Thanius biennis , a new gall-inducing weevil with a two-year life-cycle from Costa Rica ( Coleóptera : Curculionidae : Baridinae )

Thanius biennis Prena & N ishida sp. n. is described and illustrated. The larva induced galls on stems of Psychotria marginata (Rubiaceae). The complete life-cycle took two years per generation, and two alternate generations coexisted on the same plant. Each gall harbored a single immature or adult specimen. The latter stage remained dormant in the gall for nearly five months, and emerged with the beginning of the rainy season.


Introduction
During a general study of gall-inducing insects in Costa Rica, the second author made observations on a baridine weevil associated with Psychotria marginata (Rubiaceae).Subsequent investigations by the first author revealed, that the specimens represent an undescribed species belonging to an ill-defined complex near Pseudobaris LeC onte.These barids are characterized by the possession of a projecting pygidium, a distinct prosternai channel, a square basal prosternai process and notably elongate aedeagal apodemes.Our species, although larger than the type species, concurs best with Thanius Casey (1922).C hampion (1908) described four related Middle American species and assigned them to Madarus Schônherr, i. e. M. astutus, M. scaphiformis, M. tremulus and M. vitiosus.However, those species bridge character states between Thanius and Physoproctus Lacordaire.A s we do not have sufficient data for a generic revision at this stage, we assign the new species provisionally to Thanius.It should be noted, that several species included there by Bondar (1949) and H ustache (1951) belong elsewhere.In the present paper, we describe and illustrate the new species with its immature stages and report its interesting life history.Our results may help to procure further data through systematic studies in the field, which will be needed for a future revision of the entire group.The study was carried out in the wake of the ALAS 4 project (NSF 2001-05) led by J ohn Longino (Olympia, WA).Travel grants provided to the first author by the National Geographic Society (2003) and the Deutsche Forschungsgemeinschaft ( 2004) are grate fully acknowledged.INBio and MINAE provided logistic and administrative support.Lee H erman (New York), Steven Lingaeelter (Washington D.C.), Klaus Klass (Dresden) and H elen Perrin (Paris) provided access to specimens.Carlos O. M orales (San José) identified the host plant.Ian D. Gauld (London) identified the parasitoid.Paul Hanson (San José) commented on the manuscript.Our cordial thanks are extended to all of them.

Methods
Observations and collections were made by the second author between July 2002 and May 2004 in the Reserva Ecológica Leonel Oviedo (9o 56' N, 84° 03' W ), on the cam pus of the University of Costa Rica (UCR) in San Pedro, Montes de Oca, San José.The reserve is a 35 year-old secondary growth forest of approximately 0.01 km2 at 1150 m elevation.The site was used previously as a coffee plantation.According to H oldridge (1967), the habitat is considered to be a moist premontane tropical forest.However, today it is engulfed by the municipality of San José and isolated from natural habitats.Stiles (1990) provides a short description of the plant community of the reserve.Species of Psychotria are herbs, shrubs or small trees.The genus is spread widely through out the Neotropics and is notably speciose, with approximately 115 species recorded from Costa Rica alone (INBio 2004).The range of Psychotria m arginata extends from Belize southward to Bolivia (MOBOT 2004).In Costa Rica, the species can be found on both slopes of the Cordilleras Central and Talamanca, from sea level to approximately 1700 m elevation (INBio 2004).At least seven 2-3 m tall treelets occur at the study site, all of them being infested by the weevil reported upon here.
Branches of the host plant having galls were placed in plastic bags, and the insects were reared at an average temperature of 23-24 °C in the entomological laboratory of the uni versity.Approximately fifty galls were dissected and studied under the dissecting micro scope.Larvae, pupae, adults and galls were preserved in 75% ethanol.Photographs were taken with Nikon Coolpix digital cameras and edited with Adobe Photoshop®.In the field, some mature galls were bagged with a fine mesh-cloth to intercept newly hatched weevils.Ribbon-markers were used to aid in the monitoring of the galls.Measurements of length were made with an ocular micrometer in a dissecting micro scope as follows: total length, from anterior margin of eye to pygidium in dorsal view; standard length, from anterior margin of pronotum to pygidium in dorsal view; pronotal length, longest dorsal extension in lateral view; elytral length, longest extension parallel to elytral suture; length of rostrum, straight distance from apex (without mandibles) to anterior margin of eye at middle of rostrum in lateral view; apical portion of rostrum, straight distance from apex (without mandibles) to point of antennal insertion in lateral view.Drawings were made using the grid of an ocular micrometer.The larval terminol ogy follows that used by M ay (1994).
Life history.Immature and adult stages were found only in association with Psychotria marginata (Rubiaceae).Adult weevils appeared on the flowering host plant in the first half of May (1-13 May 2003;11 M ay 2004).Individual monitoring of single galls in 2004 revealed, that two adults had prepared exit holes by the third week of April, subsequent to the first significant precipitation toward the end of the dry season, but did not aban don the galls at this stage.The weevils were seen feeding on the flowers (Fig. 26), but not on other parts of the host or on any other plant species nearby.Anthers appeared to be the preferred diet (Fig. 27).The number of adults encountered declined after ap proximately 10 days, and adults disappeared shortly after the fading of the flowers in the second half of May; they were not encountered anymore on the host until the next generation showed up one year later.Oviposition was not observed, but some 20 fresh scars were found in early M ay 2003 on parts of branches previously unaffected.The scars were light brown, raised dots, approximately 2-3 mm in diameter.Noticeable swellings, 5 mm long, 4 mm wide and 1 mm high, became apparent approximately two weeks later and grew to oval surface galls with barky texture.Only lignified branches were affected, while fresh green growth was unaffected.Some galls were dissected in December, toward the end of the rainy season, and in February, and revealed mid-sized, probably secondinstar larvae.Pupae were obtained from dissected galls in the beginning of December, 18 months after oviposition, where they rested without a cocoon.The first teneral adults were found in galls in the second half of December (Fig. 24), but these and further specimens brought in over the next months remained inactive until early (2004) or late (2003) April and did not take any food during captivity.In the field, adults emerged through circular, 3 mm exit holes made in the upper one-fourth of the galls between mid April and early May (Fig. 25), thus completing the two-year life-cycle.Nothing is known about the fate of the adult weevils after having left P. marginata.Observations made on undescribed congeners in Vara Blanca, Costa Rica suggest, that they may frequent flow ering plants of other families.
Description of gall.The galls occurred primarily on 15-30 mm thick, lignified stems and branches in remarkable density, with up to 30 galls per 60 cm branch.Branches less than 6.5 mm thick were not affected.Mature galls attained the shape of an ovoid surface swelling approximately 18 mm long, 10 mm wide and 6 mm high (Fig. 25).The final size was reached in approximately one year.The outer surface of the aging gall was cov ered with light brown, longitudinally arranged lenticels or cracks.Each gall contained a single chamber that harbored a larva, pupa, or adult.Compacted blackish-brown fecal pellets were observed at the basal part (Fig. 29).Irregularly grown nutritive tissue, of granulose texture and whitish to translucent in color, covered the inner walls (Fig. 29,30).We suspect that the nutritive tissue is re-growth of parenchyma cells.The mature gall-chamber measured approximately 7 mm x 4 mm.Parasitoids.A larva of an ichneumonid wasp, Calliephialtes sp.(Pimplinae) was found in a mature gall on 21 January 2003, and was reared in the laboratory (1 male, deposited at UCR).The species is an idiobiont, i.e. the parent wasp renders the host immobile and inhibits its further growth by paralysis.The parasitized weevil larva was mature and close to pupation.
Distribution.Thanius biennis is known only from the Leonel Oviedo Reserve, on the cam pus of the University of Costa Rica in San Pedro, San José.
Discussion.The new species is remarkable for its rarity and the surprisingly long life-cycle.Adult specimens have not been collected before our monitoring of the galls commenced in the summer 2002, despite the considerable collecting effort carried out in Costa Rica.Subsequent observations of the site in general, and the host plant in particular, showed that the occurrence of the adult weevil is restricted to a small slot of time, of approxi mately two weeks, in the beginning of the rainy season, when the host plant blooms.The brief appearance of the adult stage alone would sufficiently explain this case of rarity.In addition, the population underwent a life-cycle of two years.We observed two subse quent, alternating populations on the same host.Their interrelationship remains unclear for the moment, because we have no proven case of deviation from the average cycle.We assume that the development of some individuals can deviate from the observed tempo ral pattern, perhaps under unfavorable weather conditions or at random.An exchange of individuals between local subpopulations is another plausible mechanism but rather unlikely at our study site due to its spatial isolation.Longevity of adult weevils combined with a second phase of reproduction seems the least likely explanation for the origin and presence of an alternate population.

Figs. 23
Figs. 23-30 .Life history of Thanius biennis.23, longitudinal section of one-year-old gall showing second-in star larva in situ; arrow indicates compacted fecal matter; 24, dissected gall showing fully developed dormant adult; arrow indicates compacted fecal matter; 25, recently abandoned mature galls with exit holes; arrow indicates weevil preparing exit; 26, adult T. biennis on flower of P. m arginata; 27, feeding damage on flower of P. m arginata made by adult T. biennis-, 28, longitudinal section of young gall with first-instar larva; 29, dissected gall (top part removed) showing compacted fecal matter, chamber, nutritive tissue and third-instar larva in situ-, 30, close-up of nutritive tissue (ca.75x).