Nature Notes

Vol. XII November, 1934 No. 11

The Cottonwood Leaf Beetle

red eggs
#1 Cluster of bright red eggs attached to underside of a leaf. (4 times natural size)


The most frequently encountered insect pest in the vicinity of Longmire in the summer of 1934 was the Cottonwood Leaf Beetle. During the months of June to August, larvae in various stages of development were noted on Black Cottonwood (Populus trichocarpa), Red Alder (Alnus rubra), and Willow (Salix sp). No infestation of the Sitka or Stream Alder (Alnus sinuata) was observed, even on individuals of this species growing with branches intermingling with infested ones of the Red Alder. The first brood matured about the middle of July, but larvae of the second and third generations were noticed until the first of September. The damage caused by this pest was only of average importance, although quite widespread, since only occasional leaves were affected.

#2 Larvae feeding on upper surface of leaf. (Natural size)

Larvae of the Leaf Beetle are nearly a quarter of an inch long when full grown; plump, rough, shiny black with four distinct white knobs on the back just behind the head. When alarmed, the larvae exude two rows of fluid in drops of seven on either side of the body. The composition of this fluid is apparently influenced by the type of leaf being digested, since larvae on cottonwood exude milk-white droplets having a strong odor of nitro-benzene (a usual component of shoe polish), while those on alder exude clear, straw-yellow droplets with a leafy, aromatic odor. As the larvae increase in size, they shed their skins from time to time, and may appear yellowish in color after such molts. The larvae feed side by side in compact groups of as many as ten on the upper surface of the leaf rasping away the green tissues and leaving only a fine network of veins and the transparent lower epidermis of the leaf. Usually only a part of the leaf is utilized during the ten to fifteen days required to bring a set of larvae to maturity. When ready to pupate, the larvae scatter; one or two to an individual fresh leaf, cement themselves by the last segment of the abdomen to the under surface of the leaf, and hand suspended. At this stage, the pupa is shaped like a violin, shiny black and yellow with the four persistent white spots showing as on the larvae. When the adult beetle emerges ten or fifteen days later from its discarded "flying trapeze", it resembles a ladybird beetle, although in fact a member of the Leaf Beetle Family (Chrysomelidae). Many of the adults are uniformly jet black, others are dirty yellow with 5 or 7 black spots arranged in a variety of patterns on the wing covers, while all intergrades between these two extremes are found. The adults feed for a time on the upper surface of the leaf, probably before laying eggs, but are not as voracious as the larvae and consequently do but little damage.

Two clusters of eggs of the Leaf Beetle were found on July 14. Both contained about 35 lozenge-shaped eggs fastened by one end to the surface of a fresh cottonwood leaf. The eggs closely resembled those laid by the common blowfly on decaying meat, but instead of being white were a glistening coral red. One lot of eggs of uniform color when brought to the laboratory (probably freshly laid) soon became mottled and hatched at the end of five days into small, rough, black larvae. The larvae immediately moved to the surface of the leaf and commenced feeding -- the second summer generation.

Victor Scheffer,
Season 1934

(1) Chrysomela lineatopunctate Forst. (Beller and Hatch, Colpoptera of Washington: Chrysomelidae. Univ. Wash. Publ. Biol. 1: 65-144, 1pl.1 1932)

Older name: Lina scripta (Fabr.) (Essig, Insects of Western North America. New York Macmillan, pp IX, 1035, 766 fig., 1929.)

#3 Pupa Suspended From Leaf. (Twice natural size.)

adult beetle
#4 Adult Beetle
(Size times natural size)
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