding of Nephila. Presentation of prey to the spiders that it is the spray of the bugs that attracts milichilds to the involved flipping individual insects from vials directly into spider's catch. the web and monitoring the course of events. Over the years this technique has been used by one of us (T.E.) to test for Spiders are slow eaters. Unable to swallow solids, they the acceptability of dozens of insect species to Nephila. triturate their prey,extraorally, drench it in disgorged diges- The feeding behavior of Nephila is known (4). Typically, tive fluid, and suck up its liquified contents. The process may the spider darts toward the entrapped insect, bites it with the take hours, even.;with prey substantially smaller than the chelicers to inject venom, envelops it lightly in silk, carries it spider. In nature, prey lengthily exposed is often secondarily to the hub of the web, envelops it in more silk, suspends it exploited by kle$t parasites. Vis a vis spider prey, such from the hub, and eats it. Consumption is by suctorial uptake usurpers include both other spiders and insects. Chiefamong of fluid from the ground-up predigested prey and may take the latter are certain tiny flies of the families Cecidomiidae, several hours, even with prey only a third the mass of Phoridae, Chloropidae, and Milichiidae, which have long Nephila. been known as reglar visitors to spider prey (1, 2). There is In the tests that follow, special care was taken to insure that the heteropterans did not eject spray before being offered to good evidence, in the case of cecidomiids, that the flies are collected in the field, they attracted to the prey, possibly by factors contrib- the Nephila. To this end, when chemically were coaxed gently into vials without being directly grasped. the or liberated in the course of the uted by spider prey's Nephila were given single prey presentations and not to digestion (1). Olfaitory stimuli were also suspected be reused in any tests. involved in milichiid and chloropid attraction (1-3), but the Acceptability of a Heteropteran to NephUa. Thirteen indi- most important evidence to date was an unpublished presen- viduals (nine females and four males) of a single species of tation by P. L. Mitchell, F. L. Mitchell, and J. R. Aldrich, Pentatomidae, Nezara viridula, were offered to Nephila, to reporting attraction of milichiids to insects of the het- observe in some detail, with a representative heteropteran, eropteran familiesoPentatomidae (stink bugs) and Coreidae how the spider subdues such chemically defended prey. (squash bugs).¶ The6se bugs spray defensive chemicals when Attraction of Milichiids to Heteroptera Caughit by Nephia. disturbed, and it had been noted that when they were induced An assortment of 38 bugs (three species of Pentatomidae and to spray, they attracted milichiids. We here present data that three of Coreidae) was fed to Nephila; whether or not attempts to place, this peculiar phenomenon in an adaptive milichiids alighted on the individual prey items when these context. Specifically, we demonstrate that milichiids are were being consumed by the spiders was recorded. drawn to heteropterans when the latter are being consumed For nine N. viridula (all females), the density of milichiid by orb-weaving spiders and that the spray of the bugs, assemblage was recorded as a function of time since prey emitted in futile efforts to repel the spiders, attracts the flies. presentation to the spider. The Nezara were offered one after Our experiments were done in Florida with a single species another to their respective spiders, and the webs were of orb-weaving spider, the large, widely distributed Nephila inspected thereafter to count the numbers offlies alighted on clavipes, and an assortment of stink bugs and squash bugs. the prey. Times of inspection fell within the following inter- The milichiids attracted in our tests were of several species. vals: 0-/3 hr, Y/3-l hr, 1-2 hr, 2-4'2 hr. At 41/2 hr, all spiders We show that (i) Nephila can cope with heteropterans despite had finished their meal. Time 0 was the time of the spider's the spray these eject when attacked, (it) heteropterans being first contact with the prey. eaten by Nephila attract milichiids, (iii) Nephila prey ordi- To obtain milichiids for identification, five N. viridula and narily unattractive to milichiids can be rendered attractive by two Coreidae (one Acanthocephala confraterna and one baiting it with components of heteropteran spray, and (iv) a unidentified Acanthocephala) that had been fed to Nephila chemical component of heteropteran spray can itself attract milichiids. *This is paper no. 106 in the series Defense Mechanisms ofArthro- pods. Paper no. 105 is ref. 20. fTo whom reprint requests should be addressed. The publication costs:,t&his article were defrayed in part by page charge IMitchell, P. L., Mitchell, F. L. & Aldrich, J. R., Annual Meeting of payment. This article must therefore be hereby marked "advertisement" the Entomological Society of America, December 9, 1985, Holly- in accordance with 18 U.S.C. §1734 solely to indicate this fact. wood, FL. 8194 Downloaded by guest on October 7, 2021 Ecology: Eisner et al. Proc. Natl. Acad. Sci. USA 88 (1991) 8195 were plucked away from the spiders once milichiids had Table 1. Species of Heteroptera fed to Nephila, number of assembled on the bugs and were abruptly transferred to vials individuals of each tested, and portion of this number that with ethanol, capturing a substantial fraction of the aggre- attracted milichiid flies gated flies. Heteropteran prey Offered, no. Attractive, no. Attraction ofMilichiids to Chemically Baited Prey. Previous work had revealed that a moth in culture in our laboratories, (P) N. viridula 22 19 the arctiid Utetheisa ornatrix, is highly acceptable to (P) Piezodorus guildiini 4 2 Nephila, when it has been raised on an appropriate larval (P) Euschistus sp. 6 2 diet. Raised on its natural food plant ofthe genus Crotalaria, (C) Acanthocephala confraterna 2 2 which contains toxic pyrrolizidine alkaloids, the moth is (C) Chelinidea vittiger 3 1 rejected by the spiders, but it is eaten when its diet is an (C) Acanthocephala sp. 1 1 alkaloid-free semisynthetic formulation based on pinto beans Families are Pentatomidae (P) or Coreidae (C). (5). We used Utetheisa moths raised on our alkaloid-free pinto female) did so before being bitten, by working itselfout ofthe bean diet (5) as Nephila prey to test whether addition of web in 4 min, while the spider was preening after being chemical components from stink bug spray would render sprayed. Nezara that were eaten were reduced to small such items attractive to milichiids. pellets of compacted remains, as is typical for spider prey. The two chemicals used as additives, trans-2-hexenal and Consumption times for the 12 eaten Nezara ranged from 1Y/3 hexanal, had been characterized as major components of the to >4 hr. spray of a substantial number of Pentatomidae and Coreidae Attraction of Milichiids to Heteropteran Prey. Table 1 lists (6, 7). the heteropterans fed to Nephila and the incidence of mili- We offered three categories of Utetheisa moths to Nephila: chiid visitation to these offerings. All species evidently (i) baited with 4 A.l of trans-2-hexenal (n = 8), (it) baited with showed some attractiveness. The bugs all noticeably sprayed 4 ,ul of hexanal (n = 8), and (iii) unbaited controls (n = 6). (odor apparent) when attacked by the spiders, and none The moths had one costal vein cut when offered to the escaped. No milichiids were detected on the spiders before spiders to prevent escape after being flipped from their vials prey presentation, and none appeared on the prey until after into the webs. Addition of chemical to the treated moths was spraying. These results contrasted sharply with those ob- effected with calibrated micropipettes; the fluid was trickled tained over past years by one ofus (T.E., unpublished work) onto the body of the moths as soon as these had been in comparable tests, in which Nephila were offered dozens of transported to the hub of the web and the spiders had settled butterflies, moths, beetles, flies, cockroaches, winged ter- to commence their meal. The procedure did not deter the mites, earwigs, and mayflies. Although visitation of an oc- spiders. casional milichiid could have been missed in these tests, Density of milichiid assemblage over time was monitored convergence of flies in numbers such as occurred with stink as with the Nezara feedings in the preceding test by inspect- bugs was never witnessed.