Seed predation of Sabal palmetto, Sabal mexicana and Sabal uresana (Arecaceae) by the bruchid Caryobruchus gleditsiae (Coleoptera: Bruchidae), with new host and distribution records

1 Introduction

The palms of northwestern Mexico have been poorly studied due to their inaccessibility as many palm groves remain sheltered in undiscovered canyons and hidden in remote mountains (Felger and Joyal 1999). The palm genus Sabal (Arecaceae: Coryphoideae) is endemic to the New World. It is one of the most common palm genera in Mexico and around the Caribbean region (Zona 1990). It includes 15 species, primarily in the southeastern United States and the Caribbean (including Bermuda), with an outlying species in the north of Colombia, Costa Rica, Panama, and Venezuela (Zona 1990).

Palms are among the most widely utilized plant families, especially in tropical regions (Pérez and Rebollar 2003). Leaves are the most frequently used part of Sabal palms in native habitats, which are employed in weaving, thatching, and broom-making. They sustain thatch, basketry, and hat-making industries (Bernal et al. 2011; Caballero-Nieto et al. 2001; Felger and Joyal 1999; Pérez and Rebollar 2003).

Trunks have been used for construction, fruit, and sometimes for food such as palm hearts (Felger and Moser 1985; Joyal 1996). Some species also have medical value (Jones 1999). Not only are they economically important, but because of their beauty and ornamental potential, are widely cultivated in tropical and subtropical gardens around the world (Felger and Joyal 1999; Pérez and Rebollar 2003).

Members of the tribe Pachymerini (Bruchidae: Pachymerinae), with origin from the New World, use seeds of 100 species in 34 genera of the Arecaceae family, that is why they are commonly named palm bruchids (Delobel et al. 1995; Romero and Johnson 2002). According to Janzen (1971) and Moegenburg (1996), palm bruchids usually cause a negative impact on palms by consuming seeds that are suited for human consumption, which also reduces the reproductive potential of palms.

The seed beetle Caryobruchus gleditsiae (Johansson & Linnaeus) (Coleoptera: Bruchidae) is a voracious seed predator throughout the range of Sabal species, this beetle can infest nearly 100 % of fallen mature fruit around a single palm (Fox 2015) and has been found feeding on seeds from species of other genera in the Arecaceae such as Cocothrinax, Livistona, Phoenix, Serenoa, and Washingtonia (Olvera 1981).

For Sabal mexicana Mart. only the bruchid C. gleditsiae has been reported feeding on its seeds, in Texas, United States (Nilsson and Johnson 1993), and Veracruz in Mexico (Nilsson and Johnson 1993; Olvera 1981). The following two species of bruchids have been reported for Sabal palmetto (Walter) Lodd. ex Schult. & Schult.f: C. gleditsiae in Florida, United States and Caryobruchus marieae Nilsson & Johnson in Havana, Cuba (Nilsson and Johnson 1993). Sabal uresana Trel. has only been reported as a host of the bruchid C. gleditsiae in Sonora, Mexico (Nilsson and Johnson 1993).

In this study, we examined the percentage of seeds with damage caused by the bruchid C. gleditsiae in three Sabal palm species (S. palmetto, S. mexicana and S. uresana).

2 Materials and methods

In August 2016, seeds were collected from three S. palmetto palms and four S. mexicana palms at the Benjamin Francis Johnston Botanical Garden, Los Mochis, Sinaloa, Mexico. In July 2017, seeds were collected from a S. uresana palm in a town in El Fuerte, Sinaloa, Mexico (Table 1).

Samples of the S. palmetto and S. mexicana palms were placed in clear plastic cans and stored in a laboratory, in the Benjamin Francis Johnston Botanical Garden, in Los Mochis, Sinaloa, Mexico. Samples of the S. uresana seeds palms were placed in paper bags and stored in laboratory of Coastal Ecology, of the Universidad Autónoma de Occidente, Unit, Los Mochis, Sinaloa. After 3 months of storage, the seeds were checked and found to have been infested by bruchids. Emerging insects were placed in 70 % ethanol. Subsequently, taxonomic identification was carried out using the identification keys of Nilsson and Johnson (1993). Finally, the specimens were tagged and mounted on entomological pins accompanied by their genitalia in glycerin vials. The preparation of the genitalia for the corroboration of the species was carried out following the techniques and nomenclature indicated by Kingsolver (1970) and modified by Romero and Johnson (1999). After carrying out the determination, the specimens were deposited at the Colección Entomológica del Colegio de Postgraduados, Montecillo, Estado de México, México (CEAM); further, a representative sample was deposited in the Insect Collection at the Instituto de Ciencias Agropecuarias, Universidad Autónoma del Estado de Hidalgo, Hidalgo, Mexico (ICAp-UAEH).

The percentage of seeds with damage caused by the bruchid C. gleditsiae, was calculated by counting and classifying the seeds as healthy or damaged. Seeds with bruchid emergence holes were considered damaged. Then the number of damaged seeds were divided by the total number of seeds in the sample and the result was multiplied by 100, this was done for the three palm species.

All plants were cited according to World Flora Online (2023). Also, the BRUCOL database (Romero and Johnson 2002) was checked for all information about host plants, distributions, and collections housing specimens of C. gleditsiae.

3 Results

3.2 Seed predation

The average percentage of seed predation by C. gleditsiae (Figure 1) for S. palmetto was 27.58 %, for S. mexicana was 22.36 %, and for S. uresana was 2.70 % (Table 1). In most of the 720 seeds analyzed, only a single bruchid emerged, although on rare occasions, it was observed that two specimens of the bruchid C. gleditsiae emerged from the same seed. In total 168 specimens were found in the samples.

Figure 1:

Caryobruchus gleditsiae (Johansson & Linnaeus) male: (A) dorsal view, (B) lateral view, (C) pygidium, (D) median lobe of male genitalia, and (E) lateral lobes of male genital.

4 Discussion

The percentage of predated palm seeds depends on the host, the storage time of the seeds and the bruchid species that is being studied (Parra-Gil et al. 2023). The seed beetle C. gleditsiae is a voracious seed predator mainly on the genus Sabal. This beetle can infest nearly 100 % of fallen mature fruit around a single palm (Fox 2015). However, Brown (1976) found levels of predation as high as 92 % of C. gleditsiae on S. palmetto. Zona (1990) reported 50 % predation by C. gleditsiae on S. uresana. In contrast, in the current study, C. gleditsiae damage was 27.58 % in S. palmetto, 22.36 % in S. mexicana, and 2.70 % in S. uresana. It is important to mention that in the present study, seeds were checked after 3 months of storage and with more storage time the percentage of reported seed predation would likely increase, because this beetle can infest almost 100 % of the fallen ripe fruit around a single palm (Fox 2015).

The occurrence of Caryobruchus bruchids inside palm seeds was first reported by Riley and Howard (1892) as C. gleditsiae. Currently three species of bruchids (C. gleditsiae, Pachymerus nucleorum (Fabricius), and Speciomerus ruficornis Germar) have been reported feeding on 14 species of palms, belonging to the genus Sabal (S. bermudana; S. causiarum; S. domingensis; S. etonia; S. longipedunculata; S. maritima; S. mauritiiformis; S. mexicana; S. minor; S. palmetto; S. pumos; S. rosei; S. uresana; and S. yapa), in seven countries (Bermuda, Cuba, El Salvador, USA, Haiti, Mexico, and the Dominican Republic) (Romero and Johnson 2002). The bruchid C. gleditsiae has been reported feeding on seeds of all 14 hosts in the seven countries. P. nucleorum has only been reported feeding on S. palmetto in Florida, USA, and S. ruficornis was reported consuming seeds of S. mexicana but without data on the collection site (Johnson et al. 1995).

Sabal palmetto has only been reported as host of the bruchids C. gleditsiae and C. marieae previously for Florida, USA and Havana, Cuba (Johnson et al. 1995; Moegenburg 1996; Nilsson and Johnson 1990; Woodruff 1968; Zona 1990), for Mexico there was no report so far for any bruchid eating the seeds of this palm species. Therefore, this is a new host report for Mexico.

Previously, C. gleditsiae only has been reported feeding on the seeds of S. mexicana in Texas, USA (Nilsson and Johnson 1993), and Veracruz, Mexico (Nilsson and Johnson 1993; Olvera 1981). S. uresana only has been reported as a host of C. gleditsiae from Sonora, Mexico (Nilsson and Johnson 1993). Therefore, this manuscript provides two new host reports for Sinaloa, Mexico.

After checking the hosts reported for the bruchid C. gleditsiae in the BRUCOL database (Romero and Johnson 2002), we found that Coccothrinax martii, Thrinax radiata, and Thrinax morrisii, have never been reported as hosts for Bruchidae. Therefore, these species are new hosts.

In addition, in the BRUCOL database (Romero and Johnson 2002), we found that Bridwell (1929) cited Inodes texana O.F. Cook as host of the bruchid Caryobruchus curvipes (Latreille). Although I. texana is synonymous with S. mexicana, Udayagiri and Wadhi (1989) cited Inodes sp. as host of C. curvipes, although Inodes sp. is synonymous with Sabal sp. in this context. Zacher (1952) cited Palmensamen sp. and Gleditsia sp. (Caesalpinioideae: Leguminosae), as hosts of the bruchid C. gleditsiae, but the species Palmensamen sp. is not a valid name. Gleditsia sp. is only a floral record.

Woodruff (1968) cited Sabal mauritiaeformis (Karst.) Griseb. Ex Wendl., Sabal glaucesens Lodd., and Sabal parviflora Becc. as hosts for C. gleditsiae. Although, S. mauritiaeformis is in fact, S. mauritiiformis, S. glaucesens is synonymous with S. mauritiiformis, and S. parviflora is synonymous with S. palmetto.

Nilsson and Johnson (1990) cited Sabal texana, Sabal florida Becc., Sabal  bermuda, S. parviflora and Sabal glabra (Mill.) Sarg. as hosts of the bruchid C. gleditsiae. S. texana is now synonymous with S. mexicana, S. florida is synonymous with S. maritima, S. bermuda is synonymous with S. bermudana, S. glabra is synonymous with S. minor, and S. parviflora is synonymous with S. palmetto.

Nilsson and Johnson (1993) cited the palms Copernicia torrans Leon and Copernicia torreana Leon as hosts of C. gleditsiae for Cuba. Both plants are synonymous with C. macroglossa Schaedtler, the authors also cited Copernicia inderoglossa for Cuba and Inodes neglecta for the Dominican Republic as hosts of C. gleditsiae, but C. inderoglossa is not a valid name; and I. neglecta is synonymous with S. domingensis. Also, Nilsson and Johnson (1990, 1993) cited Thrinax microcarpa as a host of C. gleditsiae, but this species is a synonym of Leucothrinax morrisii (H.Wendl.) C.Lewis & Zona.

Fernández and González (2015) cited S. florida as a host for C. marieae, although S. florida is a synonym of S. maritima, this is a new host report. In the same paper, Hemithrinax savannarum León was reported as a host of C. gleditsiae, but this palm species is a synonym of H. rivularis var. savannarum. Finally, Reyes-Zepeda et al. (2021) reported C. gleditsiae on Brahea berlandieri Bartlett in Northeast Mexico for the first time, though B. berlandieri is a synonym of Brahea dulcis (Kunth) Mart, this was a new host report.

It is essential to mention that it is likely that both the distribution and the number of hosts will increase in the future because C. gleditsiae is a voracious seed predator so as our knowledge grows and it expands its distribution new host records will be reported. In this sense, it is possible to assume that the distribution of Caryobruchus species that feed mainly on Sabal palm species will follow the distribution of their hosts given their distribution in Mexico, as well as in the United States (Brown 1976; Olvera 1981).

Therefore, when palm seed collections are carried out in places where they have not been collected before and are stored, it is likely that specimens of C. gleditsiae will be found. It is necessary to collect material from previously unsampled areas of Sabal distribution in Mexico so that in the future, it will be possible to understand the full distribution of C. gleditsiae in Mexican territory.

The importance of understanding the distribution lies in the fact that seeds are a very important component of the plant life cycle and a valuable resource for the propagation and conservation of germplasm. They not only carry the necessary genetic variation but are quite often the only means for long-distance dispersal. Hence, it is important to study the degree of damage that bruchid species cause to palms through consuming palm seeds, especially in vulnerable species like H. rivularis var. savannarum.