Effects of phyllophaga (phytallus) hoegei larval density (coleoptera:melolonthidae) on plant and herbivore growth.

Fabiola Ramírez-Corona and Alejandro Morón-Ríos

Fabiola Ramírez-Corona. Biologist, Benemérita Universidad Autónoma de Puebla (BUAP) M.Sc. in Ecology, Instituto de Ecología, UNAM, México. Lecturer. Facultad de Ciencias, UNAM, México. e-mail: framirez@ecologia.unam.mx

Alejandro Morón-Ríos. Biologist and Ph.D. in Ecology, UNAM, México. Researcher, El Colegio de la Frontera Sur (ECOSUR), México. Dirección: ECOSUR, Unidad Campeche, Calle 10x61 Nº 264, Col. Centro, CP 24000 Campeche, México. e-mail: amoron@ecosur.mx

Summary

The belowground herbivory has received relatively little attention. The plant response to belowground herbivory is variable and partially mediated by the identity and density of the root feeder. This study evaluated 1) the effects of belowground herbivory by two densities of the common root feeder, Phyllophaga (Phytalus) hoegei on the plant biomass, the nitrogen content of tillers and roots of the native, dominant grass Muhlenbergia quadridentata; and 2) the effects of these two larval densities on the survival and growth of this species. The experiment was performed in a pine forest area at 3200masl. Plants were established in pots in the field and subjected to belowground herbivory (three levels) in a completely random design with ten replicates per treatment. High densities of the root feeder significantly decreased root biomass and the root/shoot ratio, but the herbivores did not affect the nitrogen concentration of plant tissues. Survivorship of Phyllophaga hoegei larvae was not affected by growth density, but at high density the root feeder larvae decreased the relative growth rate and the weight gained.

Efectos de la densidad de larvas de phyllophaga (phytallus) hoegei (coleoptera:melolonthidae) en el crecimiento de plantas y herbívoros.

Resumen

La herbivoría subterránea ha recibido poca atención. La respuesta de las plantas a la herbivoría subterránea es variable y mediada parcialmente por la identidad y densidad del rizófago. En este estudio se evaluaron 1) los efectos de la herbivoría subterránea de dos densidades de Phyllophaga (Phytalus) hoegei en la biomasa de la planta y el contenido de nitrógeno de tallos y raíces del pasto nativo dominante Muhlenbergia quadridentata, y 2) los efectos de esas dos densidades de larvas en la supervivencia y crecimiento de esta especie. El experimento se llevó a cabo en un área de bosque de pino a 3200msnm. Las plantas se establecieron en macetas en el campo y estuvieron sujetas a herbivoría subterránea (tres niveles) en un diseño completamente al azar con diez repeticiones por tratamiento. Altas densidades del rizófago disminuyeron significativamente la biomasa de raíces y la proporción raíz/tallo, pero no varió la concentración de nitrógeno en los tejidos de las plantas. La sobrevivencia de las larvas de Phyllophaga hoegei no fue afectada por la densidad a la que crecieron, pero en altas densidades, la larva rizófaga disminuyó su tasa de crecimiento relativa y la ganancia de peso.

Efeitos da densidade de larvas de phyllophaga (phytallus) hoegei (coleoptera: melolontidae) no crescimento de plantas e herbívoros.

Resumo

A herbivoria subterrânea tem recebido pouca atenção. A resposta das plantas à herbivoria subterrânea é variável e mediada parcialmente pela identidade e densidade do rizófago. Neste estudo se avaliaram: 1) os efeitos da herbivoria subterrânea de duas densidades de Phyllophaga (Phytalus) hoegei na biomassa da planta e o conteúdo de nitrogênio de caules e raízes do pasto nativo dominante Muhlenbergia quadridentata e; 2) os efeitos dessas duas densidades de larvas na sobrevivência e crescimento desta espécie. O experimento se realizou em uma área de bosque de pinheiros a 3.200 msnm. As plantas se estabeleceram em vasos no campo e estiveram sujeitas a herbivoria subterrânea (três níveis) em um desenho completamente aleatório com dez repetições por tratamento. Altas densidades do rizófago diminuíram significativamente a biomassa de raízes e a proporção raiz/caule, mas não variou a concentração de nitrogênio nos tecidos das plantas. A sobrevivência das larvas de Phyllophaga hoegei não foi afetada pela densidade com a qual cresceram, mas em altas densidades, a larva rizófaga diminuiu sua taxa de crescimento relativa e o ganho de peso.

Key Words / Belowground Herbivory / Herbivory / Muhlenbergia quadridentata / White Grubs /

Received: 12/05/2006. Modified: 09/08/2007. Accepted: 09/08/2007.

Introduction

Belowground herbivory could play an important role in the determination of vegetal species richness and abundance (Brown and Gange, 1989; Blossey and Hunt-Joshi, 2003; De Deyn et al., 2003; van der Putten, 2003; Schädler et al., 2004) inasmuch as it can change the patterns of carbon and nitrogen allocation of the plants, and affect the recruiting process, the mortality rate and the competitive ability of the plants (Huntly, 1991; Hunter, 2001; Blossey and Hunt-Joshi, 2003). These effects of belowground herbivory on plant functioning depend on the root-feeder identity and density. Also, they depend on whether the herbivore, the plant or both are introduced or native species. Root feeders could significantly reduce plant production and even kill the plant at high densities (Ueckert, 1979; Murray et al., 2002; Blossey and Hunt-Joshi, 2003, Dawson et al., 2004). High larval densities are the result of the root feeder aggregated spatial distribution pattern (Andersen, 1987; Brown and Gange, 1990). Under this condition root-feeder larvae significantly reduce their own survival and growth rates due to intraspecific competition for food (Règniére et al., 1981; Brown and Gange, 1990).

Larvae of Phyllophaga (Coleoptera:Melolonthidae) are widespread native herbivores which consume roots of wild plants, crops, ornamentals, orchards and forest plantations, as well as decaying plant material (Morón, 1986). Their life cycle lasts one year in tropical regimes or up to three years in temperate areas. Oviposition takes place from late spring to early summer, eggs are laid near the host plant or in sites with high organic matter content. The species have three larval stages, of which the third is the longest (Ritcher, 1957; Morón, 1986). In Mexico, the larvae of Phyllophaga are common in a variety of environments, including coniferous forests and subalpine grasslands.

Muhlenbergia quadridentata (H.B.K.) Kunth is a perennial C₃ bunchgrass widespread species in the understory of pine and fir forests of Mexico, and has an adequate forage value for cattle (Beetle et al., 1995). The pine forest study site is representative of other high altitude forests in central Mexico. Here Phyllophaga (Phytalus) hoegei is the dominant species of the macroarthropod root feeding community (Morón-Ríos et al., 1997) and M. quadridentata is one of the dominant grass species of the forest understory (Obieta and Sarukhán, 1981). The response of M. quadridentata to artificial defoliation and belowground herbivory has been previously reported (Morón-Ríos et al., 1997).

In the present work the following questions were addressed: a) What are the effects of belowground herbivory by two densities of Phyllophaga (Phytalus) hoegei (Bates) on the plant biomass and the nitrogen content of tillers and roots of the grass M. quadridentata? b) What are the effects of these two larval densities on the survival and the growth of this insect?. The hypothesis to be tested were that: a) high Phyllophaga larval densities would decrease the biomass and the nitrogen content of the plant, and b) the growth of Phyllophaga larvae would decrease at high larval densities.

Materials and Methods

Study area

The study site was located in the Zoquiapan Experimental Forest Station, 98°45’W and 19°30’N. The station is in the Zoquiapan National Park, at the Neovolcanic Belt of Central Mexico, 3000-3500masl (Franco and Búrquez, 1981). Mean annual precipitation is 1169mm and mean annual temperature is 11°C. The vegetation is a pine forest dominated by Pinus hartwegii Lindl. The understory is dominated by the grasses Muhlenbergia quadridentata (Kunth) Trin. and Festuca tolucensis H.B.K.; both form a characteristic grassland-like plant community known locally as "zacatonal". These grasses are grazed by cattle all year round but there is no available information on the intensity and frequency of grazing. Other grass species of the same genera are also present along with several herbaceous species (Obieta and Sarukhán, 1981).

Experimental protocol

Thirty individuals of Muhlenbergia of 38 ±4cm height and 36 ±18 tillers (mean ± SD) were randomly collected in the study site. Plants were taken from the field and then potted in plastic containers (19cm diameter and 16cm deep) with native sieved soil mixed with sand (2:1) to improve pot drainage and to make root recovery easier. Plants were allowed to grow for one month before initiating the experiment. Plant height, basal area and number of tillers (live and dead) were recorded at the beginning of the experiment.

In November 1993, 300 third instar larvae of Phyllophaga were collected at the same site. Larvae were selected on the basis of their appearance and size. The individuals with necrotic spots or low mobility were discarded. Larvae were maintained for one week in pots with native soil to guarantee that they had not been damaged. The last instar larvae of this species as well as other macroarthropod root feeding species were kept alive to obtain adults for precise identification. Another species of Phyllophaga was found in the soils of the study site, but their larval characteristics were clearly different from the studied species (Morón-Ríos et al., 1997).

The experiment had a completely random design with a simple treatment arrangement. Belowground herbivory treatments were 1) control without larvae (C), 2) low larval density with 10 individuals per pot (D1), and 3) high larval density with 20 individuals per pot (D2).

Plants under field conditions can experience densities ranging from 0 to 25 larvae/plant because root feeders have a highly aggregated distribution pattern (Morón-Ríos et al., 1997). Mean initial weight of the 30 groups of larvae (ten larvae/group) was 1.42 ±0.006g.

The experimental units (pot with a single plant) were randomly assigned to treatments with ten replicates each. The experiment was carried out at the Forest Station within a fenced area from Nov 1993 to Jan 1994. Above-ground insect herbivores were generally absent in the experimental plot during the study period. Also, the adult phase of other root-feeding insects ended one or two months before the experiment was set.

A pilot trial showed that pots placed on the soil surface had higher larval mortality than pots located in excavated pits (Ramírez-Corona, 1995). Thus, the experimental pots were placed in soil pits of 19cm diameter and 10cm depth to provide larvae with microclimatic conditions similar to their natural environment. Plants were watered weekly. At harvest, dead and live tillers were counted. Roots were detached from the crown and were carefully washed to remove soil. Care was taken to recover fine roots. All plant material was dried at 70ºC until their weight stabilized. Larvae were counted and carefully rinsed to eliminate adhered soil on their body. They were dried with paper towels, weighed fresh and fixed with Pampel solution. Total nitrogen concentration of roots and live tillers was determined with a micro-Kjeldahl procedure (Bradstreet, 1965).

Statistical analyses

Data analysis was performed with a one way ANOVA. Variables were log transformed to satisfy homogeneity of variances and normality criteria. The Tukey test was applied when the ANOVA showed significant (P<0.05) treatment effects. An ANCOVA was done to estimate the potential effect of initial plant height on root production; the covariates were initial plant height and initial shoot number.

The biomass increase of root feeders was calculated as the mean difference between the final and the initial weights, and with the relative larval growth rate (RLGR; Ridsdill Smith and Roberts, 1976). These data were evaluated with the "U" Mann-Whitney test. Statistical tests were done using SPSS (v.3). All data are presented in their original scale of measurement.

Results and Discussion

Effects on the plant

The initial plant height and the initial shoot number did not affect root production (ANCOVA F_1.23= 0.608, P>0.05 for height data; and F_1.23= 1.23, P>0.05 for total shoot number). Root biomass significantly decreased in larval treatments (ANOVA F_2.25= 4.52, P<0.02; Table I). Root biomass decreased 46% in high larval density and 26% in low larval density under conditions of this experiment. These results are similar to those of Davidson et al. (1970) for Lolium perenne using third instar larvae of Sericesthis geminata at high densities, which damaged more than 80% of grass roots. Ueckert (1979) reported a belowground biomass reduction of 43% in perennial grasses with densities of 46.3 larvae of Phyllophaga crinita per m². However, low larval densities can also cause a major reduction in root production, as Ridsdill Smith (1977) showed for Lolium perenne.

The damage produced on plants by root-feeding insects could be severe when environmental conditions are more favorable for the attacked plants (Davidson, 1969; Davidson and Roberts, 1969). In our study, the plants were exposed to harsh environmental conditions, because they grew during the winter season on the forest floor at an altitude of 3300masl. Also, the belowground herbivory damage to plants could be overestimated, because the studies of Davidson (1969) and Davidson and Roberts (1969) were done with non native species (Radcliffe, 1971a, b; Brown and Gange, 1990).

The aboveground plant components, total shoot biomass, live shoot biomass, live shoot number, dead shoot number and total shoot number were not significantly affected by the applied treatments (Table I) under conditions of this experiment.

High density of root feeders significantly reduced the root/shoot ratio (ANOVA F_(2.25)= 5.80, P<0.025), suggesting that the plants did not markedly change their carbon distribution pattern; the plants maintained their aboveground biomass even with a 46% reduction in belowground biomass. Detling et al. (1980) showed Bouteloua gracilis distributed photosyntetically fixed C¹⁴ toward the roots, promoting fast adventitious root growth three weeks after the simulation of root herbivory. The decrease of root/shoot ratio may have adverse effects in different plant traits on a long term basis; one of them could be a poorer competition capacity compared to other grass or herb species. Decreased root biomass and shoot number by insect larvae consumption can change competitive interactions in many species (Dawson et al., 2004; Richmond et al., 2004). In the study site, the role of the belowground herbivory on the competitive interactions between the studied species and other grasses or herbs is an important and unattended research theme.

Nitrogen content

Nitrogen content of live shoots and roots was not significantly affected by the densities of belowground herbivores (Table II). This suggests that despite root loss, the plants kept absorbing nitrogen and distributing it toward the aboveground plant structures, as happens with the plants without herbivory. A lower demand of nitrogen in the plant because of a scarce production of shoots and roots in the winter could also explain the maintenance of nitrogen content in the plants even after root loss. The plant nitrogen allocation changes generated by root feeders vary according to the plant species studied, the intensity of herbivory and the type of damage produced for the root feeder (i.e. fluid removal by aphids vs. tissue removal by white grubs; Preus and Morrow, 1999; Dawson et al., 2004).

Effects on the herbivores

Larval survival was not affected by the two insect densities used in this study (Table III). Larvae grown at D1 showed a weight gain and presented a major relative growing rate (RGR) compared to larvae growing at D2. There were probably differences in their assimilation rates. Larvae did not experience changes in food quality as measured here because their feeding activity did not significantly alter nitrogen content (Table II). The survival of third instar larvae of Sericesthis nigrolineata growing on Lolium perenne roots was not affected by different larval densities. A smaller growth rate at high densities was attributed to a shortage of roots of sufficient quality (Ridsdill Smith and Roberts, 1976). Other root feeding species growing at high densities compete for food and significantly reduce their survival and growth rate (Règniére et al., 1981; Brown and Gange, 1990; Blossey et al., 2000). It has also been reported that larvae feeding at low nitrogen content conditions did not grow, but maintained body mass (Mattson, 1980). A possible consequence of the low weight of larvae under treatment D2 could be a longer time to reach the pupal stage and even smaller adults. However, since third-instar larvae can live for up to 14 months, a longer experiment would be required to evaluate this possibility.

The feeding activity of root herbivores play an important role not only for the direct effects on the plant, but also on amounts of carbon input to soil, potentially affecting microbial populations and nutrient availability (Grayston et al., 2001; Treonis et al., 2005).Our results show that Muhlenbergia quadridentata can tolerate a removal of ~30% of the root mass but that a major decrease significantly affects root biomass. The root herbivory did not change the nitrogen content and the aboveground biomass of the plant, suggesting that the plant can tolerate the recorded levels of root loss. These findings suggest that the root herbivory is not always as detrimental as may be inferred from previous studies.

Acknowledgements

The authors are grateful to Rodolfo Dirzo for valuable comments on previous stages of this document, Emerit Meléndez and Arturo Pizano for their help in the field, Octavio Riveroll for the logistic support at the forest station, and the authorities of the División de Ciencias Forestales, Universidad Autónoma de Chapingo, for allowing access to their facilities.

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