P1. Syst. Evol. 163, 35-41 (1989) --Plant

Systematics and

Evolution © by Springer-Verlag 1989

Preferential infestation of fruits within the Medicago truncatula-M, littorMis (Leguminosae) complex by the bean weevil Bruchidius bimaculatus

ERNEST SMALL, L. P. LEFKOVITCH, and BRENDA BROOKES

Received February 10, 1987

Key words" Angiosperms, Leguminosae, Medicago truncatula, Medicago littoralis, Bruchidius bimaculatus. - Bruchid infestation.

Abstract" The bean weevil Bruchidius bimaculatus was found in fruits of 68 of 217 populations of the Medicago truncatula- M. littoralis complex of Israel, Greece, Italy and Spain. In- festation was higher in fruits of somewhat larger than mean size, and in the more pubescent and tightly coiled fruits. Bean weevil size proved to be independent of fruit size, so that the "preference" for larger fruits does not seem due to adaptation to a larger food source or a larger domicile. Fruit size has been used to delimit the host "species" M. truncatula and M. littoralis, but the character proved to be unimodal, and it is apparent that the taxonomy of the plant complex requires further clarification. Of the 11 472 fruits examined, less than 4% were infested. Given that the plants have evolved indehiscent legumes in which typically only one of the approximately six seeds survives to reproduce, and that very few seeds of a fruit are destroyed, the bruchid's infestation appears to be non-harmful, and indeed the association may be mutualistic.

Medicago truncatula GAERTN. and M. littoralis ROnDE ex LOIs.-DELONG. are names used to denote allegedly different annual species indigenous to most of the Medi- terranean Circle countries. Coastal plants of sandy seaside soils have usually been labelled M. littoralis while more inland plants have been called M. truncatula. In connection with a taxonomic study of the plants, mature fruits of 217 populations were collected in the Old World, and returned to Ottawa, Canada for study, where it was discovered that live bean weevils, Bruchidius bimaculatus (OLIVlER), had emerged from many of the fruits, remaining trapped in the collection envelopes. The mature fruits are exceptionally hard-walled and spiny, and are generally im- mune to insects; B. bimaculatus was the only noticeable herbivore we observed on fruits. This stimulated a study of possible relationships of the insect with fruit characters potentially of value to clarifying the taxonomy of the plants. Bruchidius bimaculatus has been recorded on Medicago spp. and on Vicia sativa L. in the Mediterranean Circle countries, Iran, the Canary Islands, and C. Europe (CALD- ERON 1962, DECELLE 1975, ZAMPETTI 1981). It may be noted that the plants are the source of many forage cultivars, and so the biology of this phytophagous insect is of economic interest.

36 E. SMALL & al.:

Materials and methods

Fruits were collected in 1985 as follows: Israel: 1974 fruits, 1 l0 populations, May 4-19 , throughout the country, including small samples (1 - 2 fruits) from the Hebrew University of Jerusalem Collection of Wild legumes of Israel. Greece: 875 fruits, 19 populations, May 2 0 - 29, SE. to NE. Italy: 3 685 fruits, 34 populations, May 29 - June 6, W-C. to S. Spain:

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Fig. 1. Scanning electron micrographs of spirally coiled, spiny fruits of Medicago littoral& (collection E. SMALL-Israel-97) infested by Bruchidius birnacuIatus. Above, intact fruit in which pupa has caused a characteristic, transparent surface swelling though which it will exit. Below, fruit split apart between two coils, revealing emergent bean weevil in its pupal case

Bruchid infestation of Medicago truncatula agg. 37

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38 E. SMALL • al.:

4943 fruits, 54 populations, June 7 - 15, central, SW, SE. For about half of the collections, it was possible to obtain decent herbarium specimens (for the others, plants were overly mature and shrivelled), but in any event herbarium vouchers grown from the seeds for a taxonomic study are also available, at DAO. Precise localities are given on these vouchers. Appendix 1 lists the collection numbers of all of the material used for this study. Numbers of fruits per population varied from 1 to 307, with a mean of 52.9. An attempt was made to collect at least 50 fruits at every site, but in many cases only a smaller number was available.

Numbers of insects were evaluated by counting those that had not emerged but were evident by a translucent appearance of the fruit wall (Fig. 1), and by adding those that had emerged; since several insects had escaped before the present study was conceived, the number that had emerged was deduced by reference to those trapped in the seed package and by the number of emergence holes in the fruit. No attempt was made to dissect fruits to check for undeveloped insects, but none was observed during dissection of several fruits of all populations in order to obtain seeds for planting.

Our primary objective was to determine if there are any relationships between infestation with some characters of the fruit. The six fruit characters explored, and the method of scoring them, are given in Table 1. Depending on availability, 1 - 3 fruits were evaluated to .obtain estimates of means of the characters for each collection. Because of the very different numbers of fruits collected at the total of 217 sites, it was necessary to weight all statistical analyses by the numbers of fruits in the collection. The first two characters listed in Table 1 are continuous characters allowing the relationships with the presence or absence of the beetles to be examined by analyses of variance. The last four characters listed were ordered state or meristic, which were analysed by fitting generalized linear models assuming Poisson errors (McCuLLAGH & NEI~DER 1983), with log-likelihood ratio tests.

Since the above analyses suggested that there was a relationship between the presence of adult bruchids and fruits of above average size, a secondary objective became to examine whether within the infested fruits there was a preference for larger living quarters or more food. Towards this end, an analysis of covariance relating body length (for the available adults) and fruit size was conducted. The country of origin was included in this analysis as an explanatory factor, since in different climatic conditions both fruit and insects may exhibit different sizes.

To illustrate the relationships, frequency curves were prepared for infestation (based on a total of 443 infested fruits), total fruits (based on a total of 11 472 fruits), and adult bruchid length (based on 376 insects) plotted against fruit diameter in consecutive classes of 0.5 ram.

Results and discussion

Table 1 gives the means of the six fruit characters examined for fruits with and fruits without bruchids. For three of the characters the means are significantly different: infested fruits tend to be larger, more pubescent, and with the coils more adpressed. The somewhat larger mean size of infested fruits as compared to fruits generally can also be appreciated by examination of Fig. 2.

Fruit size is an especially interesting character from the viewpoint of clarifying the taxonomy of the plants since it has been alleged to distinguish the two "species". According to HEYN (1963) the pods of M. littoralis are 3 - 7 mm in diameter, while those of M. truncatula are 7 - 12 mm. LESINS & LESINS (1979) give a more restricted fruit diameter range of the two species, respectively, as 3 . 5 - 5.0 and 4 . 5 - 7.0 ram. As is evident f rom Fig. 2, the curve for size classes is not bimodal, and fruits greater than 8 mm were not encountered. Clearly the "species" are not as distinct as alleged

Bruchid infestation of Medicago truncatula agg. 39

40

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RELATIVE % INFESTATION (n = 443)

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RE'AT,VE / . .ELAT,VE F.U,TS'N \ / / B.OO.,O' NGT.

10- SIZE CLASS "~ / (n = 376)

0 . . J , ~" ~ - 3.0-3.4 3.5-3.9 4.0-4.4 4.514.9 5.0'-5.4 5.5'-5.9 6.0'-6.4 6.5-6.9 7.0-7.4 7.5-8.0

FRUIT DIAMETER (turn)

Fig. 2. Relative percentage curves for all fruits, infested fruits, and adult Bruchidius bi- rnacuIatus length, plotted against fruit diameter in 0.5 mm size classes

Table 2. Analysis of covariance of adult Bruchidius birnaculatus length on fruit diameter and country of origin

Source of Degrees of Mean Variance Significance variation freedom square ratio

Country 3 5.571 1.55 not sig. Fruit diameter 1 0.201 0.06 not sig. Interaction 3 2.344 0.65 not sig. Residual 47 3.591

in the literature. Nevertheless, to the extent that larger fruit size is more indicative of M. truncatula than M. littoralis, the preference by Bruchidius bimaculatus for fruits of larger than mean size of the plant complex as a whole suggests a slight preference for the inland M. truncatula over the coastal M. littoralis.

The adult bruchid length ranged f rom 1.9 to 3.5, with a mean of 2.71 mm. As can be seen f rom Table 2, adult length appears not to be dependent on either fruit diameter or country of origin. The constancy of bruchid body length in fruits of different size can also be appreciated f rom Fig. 2. This clearly indicates that fruit size is not a limiting factor in the beetles' growth as larvae. This eliminates expla- nations for a preference by the bruchid for fruits of larger than average size based on quant i ty of food supply or volume of space for development.

The members of the Bruchidae generally oviposit on the mature fruits, a l though some do so on young fruits or on flowers (BoRRoR 1976). We have not ascertained when oviposi t ion occurs by B. bimaculatus, and therefore cannot distinguish whether the associations found between infestation and fruit characters are attr ibutable directly to the latter (if oviposi t ion occurs in fruits) or because of relationships with other aspects, of the plants (if oviposi t ion occurs in flowers). In any case, our

40 E. SMALL ~; al. :

I I 1cm

Fig. 3. Drawing of Medicago littoralis, based on collection PHITOS 8329 (DAO). Note at base of plant persistent fruit, within which one of the seeds gave rise to the plant

observations do not permit a conclusive explanation for the "preferences" of the female bruchids for relatively large, hairy, and tightly coiled fruits.

The plants in question belong to a group of Medicago which have fruits with very hard pericarps for protection (obviously not a successful defence against Bruchidius). However, this has been at a cost: the fruit walls are so hard that they imprison the seeds. Considerable rotting of the pericarp must occur, delaying seed germination (an advantage in some respects), and generally only one of the six or so seeds survives because the seedlings develop initially inside the pericarp and the first to emerge is predominant; accordingly, most of the seeds are "wasted". As can be seen from Fig. 3, one can frequently locate the maternal fruit persisting at the base of the plant which gave rise to it. If only one of the six seeds survives, it would seem that the mean fruit infestation of 3.7% is inconsequential, especially considering that the emergence holes would allow early germination of the unaf- fected seeds in the fruit, and so increase the adaptive variability of the propagules.

We thank Dr D. E. BRIGHT for identifying Bruchidius bimaculatus, and Dr C. C. HEYN and Mr A LISTON for assisting greatly in the acquisition of material from Israel. This paper is contribution 1-897 of the Engineering and Statistical Research Centre.

References

BORROI~, D. J., 1976: An introduction to the study of insects. 4th ed. - New York: Holt, Reinhardt & Winston.

CALDERON, M., 1962: The Bruchidae of Israel. - Riv. Parassitologia 23: 207-216. DECELLE, J., 1975: Les Bruchidae (Coleoptera) des Iles Canaries. - Bull. Ann. Soc. Roy.

Belge Ent. 111: 109- 142. HEYN, C. C., 1963: The annual species of Medicago. - Jerusalem: Magnes Press, The

Hebrew University.

Bruchid infestation of Medicago truncatula agg. 41

LEsrzqs, K., LESINS, I., 1979: Genus Medicago (Leguminosae). - The Hague, Boston, London: Dr W. Junk.

McCULLAGH, P., NELDER, J. A., 1983: Generalized linear models. - London: Chapman & Hall.

ZAMPETTI, M. F., 1981 : Contributo alla conoscenza dei Bruchidi di Turchia. 1. (Coleoptera, Bruchidae). - Fragm. Entomol. 16: 73-87.

Addresses of the authors: E. SMALL and B. BROOKES, Agriculture Canada, Biosyste- matics Research Centre, Central Experimental Farm, Ottawa, Canada, K1AOC6. - L. P. LEFKOVITCH, Agriculture Canada, Engineering and Statistical Research Centre, Central Experimental Farm, Ottawa, Canada, K1AOC6.

Appendix 1. Collections (vouchers at DAO)

Israel: with bruchids: 4, 8, 19, 23, 27, 30, 33, 34, 36B, 37, 38, 43, 44, 45, 46, 50, 51, 52, 56, 73, 84, 96, 97, 98, 99, 109, 111, 143, 10080, 70178, 70216; without bruchids: 9, 10, 20, 24, 25, 26, 31, 35, 36A, 39, 40A, 40B, 41, 47, 49, 60, 64, 93, 94, 100, 106, 110, 112, 114, 10028, 10062, 10187, 10188, 10211, 10251, 10300, 10309, 10311, 10322, 10333, 10352, 10361, 10410, 10431, 10500, 10509, 10524, 10529, 10584, 10617, 10631, 10634, 10647, 10678A, 10678B, 10702, 10713, 10732, 10735, 10766, 10797, 10901, 10957, 10982, 40026, 40077, 40099, 40145, 40188, 40190, 40231, 40322, 40348, 40371, 40372, 70172, 70194, 70228, 70257, 70317, 70417, 70455, 70472, 70490.

Greece: with bruchids: 118, 136, 156, 166; without bruchids: 115, 116, 117, 121, 127, 129, 130, 130x, 132, 133, 140, 154, 164, 165, 167.

Italy: with bruchids: 173, 174, 176, 177, 178, 180, 183, 185, 186, 187, 193, 195, 196, 198, 200, 209, 213, 223, 228, 229, 231,232; without bruchids: 175, 184, 199, 203,206, 210, 212, 215, 220, 221,225, 236.

Spain: with bruchids: 260, 267, 268, 272, 276, 289, 294, 295, 298, 301, 312; without bruchids: 250, 260A, 260B, 263, 264, 265, 265x, 266, 266x, 269, 271,271 x, 274, 275, 278, 279, 280, 282, 283, 285, 286A, 286B, 287, 288, 290, 291,293, 296, 297, 299, 300, 302, 303, 304, 305, 306, 309, 310, 311,313, 314, 317, 320.