Xystus, SCHONHERR

Jones, Robert W., Cortez-Madrigal, Hipolito, García-Ruíz, Ignacio & O'Brien, Charles W., 2011, Species ofXystusSchönherr (Coleoptera: Curculionidae: Baridinae) are Seed Predators of Wild Yams (Dioscoreaceae) in Central Mexico, The Coleopterists Bulletin 65 (1), pp. 86-87 : 86-87

SPECIES OF XYSTUS SCHÖNHERR ( COLEOPTERA : CURCULIONIDAE : BARIDINAE ) ARE SEED PREDATORS OF WILD YAMS ( DIOSCOREACEAE ) IN CENTRAL MEXICO

ROBERT W. JONES Facultad de Ciencias Naturales , Universidad Autónoma de Querétaro , Juriquilla C.P. 76230, Querétaro, MEXICO

HIPOLITO CORTEZ- MADRIGAL, IGNACIO GARCÍA- RUÍZ Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional Jiquilpan, Michoacán, MEXICO

AND

CHARLES W. O’ BRIEN 2313 W Calle Balaustre, Green Valley, AZ 85622-8047, U.S.A.

Several species of wild yams, “camote de cerro”, Dioscorea spp. ( Dioscoreales : Dioscoreaceae ), are native to the mountains that surround Lake Chapala in the states of Jalisco and Michoacán. These are harvested in the field by the rural populace and command a high price in local markets. The sale of wild yams is an important source of income for the harvesters (“camoteros”), the numbers of which are increasing (Andrés-Agustín 1996; Mostul and Cházaro 1996). This has raised concern that natural regeneration of wild yam populations may not be sufficient to satisfy present or future demands. To address this concern, a project was initiated by the Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional, Jiquilpan, Michoacán, to locate populations of wild yams in Jalisco and Michoacán and determine important biotic variables and estimate the sustainable extraction levels.

The study was carried out in 2007 and 2008, consisting of explorations of the regions surrounding Lake Chapala at altitudes of 1,684 to 2,148 m to locate populations of Dioscorea spp. The vegetation included middle and low stature tropical deciduous forests, tropical scrub, and oak forests at the highest altitudes. Insects associated with plants were collected and labeled as to location and plant species. Samples were taken weekly of flower buds and fruits during 2008 and examined in the laboratory.

One of the most common and apparently important insect herbivores of wild yams of the region was species of Xystus Schönherr ( Curculionidae : Baridinae : Apostasimerini [ Prena 2009]). Two species were found and identified: Xystus gracilirostris (Champion) and Xystus pallidipennis (Champion) . Xystus gracilirostris has been previously reported from Honduras and Mexico, whereas X. pallidipennis is known only from Mexico (O’ Brien and Wibmer 1982). Based on literature searches, these are the first host records known for these species. This is not the first record of Xystus on Dioscorea , however, as Santos (2002) reports Xystus arnoldi (Kirby) on cultivated species of Dioscorea in Brazil.

Adult weevils were found externally on plants ( Fig. 1 ), and eggs and larvae were within fruit capsules of Dioscorea sparsiflora Kunth and Discorea remotiflora Robinson. Weevils were found only on female plants of these dioecious species. Both X. gracilirostris and X. pallidipennis were found on both species of plants from late September to the first week of November on Cerro Grande, Emiliano Zapata, Michoacán in 2008, and averaged 1. 8 adult weevils/plant (n = 17–25 plants/week, both species). Damage to fruit capsules was caused by adult and larval feeding. Adult feeding was evidenced by open feeding punctures on the surface of the fruit (capsule). Also, punctures for oviposition were observed on the surface of the fruit through which the eggs were deposited and later larvae moved to fruit tissues surrounding the seeds. Due to the small size of the seeds, it is assumed that individual larvae feed on several seeds for development. Larvae emerged from the capsules and pupated in the soil. Larval feeding also was accompanied by decomposition of non-damaged seeds in the vicinity of larval feeding sites. In the field, the majority of examined plants (80.4%, n =>100 plants) and between 90–100% of the fruit capsules were damaged by weevils. Of nine fruit capsules examined in the laboratory, 88.9% had damage, 77.7% had eggs, and 55.5% were found with larvae.

The impact of feeding by X. gracilirostris and X. pallidipennis on wild yams requires further study. Clearly, these weevils reduce a portion of the reproductive output of female plants through seed predation, but the relative importance of this loss to the plants is unknown. Asexual reproduction of plants is also apparently important in natural regeneration and may offset damage due to Xystus spp. and other herbivores.

Because of the high market value of the wild yams, locals are presently attempting to cultivate these plants. If these attempts at domestication of wild yams are successful, it is highly probable that Xystus weevils will become pests of these newly cultivated crop plants and management strategies will be required.

Figures

Fig. 1. Xystus gracilirostris on fruit capsule of Dioscorea sparsiflora. Oviposition puncture of weevil is present in lower right on fruit capsule.