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

Longevidade e fecundidade de Dichroplus maculipennis (Orthoptera: Acrididae: Melanoplinae) em situações de "non-outbreaking" e "outbreaking"

Abstracts

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) than in 2009 (37.44 ± 1.98 days). The number of egg-pods per female was 3.32 ± 0.44 for 2006 and 1.62 ± 0.26 for 2009. The average fecundity in 2006 (89.29 ± 11.9 eggs/female) was significantly greater (p < 0.05) than that in 2009 (36.27 ± 5.82 eggs/female). While it was observed that the oviposition rate was higher in 2006, this difference was not significant (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.

Fecundity; grasshopper; longevity; Melanoplinae

O gafanhoto Dichroplus maculipennis é um dos mais característicos e prejudiciales da Argentina, principalmente nas regiões das Pampas e da Patagonia. O objetivo deste estudo foi estimar e comparar a longevidade e fecundidade das fêmeas adultas do D. maculipennis sob condições controladas (30ºC, 14 luz: 10 obscuridad, 40% RH) a partir do indivíduos coletados em campo como ultimo estadio ninfal (VI) e con historia recente conocida de baixas e altas densidades. As densidades de D. maculipennis nos locais de coleta foram de 0.95 indivíduos por m² em 2006 e 46 indivíduos/m² em 2009, representando situações de "non-outbreaking" e "outbreaking", respectivamente. A longevidade das fêmeas adultas em 2006 (67.96 ± 3.2 dias) foi significamente maior (p < 0.05) do que as de 2009 (37.44 ± 1.98 dias). O número de oviposiçãos por fêmea foi de 3.32 ± 0.44 para 2006 e 1.62 ± 0.26 para 2009. A fecundidade média em 2006 (89.29 ± 11.9 ovos/fêmea) foi significativamente maior (p < 0.05) do que a das fêmeas de 2009 (36.27 ± 5.82 ovos/fêmea). As curvas de fecundidade mostraram que os valores mais elevados ocorreram nas 11ª e 13ª semanas de 2006, e na 6ª semana para àquelas de 2009. Comparando a taxa diária de oviposição para altas e baixas densidades, não apresentaram diferenças significativas (p > 0.05). No entanto, as baixas taxas de fecundidade das fêmeas em densidades elevadas, se devem, provavelmente à longevidade reduzida.

Fecundidade; gafanhoto; longevidade; Melanoplinae

SHORT COMMUNICATION

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

Longevidade e fecundidade de Dichroplus maculipennis (Orthoptera: Acrididae: Melanoplinae) em situações de "non-outbreaking" e "outbreaking"

Yanina Mariottini^I; María Laura De Wysiecki^I; Carlos Ernesto Lange^I,II

^ICentro de Estudios Parasitológicos y de Vectores (UNLP- CCT La Plata- CONICET). Calle 2 Nº 584, Código postal 1900, La Plata, Argentina. [e-mail], [e-mail]

^IIComisión de Investigaciones Científicas provincia de Buenos Aires. [e-mail]

ABSTRACT

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) than in 2009 (37.44 ± 1.98 days). The number of egg-pods per female was 3.32 ± 0.44 for 2006 and 1.62 ± 0.26 for 2009. The average fecundity in 2006 (89.29 ± 11.9 eggs/female) was significantly greater (p < 0.05) than that in 2009 (36.27 ± 5.82 eggs/female). While it was observed that the oviposition rate was higher in 2006, this difference was not significant (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.

Keywords: Fecundity; grasshopper; longevity; Melanoplinae.

RESUMO

O gafanhoto Dichroplus maculipennis é um dos mais característicos e prejudiciales da Argentina, principalmente nas regiões das Pampas e da Patagonia. O objetivo deste estudo foi estimar e comparar a longevidade e fecundidade das fêmeas adultas do D. maculipennis sob condições controladas (30ºC, 14 luz: 10 obscuridad, 40% RH) a partir do indivíduos coletados em campo como ultimo estadio ninfal (VI) e con historia recente conocida de baixas e altas densidades. As densidades de D. maculipennis nos locais de coleta foram de 0.95 indivíduos por m² em 2006 e 46 indivíduos/m² em 2009, representando situações de "non-outbreaking" e "outbreaking", respectivamente. A longevidade das fêmeas adultas em 2006 (67.96 ± 3.2 dias) foi significamente maior (p < 0.05) do que as de 2009 (37.44 ± 1.98 dias). O número de oviposiçãos por fêmea foi de 3.32 ± 0.44 para 2006 e 1.62 ± 0.26 para 2009. A fecundidade média em 2006 (89.29 ± 11.9 ovos/fêmea) foi significativamente maior (p < 0.05) do que a das fêmeas de 2009 (36.27 ± 5.82 ovos/fêmea). As curvas de fecundidade mostraram que os valores mais elevados ocorreram nas 11ª e 13ª semanas de 2006, e na 6ª semana para àquelas de 2009. Comparando a taxa diária de oviposição para altas e baixas densidades, não apresentaram diferenças significativas (p > 0.05). No entanto, as baixas taxas de fecundidade das fêmeas em densidades elevadas, se devem, provavelmente à longevidade reduzida.

Palavras-chave: Fecundidade; gafanhoto; longevidade; Melanoplinae.

The melanopline grasshopper Dichroplus maculipennis (Blanchard, 1851) is one of the most widely distributed species of the genus, occurring in southernmost Brazil (Rio Grande do Sul), much of Argentina and Chile, and Uruguay. In Argentina, D. maculipennis is distributed in a triangle that stretches from Buenos Aires and Entre Rios provinces in the East to southern Catamarca province in the North, and southern Santa Cruz province in Patagonia (Carbonell et al. 2006). The species is considered one of the most characteristic and damaging of Argentina, mainly in areas of the Pampas and Patagonia regions (Liebermann 1972; Lange et al. 2005). According to the categories employed for defining the pest status of grasshopper species (COPR 1982), Carbonell et al. (2006) have categorized D. maculipennis as a major pest of several crops (barley, rye, oats, wheat, flax, lucerne) and forage on natural pastures. It is a polyphagous and univoltine species that presents obligatory embryonic diapause (COPR 1982).

Although some studies conducted in the Pampas during the 90s' suggested a possible decrease in the abundance of D. maculipennis (Cigliano et al. 1995; Cigliano et al. 2000), starting in 2008 an outbreak occurred in parts of the southern Pampas, where densities reached up to 50 individuals per m² and en masse flights were widespread. Unlike other melanoplines of Argentina, adults of D. maculipennis at high densities are historically known to show significant aggregation behavior and dispersal flights, covering distances that can exceptionally reach 50 kilometers (Joan 1927; Schiuma 1938; Daguerre 1940; Liebermann 1972; COPR 1982).

Different aspects of the life cycle of grasshoppers, such as longevity and fecundity, may vary in relation to a great variety of factors, including population density (Joern and Gaines 1990). Life histories may also vary because of genetic and environmental factors acting on previous and present generations (Chapuis et al. 2010). The goal of this study was to estimate and compare the longevity and fecundity of adult female D. maculipennis under controlled conditions, but from individuals collected in the field as juveniles just immediately preceding adulthood (nymphal instar VI) with a known recent history of low and high density conditions, respectively.

Individuals of D. maculipennis employed in the experiences were collected with entomological nets as last instar nymphs (VI) in natural and improved pastures at the locality of Laprida (36º02'S, 59º06'W), Buenos Aires province, in the southern Pampas region as defined by Morrone (2006). Collecting times were late December 2006 and 2009. The state of the D. maculipennis population (non-outbreaking versus outbreaking) at collecting times was determined by estimating density (individuals/m²) through the rings method developed by Onsager and Henry (1977) and by field observations of aggregation and dispersal behavior. Since D. maculipennis is univoltine and the outbreak appeared to begin in November 2008, the individuals collected in 2009 corresponded to the second generation of the outbreaking state.

Once in the laboratory, individuals of both collections received the same treatment. Immediately after entering adulthood, 25 couples (one male, one female) were formed corresponding to each of both collecting times. Each couple was placed in a wire-screened, aluminium cage (12x12x16 cm) in a rearing room under controlled conditions (30ºC, 14L:10D, 40% RH) of wide use (Henry 1985; Hinks & Erlandson 1994; Mariottini et al. 2010). The grasshoppers were fed daily with thoroughly washed, fresh leaves of a variety of grasses, lettuce, cabbage, and wheat bran flakes. Each cage was provided with a substrate for egg-pod laying that consisted of a plastic container (10 cm deep) filled with sterilized sand. Thermoregulation and mating was stimulated with 75W bulbs suspended 15 cm above each cage. Couples were maintained until death. Once a week the containers were sifted, the egg-pods recovered, and the number of eggs per pod counted. We recorded longevity of females, number of egg-pods per female, and number of eggs per pod, and estimated the average fecundity (number of eggs/female), the oviposition rate (eggs/female/day), and the fecundity curve (m_{x =} mean number of eggs per female of x age). In order to compare longevity of females, fecundity, and oviposition rate the parametric test of ANOVA by ranks was employed. Statistical tests were performed using the XLSTAT 7.5.3 program (Addinsoft, 2005). Results are expressed as mean value ± SE.

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. As shown in table I, adult female longevity was 67.96 ± 3.2 (range = 32–90) and 37.44 ± 1.98 (25–59) days in 2006 and 2009, respectively, a significant difference (F = 64.25, df = 49, p = 0.0001). The number of egg-pods per female was 3.32 ± 0.44 (range = 0–8) for 2006 and 1.62 ± 0.26 (0–4) for 2009. The average fecundity of females in 2006 (Table I) was significantly greater than that of females in 2009 (F = 13.045, df = 49, p = 0.001). While it was observed that the oviposition rate was higher in females of 2006, this difference was not significant (F = 0.652, df = 49, p = 0.423). The fecundity curves (Fig. 1) showed that the highest values were at weeks 11 and 13 for the 2006 females, and at week 6 for those of 2009.

Results of this study show that adult females of D. maculipennis collected at high density had significant lower longevity and fecundity than adult females collected at low density. Since the daily oviposition rate of females at low and high densities was not significantly different, the diminished fecundity rate of females at high density is attributable to their reduced longevity. Some authors have reported similar results in other species. Antoniou (1973, 1978) observed under laboratory conditions that females of Ornithacris turbida (Walker, 1870) and Humbe tenuicornis (Schaum, 1853) reared at low density lived longer, laid more egg-pods, and had a greater fecundity than those reared in crowded conditions, but also displaying a similar weekly rate of oviposition at both densities. Smith (1970, 1972) reported that the number of egg-pods lay by the females of Melanoplus sanguinipes (Fabricius, 1798) decreased with crowding within a generation because females lived a shorter time and had a lower oviposition rate.

Grasshoppers exhibit life history variation in response to a variety of factors (Stauffer & Whitman 1997; Branson 2003). Increased population density frequently leads to physiological changes that are expressed in coloration, morphology, development rate, metabolism, and behavior (Applebaum & Heifetz 1999; Sword 2005; Chapuis et al. 2008; Song & Wenzel 2008). The phase polyphenism observed in locusts, an extreme case of phenotypic plasticity, is attributed to changes in population densities of both parents and offspring (Song &Wenzel 2008; Chapuis et al. 2008; Chapuis et al. 2010). Unlike locusts, grasshoppers do not exhibit phase polyphenism (Song & Wenzel 2008), but may express various changes (including those affecting reproductive output) in response to population density that differ in different species, some responding intensely to crowding, others only partially (Applebaum & Heifetz 1999).

One possible explanation for the decrease in longevity and fecundity of females that were found at high density could be the existence of a "trade-off" between early reproduction and longevity. Although in our experiments females of both groups began to oviposit after the fourth week of adulthood, the fecundity curves show that at high density the higher values of fecundity were at week 6 while at low density they were at weeks 11–13. Reproduction is assumed to carry a cost, in terms of future survival, growth or fecundity (De Souza Santos & Begon 1987; Forsman 2001). Insects can accelerate the reproduction or maturation if they inhabit seasonal environments with relatively short growing seasons. However, early reproduction has costs, such as reduced longevity or diminished investment in the number of eggs (De Souza Santos & Begon 987; Homeny & Juliano 2007). Chapuis et al. (2010) recorded that female Locusta migratoria (Linnaeus, 1758) with a crowding history reproduced earlier (by reducing oviposition times) than females with an isolation history.

Another hypothesis that could explain the decrease in longevity and fertility in D. maculipennis at high density might be related to en masse dispersal. In several species of Acrididae, significant increases in population size trigger flight dispersal or migratory capabilities (Farrow 1990; Sword 2005). One of the costs that have been associated to en masse flight dispersal is that migrants often have a decrease in reproductive fitness in relation to non-migrants (Rankin & Burchsted 1992). As it was already mentioned, D. maculipennis readily becomes migratory in outbreak situations (COPR 1982).

Results of this study represent a considerable advance in understanding the biology of D. maculipennis. However, given the ecological and economic importance of the species in Argentina, further research is envisaged, particularly regarding the central factors that govern the conspicuous size fluctuations of its populations.

ACKNOWLEDGMENTS

This research was funded in part by "Fondo para la Investigación Científica y Tecnológica (FONCYT, project PICT 2006/679)" and "Comisión de Investigaciones Científicas (CIC) de la provincia de Buenos Aires".

Received 26/7/2010; accepted 26/7/2011

Editor: Mauricio Osvaldo Moura

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Publication Dates

• Publication in this collection
  09 Sept 2011
• Date of issue
  Sept 2011

History

• Accepted
  26 July 2011
• Received
  26 July 2010