I was sitting and reading a book along the Pit River, near where it crosses California Highway 299 E in Shasta County when I noticed the alders (Alnus sp) overhead were swarming with black larvae. I could even hear their chewing. Closer inspection, and a little research, showed them to be alder flea beetle larvae (Macrohaltica ambiens).
,Alder flea beetle adults are a shiny dark blue. None were to be seen. However, the larvae which are dark brown to black dorsally and yellowish ventrally were hungrily feeding on the alder leaves. Both adult and larval alder flea beetles feed on alder leaves, the preferred host. The larvae are skeletonizers, that is, they feed on the tissue between the veins and leave the veins intact. The adult beetles chew holes in the foliage.
Alder flea beetles have one generation per year. Adults hibernate in the winter in duff at the base of trees or in other sheltered places and emerge in early spring. Dull yellow eggs are laid in clusters on the undersides of alder leaves. Once the eggs hatch the larvae begin to feed. Maturing in August, the larvae drop to the ground and pupate in chambers in the soil beneath leaf litter. After 7 to 10 days the adults emerge and begin the cycle again.
Alder flea beetles are found throughout North America, wherever their host plant occurs. There are two subspecies, an Eastern and a Western.
A synonym for M anbiens is Altica ambiens.
Now I need to return and search for adult alder flea beetles.