Mechanisms of Resistance in Eucalyptus Against Larvae of the Eucalyptus Longhorned Borer (Coleoptera: Cerambycidae)
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Mechanisms of Resistance in Eucalyptus Against Larvae of the Eucalyptus Longhorned Borer (Coleoptera: Cerambycidae) LAWRENCE M. HANKS, TIMOTHY D. PAINE, AND JOCELYN G. MILLA~ Department of Entomology, University of California, Riverside, California 92521 Environ. Entomol. 20(6): 1583-1588 (1991) ABSTRACT First instars of the eucalyptus longhorned borer, Phoracantha semipunctata (F.), were not capable of colonizing bark of vigorous standing trees of two Eucalyptus species. The lack of a kino gum reaction after the introduction of larvae into the bark of E. grandis Hill ex Maiden and E. tereticornis Small strongly indicates that this gum does not play an important role in the initial defense against borer attack. Larvae were also not able to colonize the bark of logs that were maintained at high moisture content but were able to colonize the bark of dry logs and artificially water-stressed trees that had reduced bark moisture content. We propose that bark moisture content plays a critical role in the resistance of eucalyptus trees against colonization by eucalyptus longhorned borer larvae. KEY WORDS Insecta, Phoracantha semipunctata, Eucalyptus, plant resistance ABIOTIC STRESS often renders plants more suscep adults emerge from logs by chewing their way out tible to feeding by phytophagous insects (Rhoades of the pupal cell. 1979, White 1984, Waring & Pitman 1985, Mattson Adult borers are present continuously from early & Haack 1987). Cambium feeders in particular spring through September. The sexes mate on the appear to be strongly affected by host stress (Lars host, and adult longevity is , 1 mo. Development son 1989), probably because of their intimate as from egg to adult requires ::>o3 mo during the sum sociation with host tissues. In its native Australia, mer, and there are apparently two asynchronous the eucalyptus longhorned borer, Phoracantha generations per year. semipunctata (F.), a cambium feeder, is a minor Eucalyptus species have been introduced into secondary pest that attacks severely stressed or dead other countries for wood production and as land eucalyptus trees (Duffy 1963, Pook & Forrester scape plants because of their rapid growth rate, 1984). However, the insect often becomes a dev tolerance of poor quality soils and drought, and astating pest when accidently introduced into other because they are resistant to attack by most her parts of the world where eucalyptus is planted as bivores outside of Australia (Pryor 1976). Protec an exotic species as in South Africa (Drinkwater tion of the woody tissues is believed to be through 1975, Ivory 1977), Israel (Bytinski-Salz & Neumark the production of kino, a sticky gum that is an 1952), Tunisia (Chararas 1969), Spain (Gonzalez aqueous solution of polyphenolic compounds re Tirado 1984), and Italy (Cavalcaselle 1980). In these tained in veins or pockets under the bark (Tippett situations, the beetle attacks and kills a much great 1986). It has long been believed that kino flow er proportion of standing eucalyptus than is true prevents eucalyptus longhorned borer larvae from in Australia, and it is well documented that drought penetrating the tissues of healthy trees (Tooke 1935, stress predisposes trees to borer attack (Chararas Bytinski-Salz & Neumark 1952, Hardie 1974, Scri 1969, Chararas et al. 1971, Drinkwater 1975, Ivory ven et al. 1986). Drought stress reduces the pro 1977, Scriven et al. 1986). duction of kino and in this way is presumed to At present, the eucalyptus longhorned borer is decrease the resistance to borer attack (Chararas an economically important pest in California that 1969, Scriven et al. 1986). However, Chararas (1969) attacks many Eucalyptus species (Scriven et al. reported that water stress may more directly de 1986). The biology of this species in southern Cal crease resistance by reducing the turgidity of the ifornia has been reviewed by Hanks et al. (1990). bark, suggesting that sap flow in vigorous trees may Females oviposit batches of up to 40 eggs in crev prevent the larvae from entering the bark. ices and under loose bark with an average fecun In this study, we contrast the relative importance dity of 110 eggs per female. The eggs hatch after of the kino reaction and bark moisture content on ::>o5 d, and the first instars bore through the bark the ability of first-instar P. semipunctata to colo and then mine along the cambium, also consuming nize the wood of two Eucalyptus species by arti primary xylem on one side and phloem on the ficially introducing larvae directly into the bark of other. Larval development requires as little as 2.5 standing trees subjected to two moisture treat mo. Mature larvae bore into the sapwood, packing ments. The influence of bark moisture content on the opening to the surface with frass to construct larval colonization was further examined by sub a pupal cell. Pupation lasts , 15 d. The nocturnal jecting Eucalyptus logs to dry and watered treat- 0046-225X/91/1583-1588$02.00/0 © 1991 Entomological Society of America 1584 ENVIRONMENTAL ENTOMOLOGY Vol. 20, no. 6 ments and determining the survivorship of larvae three E. tereticornis study trees by pruning the introduced into the logs. lateral roots with a backhoe, digging to a depth of 0.5 m at a distance of 0.2 m around all four sides of the bole. Five nontrenched control trees were Materials and Methods separated from the trenched trees by at least one Trials were conducted in 1990 in a research plan tree that was not included in the study. Root prun tation of mixed eucalyptus species at the Moreno ing was conducted on 20 June 1990. On 9 August, Valley field station of the University of California a strong wind left these root-pruned trees leaning in Riverside County, Calif. All trees were 8 yr old at an angle of ::::::70°; however, they maintained and were planted 3 m apart. The plantation was viable roots throughout the study. flood-irrigated biweekly, and the natural incidence To document the effect of root pruning, a mois of eucalyptus longhorned borer attack was very ture potential reading was taken of five leaves from low among standing trees. each study tree with a pressure chamber (PMS Colonization of Living E. grandis Hill ex Maid Instrument Company, Corvallis, Ore.) on 13 Sep en Trees by Eucalyptus Longhorned Borer. To tember 1990. These readings were taken at ::::::1400 document that larvae are not capable of colonizing hours (PST), at which time the ambient tempera healthy E. grandis trees, 20 first instars were in ture was 37°C. troduced into incisions made in the bark of each On 14 September 1990, 20 first instars were in of six living trees and in six 1-m-long logs of the troduced into the bark of all study trees and into same species that had been cut 2 d earlier. These three E. tereticornis logs that had been cut 2 d introductions were conducted on 21 August 1990. earlier. On 25 October 1990, the bark of the study Two weeks later the bark of trees and logs was trees and logs was dissected to determine the fate dissected to determine the fate of the larvae and of the larvae and evaluate the kino reaction. A 10- to evaluate the kino reaction. cm-square sample of the bark was also taken from In November 1990, three E. grandis trees were all trees and logs for measurement of moisture con felled and cut into 1-m-long logs to study the change tent. in bark moisture content in logs over time. Bark Rearing of Larvae and Introduction into Host samples were taken before the trees were cut down, Tissues. Adult beetles were reared from naturally and from one log of each tree 1 dafter felling. infested logs and placed in glass aquaria in a 30°C Effect of E. grandis Log Moisture Content on growth chamber, where they were provided with Eucalyptus Longhorned Borer Colonization. To sugar water and honey bee-collected pollen. Be examine the influence of bark moisture content on cause the adult female borers prefer to oviposit colonization success, larvae were introduced into under bark, a square of eucalyptus bark that was two groups of logs, one set that was maintained at weighed down on a square of paper toweling was high moisture content and a second set that was provided. Eggs laid on the paper toweling were allowed to dry out naturally. Twelve logs (92.3 ± collected on a daily basis and were kept in petri 3.1 em long and 39.6 ± 4.4 em in circumference) dishes until hatching in the same warm room. First were cut from two E. grandis on 7 September 1990 instars were stored for a maximum of 2 d at l0°C and moved to a partially shaded shed on the Riv in petri dishes. erside campus. About 1 h after felling, six of these To introduce the larvae into logs and trees, a logs (which we will refer to as the watered logs) shallow horizontal incision was cut in the bark with were randomly chosen and were placed standing a razor blade at an angle nearly parallel to the bark in 72-liter plastic tubs filled with water to a depth surface and no more than 3 mm in depth. Incisions of 10 em. These logs were leaned against a 0.5-m in standing trees were made :::::: 1.5 m from the high table to keep them standing nearly upright ground. Five larvae were gently placed into each and were interspersed with the six logs (referred incision with a damp paint brush. to as the dry logs) that stood on the floor. On 9 Determination of Larval Performance. The bark September 1990, 2 d after felling, 20 first instars was stripped in the area of the incisions with a were introduced into incisions made in the bark of hammer and chisel until larval tunnels were en each log.