Elm Leaf Beetle

ELM LEAF BeeTLE Integrated Pest Management for Home Gardeners and Landscape Professionals The elm leaf beetle, Xanthogaleruca beetle has at least one generation a (=Pyrrhalta) luteola, is a leaf-chewing year in northern California and two pest of elm trees, especially European to three generations in central and elm species. American and most Asian southern California. elm species are less severely fed upon. Most Chinese elm cultivars, zelkova, DAMAGE and many newer elm cultivars are Adults chew entirely through the leaf, infrequently fed upon. often in a shothole pattern. Larvae skeletonize the leaf surface, causing IDENTIFICATION damaged foliage to turn brown to Adults are 1/4 inch long, olive-green whitish (Figure 4). Elm leaf beetles, Figure 1. Adult, eggs, and first-instar beetles with black, longitudinal stripes when abundant, can entirely defoli- larva of elm leaf beetle. along the margin and center of the back ate large elm trees, which eliminates (Figure 1). Females lay yellowish eggs summer shade and reduces the aes- in double rows of about 5 to 25 on the thetic value of trees. Repeated, exten- underside of leaves. Eggs become gray- sive defoliation weakens elms, caus- ish before hatching. Larvae resemble ing trees to decline. However, the elm caterpillars and are black when newly leaf beetle has not been a significant, hatched and shortly after molting widespread problem in California (shedding the old skin). After feeding, since the 1990s. larvae become yellowish to green with rows of tiny dark tubercles (projec- MANAGEMENT tions). Third-instar larvae grow up to It is essential to correctly identify the Figure 2. Third-instar elm leaf beetle 1/3 inch long and have dense rows of cause of damaged elm leaves before larvae. dark tubercles down their sides that taking management action. European resemble two black stripes (Figure 2). flea weevil, Orchestes alni, also chews Pupae are orange to bright yellow. holes in elm leaves and causes elms to defoliate. This introduced beetle LIFE CYCLE has become a serious elm pest in the The elm leaf beetle develops through eastern United States, but currently four life stages: egg, larva, pupa, and does not occur in California. Also adult (Figure 3). Adults commonly from a distance, foliage severely overwinter in bark crevices and wood- chewed by beetles resembles the piles or in buildings. In spring they fly browning and dieback caused by to elm foliage and chew leaves, and Dutch elm disease fungi, Ophiostoma females lay eggs. Eggs hatch into lar- (=Ceratocystis) ulmi and O. novo-ulmi. vae that develop through three instars Report any suspected Dutch elm dis- (growth stages) over a period of several ease, European flea weevil, or other Figure 3. Elm leaf beetle stages and life weeks while chewing on foliage and new pests to the local county agricul- cycle. then crawl down the tree trunk. Ma- tural commissioner. cides, bark banding, or systemic insec- ture larvae become curled and inactive (a stage referred to as prepupae), then Where the elm leaf beetle is a prob- ticide. Recognize that elm leaf beetle pupate, sometimes in large numbers, lem, manage it with an integrated populations historically have fluctu- around the tree base. After about 10 program that incorporates good ated dramatically from year to year and days as pupae, adults emerge and fly to cultural practices, conservation of trees do not warrant treatment most the canopy to feed and, during spring natural enemies, regular monitoring, years. When management is warranted, and summer, lay eggs. The elm leaf and the use of less-persistent insecti- use a combination of methods; because PEST NOTES Publication 7403 Statewide Integrated Pest Management Program June 2014 June 2014 Elm Leaf Beetle no single action improves the health of Monitoring elm trees or kills 100% of the pests. Determine the need and effective timing for any pesticide application Cultural Control by visually inspecting leaves at about Good cultural care of trees is an essen- weekly intervals beginning in spring tial component of integrated pest man- when the first generation of eggs and agement. American and European elm larvae are expected. Watch for the species are adapted to summer rainfall. appearance of clusters of yellowish to Maintain tree vigor, and protect trunks gray eggs and caterpillar-like larvae. and roots from injury, by providing The calendar date of peak abundance appropriate irrigation in areas with and damage varies greatly from year summer drought. Check for dead or to year depending on spring tempera- dying branches and promptly remove tures. If beetles are too abundant, use Figure 4. Damage due to elm leaf beetle them. Dying branches pose a limb drop the heat accumulation method dis- feeding. (failure) hazard and provide habitat for cussed in Degree-Day Monitoring to bark beetles, Scolytus multistriatus and determine the optimal time to inspect S. schevyrewi foliage and to accurately time insecti- , which vector the Dutch beetles are too abundant) apply insecti- elm disease fungi. Avoid unnecessary cide application. cide. Generally the higher the tempera- pruning; and when pruning make cuts ture, the sooner beetles and damage properly, preferably during late fall and If you plan to spray foliage or apply appear in the spring. When spring tem- winter and when trees are young. systemic insecticide to trunks, make the application when first- and second- peratures are cooler, elm leaf beetles appear later because they do not feed, Choose species or cultivars that resist instar (small) larvae are abundant. grow, or reproduce below a “threshold both Dutch elm disease and elm leaf With bark banding discussed below, temperature” of about 52°F. beetle when planting (Table 1). Pest- band as soon as third-instar (larger) resistant elms include Accolade, Emer- larvae are observed on leaves. To predict the peak abundance of each ald Sunshine, Frontier, Prospector, and elm leaf beetle life stage, temperatures most Chinese elms, except Dynasty. If you drench or inject soil with a root-absorbed, systemic insecticide, are monitored in units called degree- the most effective application time is days. One degree-day is 1 degree above Control Action Guidelines. Healthy spring when elm trees flush new leaves elm trees can tolerate substantial dam- the insect’s lower threshold tempera- or shortly after leaf flush. The time for age to leaves; total defoliation may have ture maintained for a full day. For the action is before you know definitely little long-term effect on healthy elms, elm leaf beetle, degree-days above 52°F whether beetles will become abundant especially if leaf damage occurs late in are accumulated for each season begin- enough to warrant this preventive ap- the season. Suggested guidelines are ning March 1. The first- and second- plication. to take action when needed to prevent instar larvae of first-generation elm leaf greater than 40% defoliation (portion beetles are most abundant at about 700 Generally, insecticide application is degree-days above 52°F accumulated of leaf area chewed or leaves dropped unlikely to be warranted if: prematurely); or if damage is less toler- from March 1. If populations are high • beetles and damage were low during and damage is anticipated, foliar in- able or occurring earlier in the growing late summer the previous year; secticide spray or trunk spray or injec- season, a treatment threshold of 20% • systemic insecticide was properly tion of systemic insecticide at about defoliation may be used. For additional applied the previous growing 700 degree-days will catch susceptible information on how to predict the season; or larvae at their greatest abundance. amount of defoliation from each beetle • the winter was relatively warm or generation refer to the 1998 publication wet or both, because this weather by Dahlsten and others listed in Refer- apparently causes more “hibernat- Elm leaf beetle development can eas- ences. ing” beetles to die before elm leaves ily be predicted using the point-and- appear in spring. click, degree-day calculator online at Timing of action varies according to the If any of the above circumstances are http://www.ipm.ucanr.edu/calludt. control methods and situation. To avoid true, avoid insecticide application un- cgi/DDMODEL?MODEL=ELB&CROP unacceptable defoliation, when using less monitoring of plants reveals that =landscape. If you manage large num- methods that kill a smaller proportion elm leaf beetles are present and exces- bers of elms, you can further improve of the pests or take longer to provide sively abundant. your treatment by using degree-days control allowing beetles to continue to in combination with egg presence- feed and damage leaves, take action Degree-Day Monitoring. Monitor absence sampling, as explained in the sooner than you would when using temperatures to determine the opti- 1993 publication by Dahlsten and oth- faster-acting methods. mal time to inspect elm leaves and (if ers listed in References. ◆ 2 of 5 ◆ June 2014 Elm Leaf Beetle Biological Control Monitor beetle abundance and damage, as discussed above, to determine Elm leaf beetle populations are usu- ally low at least partly because of treatment need, choice of method, and biological control by native predators timing. Apply insecticide only when and introduced parasites. Predators beetles are present or expected to be- of the elm leaf beetle include certain come too abundant. Insecticides can bugs, earwigs, lacewing larvae, and have unintended effects, such as con- predaceous ground beetles. The easiest taminating water, poisoning natural to recognize parasite is a small black enemies and pollinators, and causing tachinid fly, Erynniopsis antennata. After secondary pest outbreaks. Completely Erynniopsis larvae feed inside and read and follow the product label in- Figure 5. Elm leaf beetle prepupae kill beetle larvae, the parasite’s 1/5 structions for the safe and effective use (left) pupae, and pupae of the Erynniop- inch long, black to reddish pupae can of the insecticide.

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