Host Specificity of the Argentine Root-Boring Weevil, Heilipodus ventralis (Coleoptera: Curculionidae), a Potential Biocontrol Agent for Snakeweeds (Gutierrezia: Asteraceae) in Western North American Rangelands—U.S. Quarantine Tests

Native snakeweeds, especially Gutierrezia sarothrae (Pursh) Britton and Rusby and Gutierrezia microcephala (DC.) A. Gray, are among the most widespread and damaging weeds of rangelands in the western United States and northern Mexico. The genus long ago spread to southern South America, where further speciation occurred. We have found several species of insects in Argentina that damage other species of snakeweeds there and are possible candidates for biological control in North America. The first of these, the root-boring weevil, Heilipodus ventralis (Hustache), was tested in Argentina and then sent to the USDA-ARS Insect Quarantine Facility at Temple, Texas, for host specificity testing on North American plants. We tested H. ventralis on 40 species of the family Asteraceae, in 19 tests of five types, using 686 adults and 365 larvae. Host specificity increased from adult feeding, to ovipositional selection, to larval development. At Temple, adults fed mostly on 6 species of the closely related genera Grindelia, Gutierrezia, and Gymnosperma, but with substantial feeding on four other genera of the two preferred subtribes Solidagininae and Machaerantherinae and on Baccharis in the tribe Baccharidinae, with lesser feeding on the subtribe Asterinae, all in the tribe Astereae, and on 1 species in the tribe Anthemideae. Females oviposited primarily on the same 6 species but very little on plants outside the 2 preferred subtribes. Larvae developed only on 9 of the 29 U.S. plant species tested, 6 within the two preferred subtribes and on Brickellia and Aster in other tribes. Only 5 species of three genera appear to be potential true hosts of H. ventralis in North America, on which all stages of the life cycle, adult feeding, oviposition, and larval development, can take place; these are Gymnosperma glutinosum (Spreng.) Less., Gutierrezia grandis Blake, Gut. microcephala, Gut. sarothrae, and Grindelia lanceolata Nutt. None of these genera contain species of economic or notable ecological value; the few rare species appear to be protected by habitat isolation from attack by H. ventralis. H. ventralis, therefore, appears sufficiently host specific for field release in North America. This is the first introduced biocontrol agent to be approved for release in a continental area to control a native weed.     

Host plant-associated genetic differentiation in the snakeweed grasshopper, Hesperotettix viridis (Orthoptera: Acrididae)

Studies of herbivorous insects have played a major role in understanding how ecological divergence can facilitate genetic differentiation. In contrast to the majority of herbivorous insects, grasshoppers as a group are largely polyphagous. Due to this relative lack of intimate grasshopper-plant associations, grasshopper-plant systems have not played a large part in the study of host-associated genetic differentiation. The oligophagous grasshopper, Hesperotettix viridis (Thomas), is endemic to North America and feeds on composites (Asteraceae) within the tribe Astereae. Previous work has shown both preference and performance differences between H. viridis individuals feeding on either Solidago mollis or Gutierrezia sarothrae. Using 222 AFLP markers, we examined the genetic relationships among 38 H. viridis individuals feeding on these plants both in sympatry and allopatry. Neighbour-joining analysis resulted in two distinct host-associated clades with 71% bootstrap support for host-associated monophyly. Analyses of molecular variation (amova) revealed significant genetic structuring with host plant accounting for 20% of the total genetic variance while locality accounted for 0%. Significant genetic differentiation was detected between S. mollis-feeders and G. sarothrae-feeders even when the two were present at the same locality. These results are consistent with observed differences in preference and performance between H. viridis grasshoppers feeding on either G. sarothrae or S. mollis and indicate that H. viridis is comprised of at least two genetically distinct host plant-associated lineages.     

Biology of Heilipodus ventralis (Coleoptera: Curculionidae), an Argentine Weevil for Biological Control of Snakeweeds (Gutierrezia spp.) in the United States

The Argentine root-boring weevil Heilipodus ventralis (Hustache) is a candidate for biological control of the perennial snakeweeds Gutierrezia sarothrae (Pursh) Britton and Rusby and G. microcephala (DeCandolle) A. Gray, poisonous native weeds of rangelands of the southwestern United States. In Argentina, the weevil occurs in semiarid regions from Tucumán south to Chubut, which are climatically similar to broad areas of the southwestern United States. Laboratory-reared females lived ca. 112 days and laid ca. 117 eggs. The eggs hatched in ca. 15 days. The larvae had eight instars; they required ca. 151 days and pupae ca. 27 days to develop. Adult weevils emerged from the taproots in early summer, fed on the leaves and terminals, and oviposited mostly in the crown near the soil line. The feeding of one or more pairs of caged adults killed medium-sized plants. The larvae tunneled downward to the taproot where they pupated. They overwintered in the taproots of these perennial host plants and pupated in the spring. A generation required 1 year but some individuals probably required two growing seasons. At Peninsula Valdés, Chubut, H. ventralis preferred Gutierrezia solbrigii Cabrera to Grindelia chiloensis (Corn.) Cabrera by a ratio of 1.9 to 1.0. In the field there, crowns of large Gu. solbrigii (average 60 cm canopy diameter) contained an average 5.0 larvae per plant, those of small plants (23 cm diameter) contained 1.1 larvae, and plants smaller than 10 cm rarely contained larvae. No insect parasitoids or predators were found attacking any stage. H. ventralis probably evolved on xerophytic, temperate Astereae, from ancestors of the genus Heilipus that fed on species of ancient, hygrophytic, tropical plant families.