Bianchi, F.A.

Proceedings of the Hawaiian Entomological Society

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245Notes on the, Abundance of the* Spiders Latrodectus mactans,L. geometricus and Argiope avara, and of their Parasiteson the Island of HawaiiBy FRED A. BIANCHI(Presented at the meeting of August 14,1944)Latrodectus mactans (Fabr.) and L. geometricus KochIn June or July of 1939 reports reached entomologists of theExperiment Station, H.S.P.A. of an unusually heavy infestation ofL.inactans at Morse Field, South Point, Hawaii. With the doubleobject of verifying the reports and of liberating a large colony ofBaeus californicus Pierce, the then recently introduced egg parasiteof the spider, I visited Morse Field on August 22, 1939, and foundthat actual conditions had not been overstated in the reports. Spiderswere in fact excessively abundant in the vicinity of the barracks,and at least one individual, usually more, could be found underevery stone that was turned over, and within almost every clump ofgrass thick enough to provide a modicum* of protection. The barracks themselves were festooned, inside and out, with numerouswebs, and it was not difficult to understand that their annoyance tothe troops had long become more than merely psychological. Inshort, South Point showed during August 1939 the thickest infestation of L. mactans that I have ever seen or read of, and provided anideal site for the liberation of the parasites I had with me.The liberation was made. The parasites were spread out over anarea of about half an acre in the vicinity of what were then themodest barracks of a small garrison. For one reason or another,mostly on account of the relative inaccessibility of South Point, nofurther observation was made again until June 23, 1944.On that date I again visited South Point and found greatlychanged conditions. As it turns out, however, they are not theentirely satisfactory conditions nor the exact changes that I expected. While the population of L. mactans has been greatlyreduced so that spiders and webs of this species can be found onlywith difficulty, Latrodectus geometricus, a closely related speciesnot heretofore seen at South Point, has appeared upon the scene,and now completely fills the niche formerly occupied by mactans.L. mactans can still be found, a scattered spider and a few eggmasses here and there under a stone or plank, but not a single spidernor an egg mass of the species is to be seen in any of the manybuildings which now constitute the greatly expanded barracks. Allthe numerous spiders which at present enjoy the protection ofUncle Sam's roofs are of the species geometricus.Proc. Haw. Ent. Sbc, Vol. XII, No. 2, June, 1945.246Following the liberation of a parasite (Baeus) which is known toattack the eggs of L. moderns only, this strikingly complete supersession of one species by the other naturally suggests great efficiencyon the part of the parasite; but unfortunately, other circumstancesof the case contradict this suggestion. In the first place, neitherBaeus itself nor any evidence of its presence—other than the circumstantial evidence provided by the observed reduction of L.mactans—have been found in the South Point area. In the secondplace, even if the parasite were definitely known to be established inthe area, it would not necessarily follow that its work had caused thesuppression of mactans by geometricus. The same result mightobtain from a different cause, i.e.: better adaptation of geometricusto the local environment. This, in fact, I believe to be the real cause.Assumption that it is the real cause agrees with previous observations—all of a more cursory nature than the present one—whichhave been made from time to time by myself and my colleagues onthe relative survival potential of the two species concerned. And itagrees also with the fact that L. geometricus was not found in SouthPoint at the time of my first visit in 1939. The species, I believe,had not found its way into the area at that time. When it later didso, some time between 1939 and 1944, it easily displaced L. mactans,and would have done so regardless of the presence or absence ofBaeus.Argiope avara ThorrelUp to some 14 or 15 years ago Argiope avara was one of the mostcommon and conspicuous invertebrates of our fields. In cane fieldsparticularly, where the strong webs sometimes made uncomfortablethe transit of men and animals, the spider was not an unmixedblessing. Then, following the establishment early in the last decadeof Tromatobia rufopectus (Cresson), a self-introduced hymenop-terous parasite, the spider practically disappeared until some fouror five years ago, since when an upward trend in the population hasagain become evident.It has been surmised that this reversal in population trend wasdue to the work of a hyperparasite of Tromatobia, But although thehyperparasite itself, a spalangid, Pleurotropis mlderi (Howard),has been known a long time and has been reported more than oncein circumstances that clearly indicated the nature of its role, nodirect evidence has been turned up heretofore on the actual importance of that role.It is this evidence which I luckily ran into on June 29 of thecurrent year, in one of the thickets of cactus plants which are to befound on the slopes immediately north of the town of Kamuela,Hawaii. It consisted, first, of hundreds of webs and egg cases ofArgiope avara, a concentration greater than I had ever seen before,even previous to the establishment of Tromatobia; second, of the247corroborative evidence provided by the dissection of a representative lot selected from the great number of egg masses available. Thelarge number of egg masses shows a very definite reduction in theefficiency of -Troinatobia rufopectus, the primary parasite of thespider. The results of dissecting the representative lot of eggmasses indicates that the reduced efficiency of Tromatobia must bedue to the work of the hyperparasite.The quantitative results of the dissections follow:Total number of egg masses examined 55Egg masses in which hyperparasites had developed or intowhich adults had crawled 55Egg masses in which all primaries had been destroyedby the hyperparasite 46Egg masses in which the hyperparasites had destroyed allbut one or two of the primaries 5Egg masses in which no primaries had been destroyedby the hyperparasite „ 1Egg masses in which no indication of the actual or formerpresence of the primary parasite was found 2Egg masses in which no primaries were found but in whicheggs had been destroyed by some other cause (desiccation?) 1

Notes on the Abundance of the Spiders Latrodectus mactans, L. geometricus and Argiope avara, and of their Parasites on the Island of Hawaii

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Notes on the Abundance of the Spiders Latrodectus mactans, L. geometricus and Argiope avara, and of their Parasites on the Island of Hawaii

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