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Cottonwood Leaf Beetle

Cottonwood Leaf Beetle

FS278E
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Bryan Carlson, School of Biological Sciences, Washington State University, R. Rodstrom, GreenWood Resources, Portland, Oregon, John Brown, , Department of Entomology, Washington State University
Grown as a monoculture for biofuel, pulp, and non-structural saw timber, hybrid poplars suffer serious insect pest infiltration. Depending on location, hybrid poplar crops can be attacked by 40 different identified species of insect pests. These pests consume foliage, burrow into stems and bole, and destroy roots. Each publication in this series focuses on an individual identified insect pest of hybrid poplars. The publications are designed to aid poplar growers and insect pest managers by characterizing an insect pest, discussing the damage it causes, and suggesting strategies for managing it.
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Introduction

Large moncultures of Populus spp. grown for pulp, non-structural timber, or biofuels are commonly attacked by Chrysomela scripta. Integrated pest management (IPM) professionals in the Pacific Northwest (PNW) need to be able to identify leaf beetle damage, prepare a monitoring program, and develop a management protocol to prevent or remedy an outbreak of this pest.

Taxonomy

Chrysomeloidea is a superfamily with four families, one of which is Chrysomelidae, the leaf beetles. The genus Chrysomela identified by Linnaeus, was further divided into a subgenus Chrysomela (Macrolina). The species name scripta was attributed to Fabricius. Two other Chrysomelidae species attack Salicaceae trees. These include the imported willow leaf beetle, Plagiodera versicolora (Laicharting), introduced into the US in 1915 from Europe and widely distributed in the eastern US and southern Canada. It is sporadically found throughout North America (Riley et al. 2003). The other species is Altica populi, a defoliating leaf beetle found in the US; however, its distribution is unknown (Charles et al. 2014).

Hosts

Cottonwood leaf beetle hosts include poplar, willow, aspen, and alders.

Range

Cottonwood leaf beetles are common from southeastern Alaska, coast to coast throughout the northern states, and south to Florida in the east.

Life History

Adults survive the winter under leaf litter or within bark crevices. They become active in April. Gravid females deposit yellowish, oval-shaped eggs in clusters (Figure 1) on leaves. Each female can deposit multiple egg masses during her lifespan. Upon hatching, first instars feed gregariously as they skeletonize the leaf surface. Larvae consume large portions of the leaf in later instars. Eversible larval glands secrete salicylaldehyde as a defensive secretion.

Length of larval development is temperature dependent, requiring 19 days at 27°C (80°F) to complete their three instars. The larval instars are followed by a pre-pupal period, when an individual attaches itself to a midrib, petiole, or small branch and pupates. All three instars and adult beetles defoliate (Figures 1 and 2) host trees. There can be five generations per year in southern states (Coyle et al. 2005), and three to four generations in northern states and in the PNW.

Poplars grown for biofuels retain a large proportion of their biomass as succulent, long-shoot foliage preferred by C. scripta (Coyle et al. 2005). Cottonwood leaf beetle defoliation can reduce tree volume in certain clones by 70% over three growing seasons (Coyle et al. 2002).

Figure 1. Clockwise from top left, eggs (1 mm each), first instar aggregated feeding that causes leaf to be skeletonized, second and third instars exuding salicylaldehyde as a defensive white liquid, and the adult cottonwood leaf beetle (6 mm) on a leaf (Photos by J. Brown).
Figure 1. Clockwise from top left, eggs (1 mm each), first instar aggregated feeding that causes leaf to be skeletonized, second and third instars exuding salicylaldehyde as a defensive white liquid, and the adult cottonwood leaf beetle (6 mm) on a leaf (Photos by J. Brown).
Figure 2. Terminals damaged by cottonwood leaf beetle (Photo by R.A. Rodstrom).
Figure 2. Terminals damaged by cottonwood leaf beetle (Photo by R.A. Rodstrom).

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