FORUM Accumulation of Pest Insects on Eucalyptus in California: Random Process or Smoking Gun TIMOTHY D. PAINE, 1,2 JOCELYN G. MILLAR, 1 AND KENT M. DAANE 3 J. Econ. Entomol. 103(6): 1943Ð1949 (2010); DOI: 10.1603/EC10214 ABSTRACT Eucalyptus spp., native to Australia, have been introduced into many parts of the world as important timber and ornamental trees. Although the trees have important silvicultural qualities, they also have generated intense dissatisfaction, particularly among groups of individuals in California. The trees have beneÞted from the lack of insect pests and diseases in their adventive ranges but that has changed over the past four decades. In California, two species of insect herbivores were introduced between the time trees were Þrst introduced to the state in the middle of the 19th century and 1983. Between 1983 and 2008, an additional 16 Australian insect pests of eucalyptus have become established in the state. The modes or routes of introduction have never been established. However, examinations of different temporal and spatial patterns suggest that the introductions were nonrandom processes. It is possible that they occurred because of increased trade or movement of people, but the hypothesis that there were intentional introductions also must be considered. The rapid accumulation of intro- duced herbivores on an ornamental plant system in a single state is a cautionary example of what could happen if a major food or Þber crop were intentionally targeted. KEY WORDS Eucalyptus, invasive species, intentional introductions, insect herbivores, adventive range Eucalyptus spp. are native to Australia and New Guinea (Doughty 2000). They have earned a unique reputation among timber species because of several desirable characteristics. For example, many species grow rapidly and produce very high-quality wood Þ- ber that can be used as timber or as cellulose stock for manufacturing high-quality paper for laser printers and copy machines. They can be planted at high den- sities in plantations and can be harvested at rotations of 10 yr, depending on location. The trees will grow on poor soils with no supplemental irrigation, giving them an advantage over other hardwood plantation species. Consequently, vast plantations of Eucalyptus are found in South America (Brazil, Chile, and Ar- gentina), sub-Saharan Africa (Republic of South Af- rica, Zambia, and Malawi), and the Mediterranean basin (Spain, Portugal, Italy, Tunisia, Algeria, and Is- rael) (Doughty 2000). Besides their value as commercial species, Eucalyp- tus also have been planted in many parts of the tem- perate and subtropical world as ornamental trees. They were introduced into California initially as a timber species, but the growth characteristics were unsuitable. However, 90 species in the genus have been widely planted as landscape and windbreak trees across California (Doughty 2000) and are also grown in Arizona, Florida, Hawaii, and into Mexico for similar purposes. Limited in distribution by low temperatures, they grow well under hot, dry summer conditions and mild winters, often where other ornamental trees do not thrive. In addition to their rapid growth, relatively high drought tolerance, and ability to grow on poor soils, the widespread adoption of Eucalyptus was enhanced by the lack of insect pests in most of the introduced ranges. The trees were widely distributed around the world from Australia in the mid-19th century at the end of the age of sail and the beginning of the era of steam ships. The long voyages made transportation of large numbers of live plants impractical. Fortunately, the trees propagate easily from seed. One of the con- sequences of seed propagation is that the foliage-, sap-, and wood-feeding insects were not moved with the seeds (Doughty 2000). Although the trees have signiÞcant commercial and esthetic value, they also have received criticism and labeled as an undesirable or invasive species. Many species have exfoliating bark and, although evergreen, they can shed large amounts of leaf litter and seed capsules that many Þnd messy and a nuisance. If the shed litter is allowed to accumulate under the trees, it can function as a critical fuel source for urban wild- Þres. In addition, the leaves have high levels of ter- penes, responsible for the characteristic Eucalyptus odor and a source of Eucalyptus oil, that make the foliage highly ßammable. The rapid growth also can enable some naturalized species to invade native land- 1 Department of Entomology, University of California, Riverside, CA 92521. 2 Corresponding author, e-mail: timothy.paine@ucr.edu. 3 Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720. 0022-0493/10/1943Ð1949$04.00/0  2010 Entomological Society of America