Pores;~;ology Management ELSEVIER
Forest Ecology and Management 102 ( 1998) 9- 12
Environmental heterogeneity as a strategy for pest management in Eucalyptus plantations Marcos Bragaqa, Og DeSouza *, Jo& Cola Zanuncio Departamento
de Biologia
Animal,
Uniwrsidade
Federal
de ViGosa. 36.571-000
Vicosa. MG.
Brazil
Received 22 October 1996; accepted 2 April 1997
Abstract The effect of environmental heterogeneity on the distribution and abundance of Lepidopterawas tested in Eucalyptus plantations interwoven with natural vegetation. Collections were carried out in Aracruz, ES, Brazil, in five sites along a 1000 m transectstartingin a native woodlandandpenetratingstandsof Eucalyptus grandis and Eucalyptus saligna. Collections were undertaken with light traps, three times a month, from April to August, 1993, recording the number of individuals per morphospecies in each site. A positive correlation was found between the number of sites where each species was recorded and its mean local abundance (r2 = 0.45; P < 0.01; n = 7901, indicating that the more ubiquitous Lepidoptera species are also those that were locally more abundant. No pest species was recorded among those very abundant. Pest species generally
presentedmoderateabundances and only threeout of ten werefound at all sites.It seems,therefore,that pestspecieswere constrained by the heterogeneity conferred by the coexistence of Eucafyptus plantations and remnants of native vegetation. 0 1998 Elsevier Science B.V. Keywordx
Forest fragments; Remnants; Insecta: Lepidoptera;
Regional distribution; Local abundance
1. Introduction Eucalypt plantations in Brazil are generally planted over huge continuous areas, thereby conforming closely to the definition of monoculture. Recently, this homogeneity inherent to monocultures have been further exacerbated, by the increasing practice of planting eucalypt clones (see Laranjeiro, 1994). Relative to diverse habitats, planted monocultures are more prone to pest attacks (Altieri and Letoumeau, 1984; Schowalter et al., 1986; Andow, * Corresponding author. Tel.: + 55-31-8992532; fax: + 55-3 I 8992537:
e-mail:
[email protected].
0378-1127/98/$19.00 0 1998 Elsevier PII s0378-1127(97~00115-1
Science B.V. All rights reserved,
1991). Mechanisms leading to such susceptibility include: (i) Resourceconcentration allowing prompt population expansion; and (ii) homogeneousspatial and temporal plant structure, increasingresourcepredictability and hence easing pest attack (Rausher, 1981; Lawton, 1983). Moreover, natural enemiesare less abundant in monocultures due to unsuitable climatic conditions (Altieri et al., 1993) and to the absence of foraging, resting and oviposition sites (Root, 1973; Price et al., 1980). It follows that, in order to diminish pest problems in eucalypt plantations, a suitable strategy could be to break the homogeneity of the system by, for instance, the preservation of remnants of native vegetation within the culture. Such a strategy is already in use in Brazil,
but its efficacy is inferred mainly from empirical results, rather than accurate scientific analysis. This paper presents one of the first formal attempts in Brazil to evaluate environmental heterogeneity in Eucu/ytus plantations and determine if this could diminish the abundance of Lepidoptera pests.
2. Material
and methods
The experiment was carried out in an Eucalyptus plantation containing remnants of native tropical moist forest. at Aracruz (ES), Brazil (19” 48’S; 40” 97’W). Mean annual temperature in the region is 24°C with an amplitude of 18.5 to 30°C and mean annual rainfall of 1364 mm. A well defined dry seasonoccurs in winter (June to September). Altitude is 27 m above sea level. Collections were undertaken three times a month. from April to August 1993, in five sites along a 1000 m transect, in the following habitats: (i) A 189.5 ha remnant of native vegetation; (ii) a 5 m wide road between the remnant and the eucalypt plantation; (iii) an Eucalyptus grandis stands; (iv) a transition zone between two stands of E. grundis and E. saligna; and (v) sameasthe latter, but 200 m further along the transect (Fig. 1). The eucalypt forest concerned was 5 years old and approximately 25 m high. No cultural practices, either mechanized or chemical. had been usedin the experimental area and the surrounding stands, for at least six months prior to the beginning of the experiment. Lepidoptera were sampled by ultraviolet light traps. Collections were undertaken during periods of waning and new moon, since light traps collect more moths within theseperiods (Mizutami, 1984; Taylor, 1986). Light traps were turned on from 16:00 to 07:OOh, following a restricted random schemewhich did not allow two adjacent traps to be active simultaneously, thus minimizing interference (Baker and Sadovy, 1978). To ensure adequate preservation of the material for subsequenttaxonomic identification. light traps were modified following the recommendations of Ferreira and Martins (1982). Lepidoptera were identified to morphospecies,by comparison with the reference collection held at the Federal University of ViGosa, Department of Animal
Fig.
I. Map
light
traps
of the e-qxriment. (black
dots)
ha-c
4lowing hcen
ES = eucalypt stands; RF = residual residual forest and eucalypt stand; stand: GRA/SALI = edge herwcen sdignct. between forest. BtZil
at 800 m from the residual 15’. grandi.r and 15. .sdi,yntr. This map is located hithin
the trxxat
installed.
Sikh
along are
which
ah fbllow~:
forest; FE == edge between GRA = hdvptrr.r p~rdi.\ E. ,~mn&s and Otcnlyptu.\ forest;
CRA/SALZ = edge m from the t&dual region of Aracrul, ES.
at 1000 the
Biology. Forest Entomology Section. Identifications were restricted to those species whose body length exceeded I .2 cm. and to those species0.6 to 1.3 cm long whose wings did not fully overlap after death. The data for each site were summed for the 15 collecting periods. The data were analyzed in terms of speciesrichness, total moth abundance, evenness index (Ludwig and Reynolds, 1988). as well as the plot of the mean number of individuals per species ()I-axis) versus the number of sites at which each species was recorded (n--axis) (Brown. 1984). A regressionline was fitted using Poissonerrors, whose residuals were investigated after the fitting in order to ensure statistical accuracy.
3. Results 790 moth species(Insecta: Lepidoptrrcr) totalling 8569 individuals were collected. Among those. IO
M. Braganca
et al. / Forest Ecology
Table 1 Number of species (richness), number of individuals starting in a residual forest and penetrating eucalypt Sites
Distance
Residual forest Edge (remnant/eucalypt) Eucc~l~pivptusgrandis stand Edge (E. grandis/Eucaliptus salignn) Edge (E. grandis/E. saligna)
0 400 600 800 1000
from the first site Cm)
abundance)
ZOO100 ;
52
0
n = 790 P < 0.01 lQ=O&
8
number
of occupied
I
sites
Fig. 2. Relationship between average local number of sites where Lepidoptera species along a gradient from a residual forest to Aracruz. ES, Brazil. Regression line fitted corrected for overdispersion.
abundance and the have been recorded, eucalypt stands, in with Poisson errors,
11
102 (199819%12
(abundance) and evenness index for Lepidoptera plantation. Aracruz, ES, Brazil, April to August
species are regarded as ‘primary pests’ in eucalypt plantations (Zanuncio et al., 1994). As shown in Table 1, the highest number of species was found in the edge between the remnant and eucalypt stand, whereas the lowest number was found in the stand containing a single eucalypt species. Abundances were highest in the modified ecosystems, with values ranging from 1418 to 2188 individuals. The lowest abundance value (1190) was found in the residual forest. The fauna of the remnant of native vegetation was more equitably balanced among species, without obvious dominans. By contrast, the fauna of the eucalypt stands was numerically dominated by few species. as indicated by the lower evenness index values (Table I). The most locally abundant species were also those more widespread, whereas the species presenting log (mean
and Management
collected 1993
in five sites along a transect
Richness
Abundance
Evenness
397 419 269 298 290
1190 1853 1418 2188 1920
0.61 0.40 0.39 0.38 0.36
index
moderate to low local abundance tended to occur at few sites (Y’ = 0.45; P < 0.01; II = 790; Fig. 2). 4. Discussion Evenness index values (Table 1) show that some species are numerically dominant in the eucalypt plantation, whereas in the residual forest, species abundances are better balanced. This is in agreement with the theoretical expectation that monocultures favor certain species, and can amplify their abundances, sometimes leading to then reaching the status of pests. Polycultures (here represented by the residual forest) promote the opposite trend. Since specific resources are not so concentrated, species tend to experience more constraints in building up their abundances. As a result. population sizes are generally better balanced among species and outbreaks are less likely to occur (Altieri and Letourneau, 1984; Vandermeer, 1989). The same reasoning is valid if one regards the whole region (eucalypt stands plus native woodland remnants) as a ‘polyculture’. Here, the native remnants contribute to creating the genetic and structural heterogeneity needed to lessen the tendency of insects to reach high abundances. Consequently, such species will be restricted to some areas, where they will hopefully not reach abundances so high as to attain pest status. Instead, a mixed landscape should be characterized by species which do not benefit from abundant food and habitat resources and should be relatively free of serious pest problems. In order to test such ideas, we analyzed the spatial variation in abundances within species. If the set of environments (woodland + eucalypt) under study is heterogeneous enough to present constraints to potential pest species, these should be restricted to few
sites and not attain high abundance values. Simultaneously, the group of species presenting high local abundances should be widespread over the region, because those are generalists which can cope with a wide range of resource variation (Brown. 1984). Therefore, in such situations, one can expect a positive, rather than a negative- or nil-correlation, between the number of sites that a species occurs at and its local mean abundance (Gaston and Lawton. 1990). Such correlation in our data was, indeed. found (Fig. 2). Moreover, no pest species were recorded among the 13 most locally abundant and widespread species. The pest species which were sampled tended to occur in moderate densities, and only three out of ten were ubiquitous. Such observations. coupled with the fact that no recent outbreaks have been recorded in the region, lead us to hypothesize that preserving remnants of native vegetation inside eucalypt plantations could be a suitable strategy to manage insect pest incidence. Manipulative experiments. exploring explicitly the cause/effect relationship here involved need to be done before our suggestions are taken as technical recommendations.
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Dr. Jonathan Majer kindly reviewed the first version of this article. Authors are grateful to Aracruz Celulose. SIF (PCMIP), CNPq and FAPEMIG, for financial support.
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