María Laura de Wysiecki

Grasshopper (Orthoptera: Acrididae) community composition and temporal variation in the Pampas, Argentina

Abstract To evaluate temporal changes in grasshopper communities over a 5-y period (1997–2001), density and species relative abundance were estimated at 27 sites in Benito Juárez county, Southern Pampas, Argentina, established across vegetation associations with different disturbance histories. A total of 22 grasshopper species were collected, Melanoplinae being the most abundant and diverse subfamily, followed by Gomphocerinae, Acridinae, Copiocerinae and Leptysminae. Results allowed us to categorize years into nonoutbreak (1997, 1998, 1999), transitional (2000) and outbreak (2001) based on the spatiotemporal characteristics of grasshopper density trends. On average, grasshopper density was over 7 times greater in 2001 than in nonoutbreak years. During nonoutbreak years, grasshopper densities showed no significant variation across disturbance categories. During the outbreak year, densities were significantly higher in pastures and halophilous steppes, with highly disturbed pastures being the most affected sites of all. Species richness changed significantly in outbreak versus nonoutbreak years: nearly twice as many species were collected during the outbreak year. Although 22 species were collected, results showed that a small number influence overall abundance from nonoutbreak to outbreak conditions. Although Dichroplus elongatus, Covasacris albitarsis, and Scotussa lemniscata were the three topranked species throughoutthe studied period, D. elongatus contributed most to overall shifts in grasshopper density.

Influence of weather variables and plant communities on grasshopper density in the Southern Pampas, Argentina.

Abstract A study was conducted to evaluate the influence of weather (precipitation and temperature) and plant communities on grasshopper density over a 14-year period (1996–2009) in Benito Juárez County, Southern Pampas, Argentina. Total density strongly varied among plant communities. Highest values were registered in 2001 and 2003 in highly disturbed pastures and in 2002 and 2009 in halophilous grasslands. Native grasslands had the lowest density values. Seasonal precipitation and temperature had no significant effect on total grasshopper density. Dichroplus elongatus (Giglio-Tos) (Orthoptera: Acridoidea), Covasacris pallidinota (Bruner), Dichroplus pratensis Bruner, Scotussa lemniscata Stål, Borellia bruneri (Rehn) and Dichroplus maculipennis (Blanchard) comprised, on average, 64% of the grasshopper assemblages during low density years and 79% during high density years. Dichroplus elongatus, S. lemniscata and C. pallidinota were the most abundant species in 2001, 2002 and 2003, while D. elongatus, B. brunneri and C. pallidinota in 2009. Dichroplus elongatus and D. pratensis, mixed feeders species, were positively affected by summer rainfall. This suggests that the increase in summer precipitation had a positive effect on the quantity and quality forage production, affecting these grasshopper populations. Scotussa lemniscata and C. pallidinota were negatively affected by winter and fall temperature, possibly affecting the embryonic development before diapause and hatching. Dichroplus elongatus and D. pratensis were associated with highly disturbed pastures, S. lemniscata with pastures and B. bruneri and D. maculipennis with halophilous grasslands. Covasacris pallidinota was closely associated with halophilous grasslands and moderately disturbed pastures. Weather conditions changed over the years, with 2001, 2002 and 2003 having excessive rainfall while 2008 and 2009 were the driest years since the study started. We suggest that although seasonal precipitation and temperature had no significant effect on total grasshopper density, these weather variables and plant communities had differential influence on the dominant grasshopper species.

Life History Parameters of the Gregarious Phase of the South American locust, Schistocerca cancellata (Serville) (Orthoptera: Acrididae), under Laboratory Conditions.

A laboratory study was conducted to determine some life history parameters of the gregarious phase of the South American locust, Schistocerca cancellata. An age-specific life table was constructed based on laboratory data. Duration of egg, nymphal and adult female and male stages was 15?1.005, 33.24?1.302, 87.60+34.84, and 89.80?26.85 days, respectively. The mean life of the cohort was 175 days, and sex ratio of emerging adults was 1:1. Survivorship curve showed a high survival during most of the life time of the cohort. 9.10 % of the cohort mortality occurred during nymphal instars V and VI and 90.90 % during adult stage, most of it during the last 5 weeks. Mean fecundity was 160.7+93.10 eggs/female and the number of eggs per pod was 73+18.94. Maximum values of the fecundity rates were reached at the age of 17 and 18 weeks. The reproductive values curve indicated that the highest contribution to the future population occurred at week 17. The values of the intrinsic rate of increase (r), the net reproductive rate ( R 0) and the mean generation time ( T ) were 0.2906 _ 0.0027, 130.43 ? 9.71, and 17.78 ? 0.2281, respectively.

The biology and some population parameters of the grasshopper, Ronderosia bergi, under laboratory conditions.

Abstract Some biological and population parameters of Ronderosia bergi (Stål) (Orthoptera: Acrididae: Melanoplinae) were estimated by monitoring five cohorts of the first generation (F1) of individuals born in captivity from grasshoppers collected in the South of Misiones province, northeastern Argentina, and held under controlled conditions (30° C, 14:10 L:D, 40% RH). The mean embryonic development time was 40.6 ± 1.7 days. Five nymphal instars were recorded. Total duration of nymphal development was 30.8 ± 0.54 days. The mean lifespan of cohorts was 22.6 ± 0.7 weeks. The number of egg-pods per female was 7.6 ± 1.44, and the amount of eggs per egg-pod was 16.45 ± 0.85. Mean fecundity was 125 ± 5.83 eggs per female with an oviposition rate of 1.55 ± 0.57 eggs/female/day. Survivorship curves showed that mortality was concentrated in the final weeks of adulthood, and the life expectancy curve decreased accordingly. The population parameters estimated gave the following values: the net rate of reproduction (R0) was 46.75 ± 11.2, generation time (T) was 18.87 ± 1.67 weeks, duplication time (D) was 3.31 ± 0.34, the intrinsic rate of population growth (rm) was 0.21 ± 0.021 and the finite rate of population increase (&lgr;) was 1.24 ± 0.026. The reproductive values (Vx) indicated that the largest contribution of females to the subsequent generation was between weeks 15 and 25.

Longevity and fecundity of Dichroplus maculipennis (Orthoptera, Acrididae) at non-outbreaking and outbreaking situations

Dichroplus maculipennis is one of the most characteristic and damaging grasshopper species of Argentina, mainly in areas of the Pampas and Patagonia regions. We estimated and compared the longevity and fecundity of adult female D. maculipennis under controlled conditions (30oC, 14L:10D, 40% RH) from individuals collected as last instar nymphs (VI) in the field and with a known recent history of low and high density conditions. Densities of D. maculipennis at the collecting sites were 0.95 individuals per m2 in 2006 and 46 ind/m2 in 2009, representing non-outbreaking and outbreaking situations, respectively. Adult female longevity in 2006 (67.96 ± 3.2 days) was significantly higher (p 0.05). The fecundity curves showed that the highest values were at weeks 11 and 13 for the 2006 females, and at week 6 for those of 2009. Since the daily oviposition rate at low and high densities was not significantly different, the diminished fecundity rate at high density is attributable to their reduced longevity.

Orthoptera assemblages associated with macrophytes of floodplain lakes of the Paraná River

Orthoptera assemblages associated with macrophytes of floodplain lakes of the Parana River. The Orthoptera assemblage composition varies considerably, depending on habitat type. This study examines the spatiotemporal relationship between plant diversity, hydrometric level, environmental variables and the Orthoptera richness and abundance in floodplain lakes connected permanently or temporarily with the main channel of the Parana River. The grasshoppers were collected fortnightly (April 2006May 2007). A total of 17 species were recorded and classified according to their frequency of occurrence in constant (7), accessory (4), or accidental (6) species. In the two lakes, the greater species richness and abundance was recorded in summer, thereby coinciding with the highest water level of the Parana River. The most significant correlation between the orthopteran richness and abundance was with the water level. The aquatic plant richness was significantly different between the lakes, but the vegetation was dominated by Eichhornia crassipes (Mart.) Solms. (Liliales, Pontederiaceae). The lake, which was connected permanently, presented the highest values of diversity and abundance, proving to be a more diverse assemblage. The beta diversity was higher in the temporary connected lake than in the permanently connected one. The orthopterans assemblages were different between the lakes, Cornops aquaticum and Tucayaca gracilis were the species that contributed more to the level of dissimilarity. C. aquaticum was more representative in the lake temporarily connected, while T. gracilis in the permanent connected one. The water level of the Parana River and the connectivity of the floodplain lakes play an important role to explain the abundance and richness of their orthopteran assemblages.