David B. Richman

Conservation strategies for the biological control of weeds

Publisher Summary The chapter reviews the factors that limit the success of insects for weed biological control, including factors that regulate control agent populations and factors that influence weed response to insects. It addresses the use of conservation biological control strategies with native agents while drawing on experiences from studies on rangeland weeds and the aquatic weed Eurasian watermilfoil. The importance of establishing the factors responsible for the success or failure of weed biological control projects is becoming more and more apparent. Most conservation strategies include population protection or informed and appropriate use of pesticides to maintain native or exotic biological control agents. In addition, strategies such as habitat protection to preserve critical habitat or refugia and plant community management to maintain and enhance the effectiveness of existing biological control agents need to be exercised. The role of resistance has rarely been considered in weed biological control because most weed biological control agents are specialists and thought to be adapted to overcome their hosts defensive systems. Properly timed range management practices can protect populations of native biological control agents, increasing their effectiveness.

Volatile Composition of Gutierrezia sarothrae (Broom Snakeweed) as Determined by Steam Distillation and Solid Phase Microextraction

Abstract Volatiles of Gutierezia sarothrae (broom snakeweed, snakeweed) were isolated from ground, composite tissues by steam distillation and by solid phase microextraction (SPME), then separated and analyzed by gas chromatography with mass spectral and flame ionization detection. Compounds detected varied in quantity between isolation protocols. In the oil, cryptone (6.4%) and β-eudesmol (5.9%) were the only compounds comprising more than 5% of the chromatographic peak area. In samples prepared by solid-phase microextraction, limonene (10.4%), β-pinene (9.6%), β-eudesmol (8.0%), sabinene (7.8%), cryptone (6.5%), α-pinene (5.5%) and o-cymene (5.2%) accounted for 53% of the extracted volatiles. The results revealed a complex volatile composition from which unique compounds may still be identified.

Comparison of the Feeding Behavior and Genetics of Beet Leafhopper, Circulifer tenellus, Populations from California and New Mexico

Abstract. The beet leafhopper, Circulifer tenellus (Baker), transmits curtoviruses that cause disease losses to several crops and many weeds in the western United States. While the host plants, life history, and possible migrations have been studied for many years, little is known about differences between leafhopper populations from different areas. Therefore, beet leafhopper populations from California and New Mexico were compared as to their morphology, feeding behavior, and genetics. The crown, wing venation, and male genitalia were indistinguishable among the leafhoppers collected from different locations. Four synonymous mutations were detected in the nucleic acid sequences of the mitochondrial cytochrome oxidase (mtcoi) region of leafhoppers between the states, but not among specimens collected from New Mexico. Feeding preferences of the leafhoppers from the two states differed when presented with cafeteria-style choice tests. With a 20-day feeding period, most (87.5%) of California leafhoppers were found on sugarbeets, Beta vulgaris L., while New Mexico leafhoppers were more evenly distributed on a variety of hosts such as Kochia sp. (46%); sugarbeets (18%); Russian thistle, Salsola tragus L. (15.3%); and red root pigweed, Amaranthus retroflexus L. (12.3%). With a 2-day feeding period, leafhoppers from both states equally chose sugarbeets, reflecting their willingness to settle on sugarbeets over most other hosts. The results suggested that while the beet leafhopper populations were similar morphologically, they showed difference in genetics and host feeding preferences.

Arboreal Spider Ecology on Saltcedar (Tamarix) at Big Spring, Howard County, Texas

Abstract. The arboreal species of spiders on saltcedar, Tamarix spp., at Big Spring, Howard County, TX, were surveyed to determine which species could pose a threat to the success of the biological control agent Diorhabda elongata (Brulle) (Coleoptera: Chrysomelidae). This resulted in a list of the spider genera and species found on tamarisk and some initial information on spider ecology in association with this weed tree. Of the 1,495 specimens of spiders collected above ground level during the 3 years of study at the site, 55 spider species in 15 families were found on saltcedar trees. The most abundant species of spiders were mapped to determine their habitat on the trees and in relation to other species. Orb-weavers (Araneidae) and jumping spiders (Salticidae) were the most diverse families, as well as being common, but mesh web-weavers (Dictynidae), while less diverse, were individually numerous. Only orb-weavers were observed to feed on D. elongata in the field.

A REVIEW OF THE SPIDER GENUS THIODINA (ARANEAE, SALTICIDAE) IN THE UNITED STATES

Abstract The jumping spider genus Thiodina in the United States is reviewed here and the common western species, T. hespera new species, is formally described for the first time. We present a species key for specimens north of Mexico.

Plant Utilization Patterns of a Guild of Leafhoppers on a Woody Plant Community in the Chihuahuan Desert

Abstract We examined plant use versus plant availability by a leafhopper guild from a Chihuahuan Desert plant community. Some native woody plants species of the Chihuahua Desert behave as weeds (e.g., Gutierrezia spp.) and could be subjects of augmentative biological control. As a first step, we wanted to know how the leafhopper guild used the woody plant community in the Chihuahua Desert to identify possible candidates to be studied. At least 37 leafhopper species were sampled on 13 woody and perennial plant species. Individual plants were sampled with sticky-traps on nine dates from June 1997 to July 1998. Leafhopper counts and plant availability were used to determine if leafhoppers used plants in proportion to their availability. Analysis of the 13 most abundant leafhopper species indicated differential use of at least one or more plant species in proportion to plant occurrence. Approximately 40% of all possible plant-insect pairs (169 pairs, 13 plants, and 13 insects) demonstrated under-utilization of plants (less than expected) by the leafhoppers. Nearly 50% of the pairs demonstrated plant use in proportion to their availability (random utilization), and in only 10% of pairs, leafhoppers used plants more often than expected. In our study site, 66.7% of the leafhopper species exhibited preference for one or two plant species, whereas the remaining 33.3% exhibited no preference for any studied plant. Our plant utilization results reaffirm published available host plant records for the leafhoppers analyzed. Qualitative temporal patterns in plant utilization were not detected. Specialist species exhibited different host uses in response to increasing leafhopper abundance. At seasonal peak abundance, specialist leafhoppers showed two different strategies: leafhoppers increased their preference toward their preferred host plants, and leafhoppers decreased their preference toward their preferred plants. The possible use of this methodology for augmentative biological control is discussed.

Revision of the jumping spider genus Sassacus (Araneae, Salticidae, Dendryphantinae) in North America

Abstract The nine species of Sassacus Peckham & Peckham 1895 known from Canada, the United States, and Mexico are described. The genus as defined here includes: Sassacus papenhoei Peckham & Peckham 1895, Sassacus paiutus (Gertsch 1934), Sassacus samalayucae Richman (new species), Sassacus cyaneus (Hentz 1846), Sassacus alboguttatus (F.O. Pickard–Cambridge 1901), Sassacus vitis (Cockerell 1894), Sassacus aztecus Richman (new species), Sassacus barbipes Peckham & Peckham 1888, and Sassacus lirios Richman (new species). Agassa Simon 1901 is synonymized with Sassacus. “Typical” (beetle-like) and more “normal” (spider-like) Sassacus range from Southern British Columbia south into Central America. The Sassacus arcuatus group, which is much more extensive in South America, possibly belongs to its own genus and is not treated beyond one species described here from Mexico.

Spiders of the Chihuahuan Desert of Southern New Mexico and Western Texas

Abstract Little information is available on spiders in the Chihuahuan Desert. In the Jornada del Muerto of southern New Mexico, we collected ca. 120 species of spiders in the past 30 years. We report four state records and five potential undescribed species. A comparison of our list of species with those from three other areas in the Chihuahuan Desert (White Sands National Monument and vicinity and Valley of Fires near Carrizozo and vicinity, both in New Mexico, and an area near Big Bend, Texas) revealed low overlap, indicating that the regional spider fauna is still not fully known. Baseline knowledge of the spider fauna from several sites could serve as a criterion for assessing climatic change.

Structure and Composition of a Thrips Community in the Chihuahua Desert, New Mexico, U.S.

ABSTRACT We examined plant used versus plant availability by a thysanopteran community on 13 woody and perennial native plants in the Chihuahua Desert. Individual plants were sampled with sticky-traps on 8 dates from May 1997 to August 1998. We sampled 5,040 adult thrips from 26 species in 19 genera, of which 16 could be identified to species. Four families were represented, Thripidae (17 species comprised 98.2 % of the collected specimens), Phlaeothripidae (5 species comprised 1.6%), Aeolothripidae (2 species comprised 0.1%) and Heterothripidae (1 species comprised 0.1%). A total of 16 species (84.2%) were phytophagous on flowers and leaves, 2 (10.5%) were predators, one (5.2%) was mycophagous. Feeding habits for 7 species are unknown. Thrips abundance was positively correlated with plant volume, but not with insect richness. Strikingly for a natural area, Frankliniella occidentalis accounted for 73.6% of the total collection of the sampled thrips, which together with Chirothrips falsus, Microcephalothrips abdominalis, Frankliniella gossypiana, and Neohydatothrips signifier, comprised 94.0% of the total number of collected thrips. Main abundances, considering all thrips species, occurred in fall and spring; no thrips were collected during winter. This seasonal pattern of occurrence was observed for the most abundant thrips species. In summary, the results of this study were: 1) few thrips species were found to be specialists; only 2 thrips species out of 12 studied, showed strong preference for host plants, 2) presence of a high percentage of positive associations, and a low percentage of negative associations, 3) the role of plant volume explained more than 80% of variance of thrips abundance. These results suggest that the studied thrips community has low plant specificity and the pattern of plant use observed could be the consequence of generalist feeding diets.

Effect of Lehmann lovegrass (Eragrostis lehmanniana) (Poaceae) on spider communities on upper bajada and basin grassland.

Abstract. Spiders were sampled in fall 2000 and spring 2001 from two areas (upper bajada and basin) of native grassland invaded by Lehmann lovegrass (Eragrostis lehmanniana Nees) on the Jornada del Muerto, Doña Ana County, NM. Sampling involved three amounts (dominant, intermediate, and minimal) of invasion by Lehmann lovegrass. Spiders were sampled in 5 × 5 m plots with three replications of the amounts of lovegrass infestation to determine if spider diversity was affected by an increase in invading grass. However, the prevalence of lovegrass did not seem to play an influential role in the numbers of species or individual spiders collected. Spider biodiversity and abundance are primarily influenced by prey availability and geometrical complexity of habitat, allowing proper hiding and building of web sites, and apparently Lehmann lovegrass compared to native grassland does not adversely affect either factor.