Eucalyptus Longhorned Borers
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EUCALYPTUS LONGHORNED BORERS Integrated Pest Management for Professional Landscapers and Home Gardeners trees suffering from stress, especially drought stress. The problem is exac- erbated in California because many eucalyptus trees are growing in unmanaged or minimally managed environments with no supplemental irrigation. Both the beetles and euca- lyptus trees are native to Australia. (actual Eucalyptus species that naturally (actual size) grow in wetter areas of Australia have size) been planted in California where they Figure 1. Adult Phoracantha semi- experience prolonged dry periods, Figure 2. Adult Phoracantha recurva. punctata. which makes them susceptible to attack by these beetles. hatching. If conditions are less than Two closely related species of IDENTIFICATION ideal, the small larvae feed outward longhorned borers (family Ceramby- Adult eucalyptus longhorned borers from the egg mass, leaving a distinct cidae) attack eucalyptus in California. have shiny, dark brown and yellow to dark trail 1/4 inch to several inches long Phoracantha semipunctata (Fig. 1) was cream-colored areas on their wing that scores the bark surface before they introduced into southern California in covers. The antennae are as long as bore into the cambial tissue on the in- the 1980s and rapidly became a pest. or longer than the body, and the an- ner surface of the bark. Once the larvae It currently occurs throughout Califor- tennae of males have prominent have bored into the inner bark, the nia wherever eucalyptus trees grow. spines. The mature larvae may be galleries they form beneath the wood This beetle readily attacks and kills large (more than 1 inch long) and are surface can extend several feet. large numbers of eucalyptus trees. cream colored and legless. Natural enemies of the eucalyptus In trees with heavy infestations, the borer were introduced from Australia, Females lay eggs under loose bark or scraping sounds made by the develop- and biological control in combination in crevices on bark surfaces. If condi- ing larvae chewing into the cambium with improved cultural care of euca- tions are ideal, larvae will bore di- are clearly audible from a distance of lyptus dramatically reduced the num- rectly into the inner bark after several feet. ber of trees killed each year by the borer. In 1995, a second species of borer, Phoracantha recurva (Fig. 2), TABLE 1. Differences between Two Eucalyptus Longhorned Borers in was discovered in southern California California. in Los Angeles, Riverside, Orange, and San Bernardino counties. Like P. semi- Characteristic Phoracantha semipunctata Phoracantha recurva punctata, this beetle is expected to color of wing mostly dark brown; zigzag mostly cream to yellowish; eventually spread throughout Califor- covers (elytra) line bisecting cream-colored dark brown areas primarily nia wherever eucalyptus grows. Not area in middle of elytra limited to rear third of elytra all of the previously introduced natu- ral enemies are effective against this golden hairs on none to sparse long, dense new borer, and P. recurva together underside of each with several other new pests is caus- antenna segment ing increased stress and mortality of Sources: Gill, R. J. 1997. An Australian eucalyptus beetle. California Plant Pest and Dis- eucalyptus. ease Report 16(1-2):12; Hanks, L. M., T. D. Paine, J. G. Millar, and C. Campbell. Another tree-killing pest of eucalyptus invades California. California Plant Pest and Disease Report Longhorned borers are attracted to 16(1-2):19-21 freshly cut wood, dying limbs, and PEST NOTES Publication 7425 University of California Division of Agriculture and Natural Resources Revised January 2000 January 2000 Eucalyptus Longhorned Borers These two Phoracantha species re- sustain beetle larvae; old dry logs are packed with larval excrement. In- semble each other but differ some- too hard for larvae to feed on and fested trees are often killed in a mat- what in biology. Distinguishing then successfully develop. ter of a few weeks. Resprouting may between P. semipunctata and P. occur from the tree base. recurva can be useful in predicting the Following pupation, adult beetles importance of biological control emerge from the same holes by chew- MANAGEMENT based on whether one or both borer ing through the plugs of frass. During The same methods are used to man- species are present in that area. Dif- spring and summer, the beetle re- age both Phoracantha species. Control ferences in wing color and hairs and quires 3 to 4 months to complete its eucalyptus longhorned borers by spines on antennae help to distin- life cycle, but starting in fall and win- reducing tree stress, properly han- guish between P. semipunctata and P. ter it may require up to 9 months. dling eucalyptus wood, planting resis- recurva (Table 1). For example, dark There are two and possibly three tant species, and avoiding activities brown predominates on the wing cov- poorly synchronized generations a that disrupt biological control. Pesti- ers of P. semipunctata, while yellow to year; adults can emerge anytime from cide applications generally are not cream color predominates on wing April through October. effective in managing these pests. covers of P. recurva. The life cycle of P. recurva differs Eucalyptus are also attacked by sev- LIFE CYCLE somewhat from that of P. semi- eral other introduced insects. Pest Several nights after emerging and punctata, and it is currently in the managers should learn how their ef- mating, the female beetles of P. process of being documented. forts, such as choosing a tree species semipunctata begin laying eggs in Phoracantha recurva may have a to plant, may affect the control of groups of 3 to 30 under loose bark of longer period of activity each year, these other pests. Eucalyptus pests eucalyptus trees. Females may live with adults emerging as early as include leaf-feeding beetles, eucalyp- one or more months and lay up to 300 February. tus gall wasp, and at least six species eggs, which hatch in about 1 to 2 of psyllids (see UC IPM Pest Notes Pub- weeks depending on temperature. In their native Australia, eucalyptus lication 7423, Psyllids and UC IPM Pest First-instar larvae may bore directly longhorned borers develop in broken Notes Publication 7460, Eucalyptus into the inner bark or they may mine limbs, pruned limbs, fallen trees, and Redgum Lerp Psyllid). Certain of these short distances in the outer bark lay- highly stressed trees. They rarely kill insects are now under effective bio- ers before turning inward to mine at healthy trees. Eucalyptus naturally logical control, including the blue the bark-cambium-xylem interface, growing in areas to which they are gum psyllid (Ctenarytaina eucalypti) leaving a dark stain on the wood sur- adapted appear more resistant to and eucalyptus snout beetle or the face. At the end of the feeding period, beetle attack, and beetles in Australia gumtree weevil (Gonipterus larvae excavate pupal chambers in are preyed upon by many natural scutellatus). Several parasites are be- the wood. They enter the chambers, enemies. ing introduced to control other pests, packing the holes behind them with such as the leaf-feeding Australian wood shavings and frass (Fig. 3). Lar- DAMAGE tortoise beetle (Trachymela sloanei). vae require about 70 days to develop Holes in the bark and stains or oozing Inappropriate actions such as spray- in fresh wood or logs during hot sum- liquid on limbs or trunks are common ing persistent broad-spectrum insecti- mer months, and up to 180 days in longhorned borer damage symptoms. cides can harm natural enemies and drier logs. Only relatively fresh logs Foliage may discolor and wilt, and might cause outbreaks of these other limbs may die back. Longhorned bor- pests. Conversely, uncontrolled popu- ers mostly attack stressed or dam- lations of pests such as eucalyptus aged plants; vigorous, well-watered redgum lerp psyllid (Glycaspis trees are rarely attacked. However, in brimblecombei) can seriously stress California many eucalyptus trees are trees, increasing eucalyptus suscepti- seasonally water-stressed during hot bility to longhorned borers. summer months, rendering a signifi- cant proportion of them susceptible Cultural Management to the beetle. Choose eucalyptus species that are well adapted to each location and Extensive larval feeding at the inner provide them with proper cultural (actual size bark-cambium-xylem interface can care. Certain species of eucalyptus of larva) effectively girdle the tree. Trees at are more resistant to longhorned bor- this stage of infestation are character- ers (Table 2). Most eucalyptus spe- ized by a thin canopy with wilted or cies that are adapted to wetter Figure 3. Mature larva exposed in wood. dry leaves; the bark is cracked and growing conditions are more suscep- ◆ 2 ◆ January 2000 Eucalyptus Longhorned Borers Biological Control TABLE 2. Relative Susceptibility of Eucalyptus spp. to Phoracantha semipunctata. Biological control with natural en- More susceptible More resistant emies may provide the best long-term solution to longhorned borer prob- blue gum, E. globulus hybrid, E. trabutii lems. If beetle populations are re- flooded gum, E. grandis lemon gum, E. citriodora duced to low levels, natural enemies Karri gum, E. diversicolor mountain gum, E. dalrympleana are better able to keep populations in manna gum, E. viminalis red ironbark, E. sideroxylon check, and vigorous trees can survive shining gum, E. nitens river red gum, E. camaldulensis a few attacks. At least one native Cali- Sydney