Elm leaf beetle

Appearance

The imago is 6–8 mm in length, and ranges from yellow to green in colour, with a spot on its head, an hourglass mark and two spots on the pronotum, and a broad, dark stripe along the edge of each elytron. The larvae are usually black, occasionally black and yellow, with multiple rows of dots on the back and on the sides and < 13 mm long. The pupae are orange-yellow with black chaetae. The ova are yellow, and laid in spindle-like clusters of < 25 on the undersides of the elm leaves.

Distribution

These beetles are common in the Western Palearctic realm from Portugal to Central Asia.
Indigenous to Europe, they were accidentally introduced to North America and Australia, and are now widespread, and are serious pests in Australia and parts of North America.

Habitat

"X. luteola" is a serious pest of the elm. Both the adults and larvae feed on the emergent leaves of the elm. Repeated heavy infestation rarely kills the tree outright, but weakens it, rendering it vulnerable to disease. Falling branches encourage elm bark beetles to proliferate; the beetles are vectors of Dutch elm disease.

Elm leaf beetles become active in the spring once temperatures rise above 52 °F. In fact, weather is one of the most limiting factors in their population growth. A late spring freeze or long winter can kill off entire colonies. The imagines depart their hibernation sites, the females laying their ova on the underside of the elm leaves. The ova hatch after one week, and the larvae immediately feed on the underside of the leaves. This larval stage lasts between two and three weeks, at the end of which larvae migrate to the lower part of the trees in search of bark crevices in which to pupate. The next generation emerges in midsummer after two to three weeks' pupation, and begins feeding on the leaves. The female can lay about 800 ova, but this ovipositing may be interrupted by shortening of the photoperiod to less than 14 hours, inducing a brief feeding bout before the search for a hibernation site.

Hosts are known to include English elm, wych elm, American elm, Siberian elm, and Chinese elm, as well as complex hybrids such as 'Homestead'. The beetle has also been reported on "Zelkova serrata".

When "X. luteola" oviposits, "U. minor" releases plant volatiles which recruit the egg parasitoid "Oomyzus gallerucae". "U. minor" is able to distinguish between oviduct secretions and damage which occurs in the natural course of oviposition, releases different volatiles upon detection of the oviduct compounds. "O. gallerucae" is then able to distinguish between these different sets of volatiles and is only attracted to oviposition. Gravid "X. luteola" females also respond, but inversely: They are attracted to volatiles indicating undamaged and/or unoviposited "Ulmus" material. Gene expression studies have shown a large number of genes to be involved in the oviposition response process.

Jasmonic acid is released during gravid females' herbivory on "Ulmus" spp.