In recent episodes of Bug of the Week, we learned that life is risky when you make a living feeding on the surface of plants. Recall the gruesome demise of rose aphids at the mouths of maggots and beetle larvae in “Death by Fly” (LINK TO MAY 12, 2008) and “Death by Beetle”(LINK TO MAY 19TH, 2008). Some insects have evolved a clever life style to reduce risks associated with feeding on the surface of a plant. Immature stages of many insects tunnel between the upper and lower surfaces of a leaf consuming nutritious plant cells along the way. In this way, they dine out of sight of hungry, predatory eyes.
Leafminer larvae scramble for a place to hide when exposed to the sunlight.
These insects are leafminers. As they eat and grow, they leave behind conspicuous trails. Sometimes the trails are serpentine and winding, as was the case with the native holly leaf miner (The Holly and Ivy, Part I) (LINK TO DEC 10TH 2007). Sometimes the mines are irregular and blotchy. Bug of the Week visited a meadow with a handsome patch of goldenrod wherein several plants wore the telltale marks of leaf-mining insects. Large irregular blotches discolored many leaves. Removing the upper surface of mine revealed small creamy colored larvae that scrambled to seek cover from the bright sunlight. These marvelous miners were larvae of the goldenrod leafminer. Goldenrod leafminers belong to a large and diverse group of beetles called leaf beetles.
Adult leafminers remove soft tissue and leave behind veins as they feed.
We met other leaf beetles like the dogbane beetle, in the meadow where it ate the leaves of the poisonous dogbane plant (Gold in the meadow) (LINK TO JULY 18, 2005) and the locust leafminer that scorched the leaves of black locust (Scorched locust) (LINK TO JULY 24, 2006). The adult goldenrod leafminer is a gorgeous insect with a black body and jazzy orange stripes on the side. The adult beetle also eats the leaves of goldenrod. As it dines, it etches linear trails in the upper surface of the goldenrod leaf. The beetle’s tiny jaws are not powerful or large enough to chew entirely through the leaf. Instead, it feeds between the lines, removing tender leaf tissue and leaving behind tough leaf veins. This type of feeding is skeletonization.
The female leafminer deposits eggs in small batches on the underside of a leaf.
The female goldenrod leaf miner lays several small clusters of eggs on the lower surface of the leaf. After depositing her brood, she engages in the rather unorthodox behavior of defecating on her eggs. No, this is not some bizarre form of maternal dominance. The nasty coating probably discourages would-be predators from making a meal of her tasty young. After hatching, the tiny larvae chew through the surface of the leaf and begin their lives as miners of goldenrod.