Why might hedgerows lit by streetlamps contain fewer moth caterpillars than unlit hedgerows?

Answer: Female moths lay fewer eggs in hedgerows lit by streetlamps.

Streetlights make the leaves of hedgerow plants poisonous to moth caterpillars. — Wrong. Light can alter how plants grow and how much caterpillars eat, so a plant effect is a reasonable hunch. But there is no need for the leaves to become poisonous; that is a specific toxic step rather than the simplest explanation. A plant effect can be real without turning the leaves into a poison that harms caterpillars.

Adult moths all die at the lamp before they can lay their eggs. — Wrong. Moths are genuinely drawn to artificial light, so adult harm is a reasonable hunch. But saying that every adult dies at the lamp is too absolute and misses what happens across the hedgerow. Adults can survive yet lay fewer eggs in lit stretches, so the next generation of caterpillars is smaller even without a pile of dead moths.

Female moths lay fewer eggs in hedgerows lit by streetlamps. ✓ — Correct. Artificial light at night can disrupt adult moth activity, including mate-finding and egg-laying. Female moths may avoid illuminated stretches of hedgerow, so fewer eggs are laid there and fewer caterpillars hatch. Predators and plant quality may also play a part, but reduced egg-laying is a direct step in the life cycle.