Ecology (1974) 55: pp. 1096-1103

COEVOLUTION OF SOME SEED (COLEOPTERA: BRUCHIDAE) AND THEIR HOSTSl

TED D. CENTER2 AND CLARENCE D. JOHNSON Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86001

Abstract. Original observations and a review of the literature revealed many adaptations evolved i!l se.ed beetles (Br~chidae) to circumvent the defenses of leguminous plants against seed-feeding . A~~lysls was based on 38 bruchid species and 44 species of host plant. In response to seed toxIcity the beetles either avoid the toxins or develop a resistance to them. !f t~e plant produces a greater number of seeds a is apt to feed on several seeds during Its hfe. Other defenses such as rapid dehiscence of fue pod, special characteristics of the husk synchronization of seed set, and special characteristics of the seeds are also circumvented b; the Bruchidae. This pattern of coevolution probably rarely allows the host plant to entirely exclude fue seed predators.

Key .wor~s: .Bruchidae; coevolution; feeding behavior; host plants; legume seeds; Legumi­ nosae; life histories; seed beetles; seed predators.

INTRODUCTION attachment. Characteristics of the seeds and pods The seed beetles (Bruchidae) are best known for were also examined. Our findings from 13 species their habits of attacking the seeds of cultivated of bruchids were compared with similar information legumes. Several species, however, feed in the seeds published on other species of Bruchidae (Table 1). of wild Leguminosae and about 30 other families of In all, we compared traits from 38 species of bruchids plants. Females oviposit on fruits (pods) or seeds. and 44 species of host plants. The larvae feed inside a seed or seeds, and then RESULTS AND DISCUSSION pupate inside one seed or in a protected place. These insects are, for the most part, highly host-specific. Our study showed that (1) the substrate on which In several recent papers Janzen (1969, 1971a, b, oviposition occurred was more often the pod than e, 1972) discussed some possible evolutionary re­ the seed; (2) although about two-thirds of the sponses of plants to seed predation by seed beetles. bruchids fed hidden inside a single seed then pupated Janzen's emphasis was on the development of pro­ there, about one-third consumed one seed almost tective devices by the host plants but he stated completely or fed on several seeds, then pupated out­ (1969: 18) that "virtually all [defensive traits] seem side of a seed; (3) 75% of the host plants had in­ effective against at least one species of bruchid but dehiscent or tardily dehiscent pods and 25 % had only rarely against all bruchids." This phenomenon dehiscent pods; and (4) 11 % of the plants had only is our concern here. We give examples of some one-seeded pods while the remainder had an average bruchid traits that have apparently evolved in of more than one seed per pod. Table 1 summarizes counteracting the defensive mechanisms of plants. these traits of the bruchids and their host plants.

METHODS Seed toxicity From 1968 to 1972 we collected 58 lots of seeds Janzen (1969) found that of the 36 species of of 19 species of plants from California, Arizona, and plants he studied, those that were free from bruchid northern Mexico, from which we reared 13 species infestation had toxic deterrents in their seeds. The of bruchids. We isolated a sampling of seeds from one plant he cited without toxins and not fed upon, each lot by placing individual seeds, pods, or seed Indigofera sutfrutieosa, has since been found to have clusters in gelatin capsules. After emergence of the its seeds eaten by bruchids (Table 1, No.6; Johnson adult bruchid we dissected the seed and pod to deter­ 1973). mine the location of the pupal chamber, the path Several species of toxic seeds are attacked by of the larva through the seed, the number of seeds bruchids. Trelease and Trelease (1937) and Johnson fed upon, the point of entry through the pod and (1970) cite collectively numerous examples of spe­ seed coat, the oviposition site, and the mode of egg cies of Astragalus which have poisonous seeds and are fed upon quite commonly by several species of the seed beetle genus Aeanthoseelides. Seeds of 1 Manuscript received February 22, 1973; accepted December 3, 1973. Present address: Department of En­ species in the genera Erythrina, Abrus, Dioclea, and tomology, University of Florida, Gainesville, FL 32611. Sarothamnus which are known to contain toxins are Late Summer 1974 COEVOLUTION OF SEED BEETLES AND HOSTS 1097

TABLE 1. Traits of some Bruchidae and their host plants

Oviposition Feeding Pupation Pods Seeds

'" ,., ,.,'g -a 0- 'il'" 0 9' ~i 'g .... ~ 0 S':l ~ 'tI 'g 0 8~ §'" 'tI § Q, 'tI

TABLE 1. ( Continued)

Oviposition Feeding Pupation Pods Seeds

>.'§ b ...... "

also eaten by bruchids (Janzen 1971c). Rotenone in which occurs in pods of Amorpha fruticosa, acted as the seeds of Cracca virginiana has no ill effect upon both a stomach and contact insecticide. He deter- the larvae of Acanthoscelides obsoletus (Bridwell mined this substance to be much more abundant 1938) . in glands (or "pustules") scattered over the surface Brett (1946) reported that the toxin "amorpha," of the fruit. The seeds of the plant apparently con- Late Summer 1974 COEVOLUTION OF SEED BEETLES AND HOSTS 1099 tain little or no toxins. The one-seeded, indehiscent energy have a reduced chance of survival even if fruits of A. fruticosa are used as food by at least they do escape. three species of bruchids. We found that one of The problem for the bruchids, then, was to counter these species, Acanthoscelides submuticus (Table 1, this increase in seed number, gradual reduction in No.9), oviposited into the space between the calyx seed size, and rapidity of dispersal with an evolu­ and the pod surface. After eclosion the larvae then tionary strategy of their own. The bruchids feeding emigrated from under the calyx to glands on the in seeds of plants gradually evolving the strategy pod surface and entered the seed through these of producing more and smaller seeds must either (1) glands. Because these seeds are commonly infested feed on more seeds, (2) become smaller themselves, by bruchids it is apparent that the bruchids are in (3) move to a different host with seeds of a suitable some way circumventing "amorpha." Apparently food quality and quantity, or (4) become extinct. A. submuticus has developed a physiological means We think we have observed several kinds of be­ to resist this toxin because it does contact, and per­ havioral responses of alternatives (1) or (2) which haps consume, the toxin. the bruchids have probably evolved in response to Acanthoscelides collusus (Table 1, No.2) is known the strategy of predator satiation. to feed in the seeds of Amorpha fruticosa as well It is known that the size of many species of as in the two host plants that we studied, Parry ella bruchids depends on the size of the seed in which filifolia and Errazurizia rotundata. The three plants it develops (e.g., Acanthoscelides aureolus in Astrag­ are closely related and have similar pods with many alus spp. is usually much larger than when in the pustules (glands) on their surfaces. All three have smaller seeds of Lotus scoparius, Johnson 1970) . a similar aromatic odor and are reported to have Because of this plasticity, a reduction in seed size insecticidal properties (Kearney and Peebles 1960). does not preclude bruchid infestation. Sennius mo­ It seems probable that P. filifolia and E. rotundata rosus normally feeds on a seed cluster. When only concentrate toxins in glands on their fruits as does one seed is available it still develops fully but does Amorpha fruticosa. The eggs of Acanthoscelides not attain normal adult size. Whether these smaller coli usus are glued directly to the pod surface or adults are as fecund and able to compete as the larger wedged between the calyx and pod surface. We individuals is a matter for further investigation. The found that first-instar larvae of A. collusus, unlike reduction of adult size in response to food quantity A. submuticus, enter through the pod surfaces rather may not be an evolutionary response but this ability than through glands of P. filifolia and E. rotundata. certainly enables the species to survive a reduction We believe this to be a different kind of evolved in seed size (Acanthoscelides guazumae, Sennius mechanism: avoidance of the toxin. Because 62% medialis, Table 1, Nos. 5, 31). of the species in Table 1 oviposited on pods rather Probably a more common bruchid evolutionary than seeds, more deterrents to the bruchids might reaction to the strategy of predator satiation is that be expected in the pod walls than in the seeds; in one larva utilizes almost the entire seed instead of this case, however, the bruchids seem to have cir­ only a small portion (e.g., Acanthoscelides compres­ cumvented the pod toxins. sicornis) or that one larva feeds on several seeds. All of these examples we interpret as a specific Of the species studied 34% were in one of these bruchid's having evolved a resistance to a specific categories (Table 1). toxin; that is, each bruchid would probably be sus­ Most bruchid larvae that feed on only one seed ceptible to toxins found in seeds of plants other than pass through the seed coat upon entry and exit from its hosts. the seed leaving a large portion intact. An Acan­ thoscelides compressicornis (Table 1, No.3) larva, Predator satiation however, devours only one seed almost completely, Janzen (1969) suggested that the plants had leaving only bits of the seed coat. The larva then evolved another strategy, which he termed "predator spins a fragile cocoon amongst the remnants of the satiation"; it involves a set of adaptations. By pro­ seed it devoured. The larvae of four of the species we studied fed ducing smaller seeds the plant is able to produce a upon more than one seed during their development greater number of seeds. Hence, seed size and num­ (Acanthoscelides fraterculus, A. lobatus, Sennius ber are complementary phenomena. Synchronization morosus, and S. simulans) and this behavior has of seed set is another adaptation that may be con­ been reported by several other authors in other sidered a part of this strategy. The production of species of Bruchidae (Table 1, Nos. 4, 7, 32, 33). many small seeds allows them to be dispersed rapidly If a larva feeds on several seeds then it must before total infestation can occur. Seed mortality is build a pupal chamber outside of a single seed. very high and the small seeds with little reserves of Caryedon gonagra (Davey 1958) and Acanthoscelides 1100 TED D. CENTER AND CLARENCE D. JOHNSON Ecology, Vol. 55, No.5 TABLE 2. Traits of Leguminosae that may be functional in eliminating or lowering bruchid destruction of seeds, and a summary of some corresponding adaptations of bruchids to circumvent these plant defenses

Plant defense (after Janzen 1969) Bruchid adaptation 1. Gum production by seed pods following penetra­ A period of quiescence in embryonic development in tion of first larva from egg mass; this may push off the egg until seed maturation is completed (Prevett remaining eggs (Bridwell 1920a, Prosopis juliflora) 1966, Caryedon albonotatum). Resistance to these or drown or otherwise obstruct young larvae (Hinckley gummy fluids (Acanthoscelides submuticus, Alga­ 1960) (Acacia, Pithecolobium, Cassia). robius prosopis). Eggs laid singly instead of in clusters (many species). 2. Dehiscence (Leucaena), fragmentation (Mimosa) Oviposition on seeds only after they have been scat­ or "explosion" (Canavalia) of pods, scattering the tered (Caryobruchus buscki, Pachymerus sp., Janzen seeds to escape from larvae coming through the pod 1971a; Acanthoscelides obtectus, Callosobruchus mac­ walls and from ovipositing females (Cushman 1911, ulatus, Larson and Fisher 1938; Stator pygidialis). Bridwell 1918). Attachment of seeds to one another or to the pod valve (Merobruchus julian us, Johnson 1968; Sennius morosus). 3. Production of a pod free of surface cracks Oviposition into the soft flshy exocarp (Acantho­ (Bruch us prosopis oviposits only in cracks on Prosopis scelides compressicornis). Chewing holes in the pod juliflora and cannot glue its eggs to smooth surfaces walls by the female in which to oviposit (Acantho­ as do many other Bruchidae, Bridwell 1918). scelides obtectus, Pygiopachymerus lin eo la, Janzen 4. Indehiscent pods, excluding those species that 1972). Gluing the eggs to the pod surface (Bruch us, deposit only on exposed seeds (Stator pruininus on Mimosestes, Caryedon). Prosopis juliflora when pods opened artificially, Brid­ well 1918). 5. A layer of material on the seed surface that Attachment of the eggs by anchoring strands to allow swells when the pod opens and detaches the attached for substrate expansion (Merobruchus julianus, Foris­ eggs (Bauhinia monandra, Bridwell 1918). ter and Johnson 1970; Sennius morosus, S. simulans). Entry of the larva through the pod into the seed before the pod opens (Bridwell 1918). 6. Poisonous or hallucinogenic compounds such as Mechanisms for avoidance and detoxication. Loss of alkaloids, saponins, pentose sugars, and free amino inhibitable endopeptidases (Applebaum 1964). acids (Bridwell 1918, Farnsworth 1968, Howe and Currie 1964, Applebaum 1964, Applebaum et al. 1965, Ishii and Urushibara 1951, Bell, pers. comm.). 7. Rich endopeptidose inhibitors, making digestion of the bean by the bruchid very difficult (Applebaum 1964 ). 8. Flaking of seed pod surface which may remove Oviposition beneath the flaking exocarp (Acantho­ eggs laid on that surface (several swollen-thorn scelides fraterculus). Rapid embryonic development acacias) . or feeding on immature seeds to enter the pod before flaking occurs. Oviposition under the calyx away from the pod surface (Acanthoscelides col/usus, A. submuticus). 9. Immature seeds remaining very small throughout Entry and feeding upon immature seeds (Bruchus, the year and then abruptly growing to maturity just Algarobius, Mimosestes). Delay of embryonic ma­ before being dispersed (A cacia). turation until seeds are mature (Caryedon albonota­ tum, Prevett 1966). 10. Seeds so thin that bruchids cannot mature in Utilization of the seed contents more completely them (Cassia siamea, Bridwell 1918). (Acanthoscelides compressicornis). Feeding on sev­ 11. Seeds so small that bruchids cannot mature in eral seeds (several species). them (many wild herbaceous legumes).

fraterculus (Table 1, Nos. 24, 4) are known to leave behavior has been recorded in some species of the pod to spin a cocoon. More often, however, in Bruchidius (Prevett 1967a) and Merobruchus (John­ most species that consume several seeds, the cocoon son 1968). Possibly these species have responded is constructed inside the pod. Apparently the re­ to an increase in seed number and a reduction in duction in seed size has affected both the larval seed size by evolving this method of pupation rather behavior and pupational behavior. than spinning a cocoon outside seeds. We have noted a unique behavior that includes Other seed-protecting traits feeding on several seeds but appears also to be a response to rapid dehiscence of seed pods. Sennius Thirty-one "traits of Leguminosae that may be morosus (Table 1, No. 32) larvae glue several seeds functional in eliminating or lowering bruchid de­ to each other and to the pod as they feed upon struction of seeds" were listed by Janzen (1969, them. Unattached seeds fall from the dehiscent pod Table 2). We have discovered in our own studies, while those glued together remain in the pod and and in the literature, countermechanisms that the serve as a feeding and pupal chamber. A similar bruchids have apparently evolved in response to at Late Summer 1974 COEVOLUTION OF SEED BEETLES AND HOSTS 1101 least 11 of these traits (Table 2). Janzen's protective found that Sennius morosus (Table 1, No. 32) also traits are for the most part found in a limited num­ produced eggs with anchoring strands on green pods ber of species of plants. We will discuss mostly of Cassia bauhinioides, as does S. simulans (Table 1, countermechanisms evolved by bruchids that exist No. 33) on the green pods of C. leptadenia. Ap­ in plant species other than those cited by Janzen. parently an egg with anchoring strands will adhere Thus, protective traits are apparently functional in better to a green, still-growing pod. Also if the eggs some plants but have been overcome by bruchids in were laid on the pods and the larvae entered the others. seeds before the pod dehisced, this plant protective A countermechanism to trait 1 (Table 2) may be device would not work (Bridwell 1918). that which Prevett (1966) reported concerning the Mechanisms of bruchid avoidance and detoxication eggs of Caryedon albonotatum (Pic). It oviposited of trait 6 were mentioned earlier. Applebaum [1964] on the pods of Acacia nilotica, which has seeds sur­ states that the presence of endopeptidase inhibtors rounded "by a gummy fluid." The eggs do not in many leguminous seeds "could then have been mature for some 2 mo after oviposition. Apparently followed in the Bruchidae by the subsequent loss embryonic development is followed by a period of of inhibitable endopeptidases, obligating them to quiescence. At the end of the 2-mo period "the such seeds which contain either a sufficiently high seeds have enlarged so that, although there is still level of initially available peptides or, at the least, fluid at the sides of the pod, there is probably little containing prot eases capable of adding appreciably danger to a larva boring straight through to a seed to this level of integrating with the residual protease from the region of the septum." Our earlier report activity of the Bruchids." This could be an evolu­ of the entry of Acanthoscelides submuticus into pods tionary response by the bruchid to trait 7. through a gummy material in the surface glands is Apparently in response to trait 8, Acanthoscelides another example. Larvae of Algarobius prosopis are fraterculus (Table 1, No.4) took advantage of the bathed in fluid in Prosopis pods and survive (Brid­ cracks caused by flaking of the exocarp of pods of well 1920b). Lotus crassifolius by ovipositing beneath them. One method that bruchids may use to overcome Species of bruchids that have a very rapid embryonic trait 2 is to oviposit on seeds after they have been development could enter the pod before flaking oc­ scattered. Nuts of Scheelea rostrata (Palmae) are curred. Acanthoscelides collusus and A. submuticus oviposited upon by Caryobruchus buscki and Pachy­ (Table 1, Nos. 2, 9) are known to oviposit beneath merus sp. after they have fallen to the ground and the calyx, well away from the surface of the pod. are cleaned of pulp by rodents (Janzen 1971a, Wilson All are possible devices to overcome this strategy of and Janzen 1972). Beans and cowpeas are oviposited the plant. upon while on the ground by Acanthoscelides ob­ Trait 9 is probably countered by species of tectus and Callosobruchus maculatus (Table 1, Nos. Bruchus, Algarobius, and Mimosestes which enter 8, 20; Larson and Fisher 1938). We have found and feed upon immature seeds. The previously cited that Stator pygidialis (Table 1, No. 35) oviposits case of Caryedon albonotatum (Prevett 1966) whose on seeds of Calliandra humilis only after they have first-instar larva undergoes an apparent quiescent fallen to the ground. If the pods are more tardily period for some 2 mo before leaving the egg could dehiscent, then the attaching of several seeds to one also be functional in attacking seeds that abruptly another and to the pod valve by bruchids can keep grow to maturity. On the other hand, the plant the seeds from being scattered (i.e., by Sennius Amorpha fruticosa may be favored by delayed seed morosus and Merobruchus julianus; Table 1, Nos. maturation. Seeds are present on the plant through­ 32, 26). out most of the summer but do not mature until In response to traits 3 and 4, Acanthoscelides com­ mid-August. Acanthoscelides submuticus (Table 1, pressicornis (Table 1, No.3) is thought to oviposit No.9) feeds only in the mature seeds of this plant; into the soft, fleshy exocarp of Desmanthus spp. therefore, by the time the first generation of bruchids Females of A. obtectus are known to chew holes in emerges it is apparently too late in the season for fleshy pods and then oviposit into them, as does reinfestation, since no second generation appears. Pygiopachymerus lineola (Table 1, No. 28) on pods Perhaps in this way the plant has evolved a trait of Cassia grandis (Janzen 1972). There are numer­ that prevents reinfestation of its seeds and allows a ous examples of species (Bruchus, Mimosestes, Carye­ maximum period of time for their dispersal. don) that glue eggs to fleshy pods. All are counter­ In response to traits 10 and 11 we cite the very mechanisms apparently evolved by bruchids. flat and thin seeds of Hoffmanseggia drepanocarpa. Trait 5 is possibly countered by eggs laid with Acanthoscelides compressicornis (Table 1, No.3) radiating anchoring strands (Merobruchus julianus, devours almost the entire seed of this plant. The Table I,No. 26; Forister and Johnson 1970). We shape seems to have little or no deterrent effect. We 1102 TED D. CENTER AND CLARENCE D. JOHNSON Ecology, Vol. 55, No.5 have already cited elsewhere in this paper (and in when large numbers of eggs are oviposited on single Table 1) numerous examples of bruchids that feed seeds, thereby satiating the female bruchid. For on several seeds during the course of their develop­ example, many individuals of Caryedes brasiliensis ment. (Table 1, No. 21) often emerge from only one of A defensive trait cited by Wessling (1958), Mar­ two seeds in a pod, or two of three seeds, etc. (D. H. shall and Jain (1970), and Janzen (1971c) was that Janzen, pers. comm.). of hairiness of husk. We found that heavy pubes­ Protective devices in seeds and pods.-Of the 31 cence sometimes may prevent the first-instar larva protective traits for seeds (Janzen 1969: 19) only 7 from reaching the pod surface (e.g., Astragalus are concerned with protective devices in the pods newberryi pods are densely pubescent and are not (numbers 1, 2, 5-8, 14) and the remaining 24 have infested by bruchids). However, Astragalus mollis­ to do with seeds. Because about 75% of plants we simus has a pubescent pod but Acanthoscelides fra­ studied had indehiscent or tardily dehiscent pods it terculus (Table 1, No.4) successfully counters this seems logical that most oviposition-over 62 % of by inserting the ovipositor between the hairs and the species we studied-is on the pods. It also seems placing the egg as near as possible to the pod sur­ logical that most of the protective devices would face. The larva then 'crawls to the pod surface and be associated with the pod, but this is not the case, if enters. Janzen's citations are indicative of most traits for These defense mechanisms of the plant and the protecting seeds. corresponding adaptations of the seed predator rep­ A possible explanation is that, since the seeds are resent a dynamic balance. The plant probably very the most important components of a fruit, protective seldom completely eliminates the seed predators traits have evolved in them rather than in the husk. (Janzen 1969). By the same token, the seed preda­ A more likely explanation is that we have insufficient tors are seldom able to utilize the total seed crop. information at this time about the number of husk The defensive mechanisms developed by the plant protective mechanisms. reduce the effect of the infestation to insure the ACKNOWLEDGMENTS survival of the plant species, which in turn permits the survival of the bruchid. We wish to thank the following persons for construc­ tive criticism of this paper: R. P. Balda, D. H. Janzen, Unexplained phenomena J. M. Kingsolver, G. S. Pfaffenberger, P. F. Prevett, C. N. Slobodchikoff, B. J. Southgate, S. R. Swier, A. L. Pods with a single seed.-Eleven percent of the Teran, T. J. Walker. Partial support for this research plants in Table 1 have fruits with only one seed. was obtained from Grants 12-14-100-9187 (33) and 12-14-100-9970 (33) from the Entomology Research All are of sufficient size for one small bruchid to Division, U.S. Department of Agriculture. develop inside them, but only Errazurizia rotundata seeds are large enough for development of several LITERATURE CITED bruchids. Some of the fruits have toxic glands on Applebaum, S. W. 1964. Physiological aspects of host their surfaces but all are fed upon by bruchids. specificity in the Bruchidae-l. General considerations of developmental compatibility. J. Physiol. 10: Conceivably the one-seeded pods represent a tran­ 783-788. sitional stage to the ultimate mechanism to exclude Applebaum, S. W., B. Gestetner, and Y. Birk. 1965. seed predators: very small one-seeded pods dispersed Physiological aspects of host specificity in the Bruchi­ over a plant. With seeds so small that one bruchid dae. IV. Developmental incompatibility of soybeans could not develop in one seed, the larva would have for Callosobruchus. J. Insect Physiol. 11:611-616. Bondar, G. 1931a. Notas biologicas sobre alguns to migrate from one pod to another, a phenomenon Bruchideos brasileiros do genero Pseudopachymerus. not yet observed in the Bruchidae. Rev. Entomol. 1:417-422. Large nontoxic seeds.-Large seeds often harbor 1931c. Notas biologicas sobre Bruchideos several bruchid larvae at the same time (Acantho­ brasileiros do genero Spermophagus. 0 Campo 11: 86-88. scelides obtectus, species of Stator, Caryedes, Pygio­ 1937. Notas biologicas sobre Bruchideos ob­ pachymerus, Specularius, Callosobruchus and Za­ servados no Brasil. Rio de Janeiro Instituto de Bio­ brotes; Table 1). Although some of these large seeds logia Vegetal. Arquivos 3:7-44. contain toxins and are fed upon (e.g., by Specularius Brett, C. H. 1946. Insecticidal properties of the indigo­ erythrinae in Erythrina), most are not known to bush (Amorpha fruticosa). J. Agric. Res. 73:81-96. Bridwell, J. C. 1918. Notes on the Bruchidae and contain toxins. Perhaps predator satiation is func­ their parasites in the Hawaiian Islands. Proc. Hawaii tioning here but it seems to us that the production Entomol. Soc. 3:465-505. of large seeds that could be destroyed by one small 1920a. Notes on the Bruchidae (Coleoptera) bruchid larva would be an excellent example of prey and their parasites in the Hawaiian Islands. 3rd pap. Proc. Hawaii Entomol. Soc. 4:403-409. foolhardiness. Perhaps if this is a seed escape mech­ 1920b. 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