The use of host preferences as taxonomic characters of bruchid beetles (Coleoptera: Bruchidae) feeding in the seeds of Cassia (Leguminosae).

Abstract

Host preferences of bruchids that feed in Cassia seeds are useful as taxonomic characters primarily at the species group level. Many species of Cassia bruchids feed in the seeds of only one host but usually the host supports more than one species of bruchid. Thus, few Cassia bruchids have unique host charac ters. More often, the species groups of Sennius feed in a subgenus, section or series of Cassia in common. Species groups 1, 5, and 6 of Sennius especially show host specificity to a category of Cassia. A valuable character for Pygiopachymerus lineola is that it exhibits remarkable specificity to the subgenus Cassia. Species groups 2 and 3 of Sennius are not classified as well as the other groups using host preferences because several species in these groups have 5 to 10 hosts, have a wide distribution, and thus are less specific in their choice of hosts. Most bruchid beetles feed in the seeds of the Leguminosae and recent studies have shown that congeneric species of bruchids feed in seeds of plants which are in the same taxonomic group. Johnson and Kingsolver (1976) showed that species of the genus Stator attacked seeds of species of plants in the subfamily Mimosoideae (Leguminosae) much more often than they did those of other plants. Kingsolver and Johnson (1978) also showed that species of the genus Mimosestes preferred seeds in the same subfamily. Johnson (1970, 1977a, 1977c) published data that showed that species of the genus Acanthoscelides preferred seeds of species in the subfamily Papilion oideae and that some species of Acanthoscelides are restricted to individual genera and species. However, Johnson and Kingsolver (1971) and Forister and Johnson (1971) showed that some Acanthoscelides feed only in non leguminous seeds (e.g., Rhamnaceae, Sterculiaceae). The above indicates that host preference data are often useful in evalu ating relationships. Unfortunately, as with other characters, characters such as food preference sometimes are so variable that they are not of great value. Interesting evolutionary problems are suggested by the studies of Received for publication 6 February 1979. This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 05:51:25 UTC All use subject to http://about.jstor.org/terms 28 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Forister and Johnson (1971) on Acanthoscelides prosopoides (Schaeffer), a species whose morphology places it in the genus Acanthoscelides. Most species of Acanthoscelides feed in the Leguminosae, but A. prosopoides feeds in the seeds of a rhamnaceous species. Johnson and Kingsolver (1975) have pointed out that the bruchid Amblycerus vitis (Schaeffer) feeds in Arizona wild grape seeds (Vitaceae), not in a legume. Perhaps the best current example of a very interesting evolutionary problem involving a bru chid is a species in the Southwestern U.S. genus Neltumius. Two of the three species feed in mesquite seeds. Neltumius arizonensis (Schaeffer) feeds only in the seeds of the velvet mesquite, Prosopis velutina Wooton and N. gibbithorax (Schaeffer) feeds only in seeds of screwbean mesquite, P. pubescens Bentham. The third member of the genus, N. texanus (Schaef fer), is also restricted in its feeding habits but feeds in seeds of the rham naceous genus Condalia (Johnson, 1978). Because the mesquite hosts were discovered much earlier, the initial unpublished predictions of bruchid spe cialists were that the host of N. texanus was a mesquite (Prosopis) or at least a host in a genus closely related to Prosopis. Fortunately, however, host preferences for most bruchids are much more predictable than the above examples. The reasons for the host preferences of bruchids are very complex and are surely the results of long periods of coevolution with the host plants. We do know from recent studies (Center and Johnson, 1974; Janzen, 1969; Johnson and Slobodchikoff, 1979) that seed chemistry, seed and pod mor phology, and behavior of the plants and insects are some of the primary reasons for host preferences. An excellent example of restriction of feeding by a genus of bruchids to a genus of plants is that of species of the genus Sennius and its preference for seeds of Cassia (Leguminosae: Caesalpinioideae). Twenty-five of the 26 species of Sennius whose hosts are known, feed in the seeds of species of Cassia (S. willei Kingsolver was reported by Kingsolver (1968) to feed in seeds of alfalfa). Recent research on the host plants of Sennius (Johnson and Kingsolver 1973; Johnson 1977b) has enabled me to analyze the host preferences of species and subgroups of Sennius and these relationships to the subgroups of Cassia. This analysis and its results are the subject of this paper. Host Preference and Classification of Cassia Bruchids Johnson and Kingsolver (1973) placed the species of Sennius they treated into groups. Whitehead and Kingsolver (1975) and Johnson (1977b) added new species to these groups (Table 1). Table 2 lists a classification of Cassia by Bentham (1871) together with the bruchids that attack the seeds of each species of Cassia. The use of host preferences for the relationships of bruchids at the species This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 05:51:25 UTC All use subject to http://about.jstor.org/terms VOLUME 53, NUMBER 1 29 Table 1. Species groups of the seed beetle genus Sennius whose hosts are known. Group 1. guttifer, russeolus, inanis, breveapicalis, colima, panama Group 2. abbreviatus, instabilis,* lawrencei, medialis, durangensis, celatus,* leucostauros, trinotaticollis Group 3. fallax,* auricomus, biflorae, discolor Group 4. incultellus Group 5. cruentatus, simulans, obesulus, morosus,* ensiculus

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