Pyrrhalta luteola
(elm leaf beetle)

The elm leaf beetle P. luteola is a serious pest of ornamental elms in most areas where they are grown. This insect is native to southern Europe and was introduced to the USA in the 1800s, but has also reached Central Asia, the Middle East, North Africa and South America and was recently detected in Australia. It feeds on all species of native and introduced elms, including American (Ulmus americana), English (U. procera), Chinese (U. parvifolia) and Siberian elm (U. pumila).

Wu et al. (1991) identified elm leaf beetle as the second most important urban tree pest in the western United States and third most important nationally.

The elm leaf beetle is of Palearctic origin. It has been progressively introduced into North America, Argentina (Dahlsten et al., 1998), Chile (Dreistadt, personal communication), Central Asia, the Middle East, North Africa (Algeria), Siberia (Dreistadt, 2008) and South Africa (Aslan et al., 2000; Dreistadt, personal communication). Most reports and publications mention regions where the elm leaf beetle is present (e.g. Central Asia), but rarely countries within regions.

The elm leaf beetle is native to temperate climates. In its introduced range it is usually found anywhere elm trees are grown.

The beetles overwinter as adults in sheltered places such as woodpiles, garages, and attics. When ready to pupate, the larvae crawl into holes in the trunk of the tree or limb crotches, beneath loose bark, or commonly at the base of the tree, where they gather in large numbers.

Elm leaf beetle has become a significant pest in urban landscapes in all countries it has become established in (Kielbaso and Kennedy, 1983; Nielsen et al. 1985; Wu et al., 1991).

The elm leaf beetle is a pest only of elm trees (Ulmus sp.). Damaged leaves cause physiological stress for the trees and are an aesthetic problem (Kaya et al., 1981).

Several studies have been carried out to evaluate the impact of elm leaf beetle on various elm species and hybrids in North America (Luck and Scriven, 1979; Hall, 1986; Hall and Young, 1986; Young and Hall, 1986; Hall et al., 1987; Miller and Ware, 1997; 1999; 2001a,b; 2002; Miller et al., 2003). This extensive body of work has demonstrated that the Siberian elm (Ulmus pumila L.) and hybrids with Siberian elm parentage, as well as most European elm species such as U. glabra Huds. and U. hollandica Mill., are highly suitable to elm leaf beetle feeding and oviposition (Miller and Ware, 2002) and therefore very susceptible to damage by this pest. Asian elms, including Chinese elm (U. parvifolia Jacq.) and members of the U. davidiana Planch. complex (U. davidiana, U. propinqua Koidz., U. wilsoniana Schneid and U. japonica Sang.), have high tolerance to elm leaf beetle feeding and reproduction and are good candidates for breeding programs (Bosu and Wagner, 2007). American elms (U. americana L.) seem to have intermediate tolerance, although the American elm ‘Valley Forge’ tends to show high tolerance to elm leaf beetle defoliation (Miller and Ware 2002; Bosu et al., 2007).

The Host Plants or Crops Affected table is not exhaustive. Many species and their hybrids have been tested over the past 20 years. For complete lists of hosts, refer to the work of Miller and Ware (1993 to 2002), Mahani et al. (2003) and Bosu et al. (2007). Another Ulmaceae, Zelkova serrata (the Japanese zelkova or saw leaf zelkova), has been reported occasionally as a host of the elm leaf beetle but tends to show good resistance to this pest (Dreistadt, 2008).

Reproductive Biology

As foliage begins to develop in early spring, the adults leave their protected overwintering sites to mate and lay eggs on the underside of the new leaves. Adults feed on the foliage for 1 to 2 weeks before they start to lay eggs. Eggs hatch after 7 to 10 days and the larvae feed by skeletonising the undersides of leaves. Larvae feed for three weeks, during which they develop through three instars.

Mature larvae migrate down the bole of the tree to pupate at the tree base or in bark fissures on the lower tree bole. Some larvae also may drop directly from the tree crown and migrate to protected sites to pupate. The pupal stage lasts around 10 days to two weeks.

In mid-summer, the adults emerge and return to the foliage of the same elm or adjacent elms to produce the second generation. One female may produce as many as 600 to 800 eggs during her lifetime. When the days shorten to less than 14 hours, egg production stops and the adults feed for a brief period before they search for an overwintering site (Bosu et al., 2007; Morton Arboretum, 2013).

Physiology and Phenology

The elm leaf beetle has 2 to 4 generations per year, depending on the region and climate where they occur. Two generations have been observed in east-central Arizona (USA), with peak populations in July and August. Depending on climate, there can be one to three generations per year in the northern part of California and even more in the southern part of the state (Dahlsten et al. 1998; Lawson and Dahlsten, 2003).

The life cycle of one generation was found to be around 50 days in Tuscon, south-central Arizona. Densities vary between generations and years, such that the beetle may occur in low numbers in the first generation and high numbers in the second or vice versa. Details of elm leaf beetle life history and related information can be found in, for example, Wene (1968) and Weber and Thompson (1976) (Bosu and Wagner, 2008).

Life Cycle

Its life cycle can be affected by a cold climate and lengthy, frosty winters. Overwintering adults can be killed by long winters or late springs. Strong winds can blow larvae off trees (Morton Arboretum, 2013).

Environmental Requirements

The elm leaf beetle requires a minimum of 300 growing-degree days and a maximum of 912 growing-degree days for development (Adams, 2013).

There are many natural enemies of elm leaf beetle, including two eulophid wasps in the genus Tetrastichus, Oomyzus (=Tetrastichus) gallerucae and Tetrastichus brevistigma.

During feeding, the larvae skeletonize the leaf, leaving the upper surface and veins intact. This gives the foliage a net-like appearance. Areas of the leaf around the feeding site will dry up and die, causing the leaf to drop prematurely. Adults, on the other hand, chew small, irregularly-shaped holes in the expanding leaves. Trees that lose their leaves often develop a second flush, only to have them consumed when the next generation is produced. The larval stage is the more destructive part of the elm leaf beetle’s life cycle, and the presence of many egg clusters can give an indication of the extent of the defoliation that will occur later on. Most trees will not be killed outright as a result of this destruction, unless it is nearly complete and is repeated for 2-3 consecutive years (Morton Arboretum, 2013).

The elm leaf beetle can be easily confused with other beetles, such as the western corn rootworm (Diabrotica virgifera virgifera LeConte), the striped cucumber beetle (Acalymma vittatum (Fabricius) and other species of elm beetles such as the European elm bark beetle (Scolytus multistratus) and the native elm bark beetle (Hylurgopinus rufipes). The elm leaf beetle can be positively identified by the distinctive black spots on the body segment behind the head (Walker, 2009).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Host Resistance

The use of insect-resistant trees is one of the current control methods. Previous efforts to control this pest have focused on the preference for, and suitability of, various elm species and their hybrids for feeding by the elm leaf beetle (Miller and Ware, 1999). Townsend (1979) reported that 175 elm species and hybrids were then available for planting in the USA. Many of these elms were hybrids developed for resistance to Dutch elm disease and originated from Asia and Europe. New species have been introduced to the USA from China in recent years and this has accelerated hybridization (Miller and Ware 2001a,b,c; 2002). It was estimated by Bosu and Wagner (2008) that over 200 elm genotypes are now available in the United States.

Early Warning Systems

The elm leaf beetle is a ‘List A pest’ declared under section 101(a) of the Plant Quarantine Act 1997 in Australia, and measures must be undertaken by landowners to control or eradicate the pest.

Biological control

Foliar sprays with Bacillius thuringiensis ssp. tenebriosis have been used (Dreistadt, 2008). Several attempts to release natural enemies of the elm leaf beetles have been only partially successful. In the eastern and midwestern United States and California, biological control was attempted from 1908–1936 (Dowden, 1962; Clausen, 1978) and during the 1970s and 1980s (Clair et al., 1987; Dreistadt and Dahlsten, 1991; Dahlsten et al., 1998).

The eulophid egg parasite Oomyzus (=Tetrastichus) gallerucae (Fonscolombe) was the primary organism introduced and released during these periods (Clausen, 1978; Clair et al., 1987). These programs in California resulted in limited establishment of O. gallerucea. Impact was only significant when releases of the parasite were made each year, allowing it to become marginally established (Ehler et al., 1987; Dahlsten et al., 1998). Some success was obtained with O. gallerucae in southern California (Luck and Scriven, 1976).

Tetrastichus brevistigmaGahan (1936) is apparently a native of the northeastern USA. This other species of eulophid wasp attacks elm leaf beetle pupae. It is a gregarious internal parasite. First described in 1932 in Massachusetts, it is widespread throughout the eastern states and is present as far west as Indiana and Kentucky. In 1934 it was shipped to California, released, established immediately and became widely distributed and abundant in some areas and during some years, but seems to be unimportant compared with other elm leaf beetle parasitoids introduced there. The females lay from 1 to 8 eggs at a time in beetle pupae or prepupae. T. brevistigma commonly parasitizes 50-80% of the beetle pupae in midseason. In areas where parasitization is less, augmentative releases might prove effective. Although rearing techniques have been developed, neither T. brevistigma nor T. gallerucae is currently commercially available (Marh, 1998).

Chemical Control

For many years, chemical treatment has been the primary method of control for the elm leaf beetle. Both foliar and systemic insecticides have been used with various levels of success. Systemic insecticides have several advantages over foliar applications when used on large trees in an urban setting: they eliminate the problem of pesticide drift, which is of particular concern in densely populated urban areas, and much lower amounts of pesticide per treatment can be used.

In recent years, new systemic insecticides have appeared on the market for control of elm leaf beetle, including trunk injection formulations of imidacloprid (a synthicic chloronicotinyl insecticide) and abamectin (a fermentation product of a soil bacterium). These insecticides have been integrated in a city-wide integrated pest management (IPM) in Sacramento, California, by Lawson and Dahlsten (2003), who showed that trunk injections of abamectin and imidacloprid reduced the defoliation caused by elm leaf beetle when applied at the peak density of eggs. The problem with this treatment is the potential scarring and wounding of trees, which may serve as entrance points for pathogenic or decay fungi.

Bark banding is also used (with broad spectrum insecticides) to stop migration of larvae down the trunk of the trees (Dreistadt, 2008).

Intergrated Pest Management (IPM)

Current recommendations are to use an integrated program that incorporates resistant elm hybrids, good cultural practices, conservation of natural enemies and regular monitoring for the presence or absence of eggs for better timing of insecticide applications. 

11/08/10 Original text by:

CRCNPB Australia, CRC for National Plant Biosecurity, Canberra, Australia