11 Insect Outbreaks in Tropical Forests: Patterns, Mechanisms, and Consequences Lee A. Dyer, Walter P. Carson, and Egbert G. Leigh Jr. 11.1 Introduction “First of all, the statement that there are no outbreaks in tropical forests (Elton 1958) is wrong.” Wolda (1983) issued this verdict when he demonstrated that over six-year periods, tropical insect populations fluctuated as strongly as temperate ones. Nevertheless, the paradigm that tropical forests are free of outbreaks (Elton 1958) has persisted in the face of strong evidence to the contrary (Nair 2007). In fact, the term “tropical outbreaks” is usually associated with disease outbreak literature or, to a lesser extent, with insect outbreaks in agricultural ecosystems or tree plantations (Nair 2007). However, as studies of insect natural history and population dynamics in the tropics accumulate, it is clear that insect outbreaks occur in most natural tropical terrestrial ecosystems and that these outbreaks may occur moderately frequently (Table 11.1). Our goal is to review what is currently known about the frequency, causes, and consequences of outbreaks both within and among tropical forests worldwide. In this chapter, we do the following: (1) define an outbreak, identify different general types of outbreaks, and outline challenges associated with quantifying and detecting outbreaks in tropical forests; (2) point out potential biotic linkages between outbreaks in managed landscapes and those in more natural forests; (3) identify taxa that are known to outbreak and traits that make species prone to outbreak; (4) predict where outbreaks are likely to occur both within a stand and across gradients from dry to wet forests; (5) discuss the consequences of outbreaks for plant communities and species coexistence; (6) make predictions about how global climate change will alter the frequency and severity of outbreaks; and, finally, (7) propose a series of testable hypotheses concerning outbreaks in the tropics to serve as a guide for future research. We focus mostly Insect Outbreaks Revisited, First Edition. Edited by Pedro Barbosa, Deborah K. Letourneau and Anurag A. Agrawal. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd. BBarbosa_c11.inddarbosa_c11.indd 221919 112/8/20112/8/2011 44:41:08:41:08 AAMM BBarbosa_c11.indd 220 a r b o s a _ c 1 1 . i n d d 2 2 0 Table 11.1 Examples of tropical insects for which outbreaks of native herbivores in natural ecosystems have been documented. The herbivores in these examples are assumed to be native, but natural history data for most tropical herbivores are so poor that it is difficult to determine the recent origin of most herbivores within a tropical forest. Additional references are provided for outbreaks of other species within the same broad taxonomic feeding guild. Diet breadth for guilds is categorized based on host family – generalists consume greater than one family of host plant, and the facultative generalist increases its diet breadth when outbreaking. Species categorized as “solitary” may be gregarious in earlier instars, or adults may oviposit large clutches. “Long-term” observational or experimental studies utilized at least 5 years of data; all others were short-term data sets. The Van Bael et al. (2004) and Torres (1992) references document one large outbreak in Panama and Puerto Rico, respectively. Only outbreaks recorded in tropical forests and measured or described as outbreaks or eruptions are included here. Other sources summarize outbreaks in tropical agriculture or of invasive insect species in the tropics (e.g., Nair 2007; Food and Agricultural Organization (FAO) 2011). Order Family Example Guild Documentation References Outbreak damage Coleoptera Buprestidae Agrilus kalshoveni Specialist; Long-term observational; Kalshoven 1953 Large-scale tree in Indonesia wood borer tree mortality mortality Coleoptera Buprestidae Sphenoptera Specialist; Observational; tree Singh et al. 2001 Large-scale tree aterrima in India wood borer mortality mortality Coleoptera Cerambycidae Hoplocerambyx Specialist; Long-term observational; Roonwal 1978 3–7 million trees spinicornis in India wood borer tree mortality attacked Coleoptera Curculionidae Ambates spp. in Specialist; Long-term observational, Letourneau and Dyer 0.58 ± 0.1% shrub Costa Rica stem borer experimental; stem 1998; Letourneau mortality damage 1998 Coleoptera Curculionidae Cryptorhynchus Specialist; Observational; tree Singh et al. 2001 Large-scale tree rufescens in India wood borer mortality mortality Coleoptera Curculionidae Dendroctonus Specialist; Long-term observational, Billings et al. 2004 1700–2 million ha frontalis in Central wood borer experimental; tree America mortality Coleoptera Curculionidae Platypus biformis Specialist; Observational; tree Singh et al. 2001 Large-scale tree in India wood borer mortality mortality Coleoptera Curculionidae Polygraphus Specialist; Observational; tree Singh et al. 2001 Large-scale tree longifolia in India wood borer mortality mortality Coleoptera Various Various species Generalist and Observational, leaf Janzen 1981 Localized defoliation, > 112/8/2011 4:41:08AM in Costa Rica specialist leaf damage 90% herbivory 2 / 8 / chewers 2 0 1 1 4 : 4 1 : 0 8 A M BBarbosa_c11.indd 221 a r b o s a _ c 1 1 . i n d d 2 2 Hemiptera Pentatomidae Udonga montana Generalist, Observational; adult Reviewed by Nair Millions of individuals 1 in India seed predator, numbers 2007 leaf chewer Hemiptera Psyllidae Phytolyma spp. Specialist gall Observational; gall Wagner et al. 1991 Large-scale mortality In Africa numbers Hymenoptera Agridae Shizocera spp. Generalist; Observational; leaf Tin 1990 Severe leaf damage In Vietnam gregarious damage leaf chewer Hymenoptera Formicidae Atta and Generalist; Long-term observational, Holldobler and Wilson Doubled rates of Acromyrmex leaf harvester experimental, leaf 1990; Terborgh et al. colony growth; 5–11 in the Neotropics damage, nest density 2001; Feeley and times increases in Terborgh 2008; density Meyer et al. 2009 Lepidoptera Arctiidae Cosmosoma Specialist; Observational; rough Torres 1992 Herbivory per tree up myrodora in Puerto gregarious estimates of larval to 99% Rico leaf chewer densities and leaf damage Lepidoptera Arctiidae Eucereon tesselatum Generalist ; Long-term observational; Van Bael et al. 2004 0.32 larvae per young in Panama solitary leaf larval densities; leaf leaf chewer damage Lepidoptera Arctiidae Munona iridescens Generalist; Long-term observational; Van Bael et al. 2004 0.4 larvae per young in Panana solitary leaf larval densities; leaf leaf chewer damage Lepidoptera Epiplemidae Syngria druidaria Specialist; Long-term observational; Van Bael et al. 2004 1.09 larvae per young in Panama solitary leaf larval densities; leaf leaf chewer damage Lepidoptera Geometridae Cleora injectaria Specialist; Observational; larval Piyakarnchana 1981 Up to 100% defoliation in Thailand solitary leaf densities; leaf damage on trees in a large area chewer Lepidoptera Geometridae Eois spp. in Costa Specialist; Observational and Letourneau and Dyer 47 ± 10% herbivory Rica solitary leaf experimental; larval 1998; Letourneau chewer densities and leaf 1998; Dyer and damage Gentry 2010 112/8/2011 4:41:09AM 2 / 8 / 2 0 1 1 4 : 4 1 : 0 9 A M BBarbosa_c11.indd 222 a r b o s a _ c 1 1 . i n d d 2 2 2 Table 11.1 (Cont’d). Order Family Example Guild Documentation References Outbreak damage Lepidoptera Geometridae Miliona basalis in Specialist; Observational Reviewed by Nair Repeated outbreaks Indonesia solitary leaf 2007 chewer Lepidoptera Geometridae Scotorythra Specialist; Long-term observational; Haines et al. 2009 Repeated outbreaks; paludicola in Hawaii gregarious larval and adult up to 16 km2 full leaf chewer densities; leaf damage defoliation Lepidoptera Hesperiidae Perichares philetes in Specialist; Observational; rough Torres 1992 Herbivory per plant up Puerto Rico solitary shelter estimates of larval to 75% builder densities and leaf damage Lepidoptera Hyblaeidae Hyblaea puera across Specialist; Reviewed by Nair 100% leaf herbivory, southern Asia shelter- 2007 up to 100% herbivory building leaf per tree or shrub, large chewer scale out breaks of up to 350 million larvae Lepidoptera Lasiocampidae Euglyphis spp. in Generalist; Long-term observational; Van Bael et al. 2004 1.54 larvae per young Panama gregarious larval densities; leaf leaf leaf chewer damage Lepidoptera Lasiocampidae Voracia Generalist; Observational; leaf Kalshoven 1953 Up to 100% defoliation casuariniphaga in gregarious damage in 800 ha of forest East Java. shelter builder Lepidoptera Limacodidae Acharia hyperoche in Generalist; Observational; larval Dyer and Gentry 2010 90 ± 12% herbivory per Costa Rica gregarious densities, leaf damage shrub leaf chewer Lepidoptera Lymantriidae Lymantria galinaria Generalist; Observational; larval Kalshoven 1953 100% of host trees in Indonesia gregarious densities attacked leaf chewer 112/8/2011 4:41:09AM 2 / 8 / 2 0 1 1 4 : 4 1 : 0 9 A M BBarbosa_c11.indd 223 a r b o s a _ c 1 1 . i n d d 2 2 Lepidoptera Noctuidae Condica cupentia in Specialist; Observational; rough Torres 1992 Herbivory per vine up 3 Puerto Rico gregarious estimates of larval to 95% leaf chewer densities and leaf damage Lepidoptera Noctuidae Dyops dotata in Specialist; Long-term observational; Van Bael et al. 2004 5.07 larvae per young Panama solitary leaf larval densities; leaf leaf chewer damage Lepidoptera Noctuidae Eulepidotis spp. in Generalist; Long-term observational; Wong et al. 1990; 95% trees with