Insect eggs on leaves, walls and windows: what they look like, whose they are, and which ones to leave alone

Insect eggs are one of the most common “what is this?” photos, and also one of the hardest. They are tiny, they all look roughly like dots, and the insect that laid them is usually long gone. The good news is that where they are and how they are arranged tells you more than the eggs themselves. A tidy cluster under a leaf, a smear on a fence and a capsule behind the fridge are three very different stories.

Eggs on the underside of leaves

Most plant-feeding insects, and many of the insects that eat them, lay on the underside of leaves, where the eggs are shaded and out of the rain. Turn a leaf over in summer and you will often find one of these.

Stink bugs are the tidiest layers. Penn State Extension describes the brown marmorated stink bug’s eggs as light yellow to yellow-red, about 1.6 mm long, laid side by side in groups of about 20 to 30 between May and August. After hatching, the empty shells remain as a pale honeycomb, often with the newly hatched nymphs sitting on top of it.

Ladybirds lay smaller groups of upright eggs. Ohio State University Extension says they are yellow and football-shaped, laid in clusters of 10 to 50 among aphids, and hatch in three to five days into larvae that the factsheet compares to tiny black alligators. Both the seven-spot ladybird and the Asian lady beetle lay this way, and from the eggs alone you usually cannot tell them apart.

Butterflies usually lay one egg at a time. A note from UC Agriculture and Natural Resources puts monarch eggs at about 0.9 to 1.2 mm, creamy yellow with fine ridges running from top to bottom, usually one to a leaf and hatching in about three to four days. The monarch only lays on milkweed, which narrows things down a great deal. iNaturalist research-grade records on card #27 peak from July to September, so that is when single eggs on milkweed are most likely to be monarch eggs.

Green lacewings do something unusual. The University of California IPM programme describes each egg perched on the end of a slender silk stalk, pale green or white when laid and turning grey before it hatches. A lacewing egg looks like a tiny pin stuck into the leaf. The larvae eat aphids, so this one is a guest worth having.

Eggs on bark, walls, fences and windows

Eggs laid on hard surfaces are often laid in autumn and left to survive the winter, which is why they come wrapped in something.

The spotted lanternfly covers its eggs in a grey-brown coating that looks like a smear of putty or dried mud. Penn State Extension says the masses are laid from about September to December and hatch from late April to June, with about 35 eggs in each. They are laid mostly on trees, but also on decks, house walls, stones, fences, rusty metal, outdoor furniture and vehicles, which is how they travel. iNaturalist research-grade records on card #121, the spotted lanternfly, peak from June to September, with about 60 times as many records in July as in January. The adults you saw in summer are the ones laying now.

Praying mantises lay a foamy mass that hardens into a tough, papery case. Colorado State University Extension says the cases are fixed to rocks, buildings, fences, trees and dry stems, that each holds several dozen to over a hundred eggs, and that the eggs spend the winter inside and hatch in late spring. It also describes how to tell two of them apart: the native Carolina mantis makes a longer, slimmer case of about 2 to 3.5 inches with a pale stripe down the middle, while the European mantis makes a larger case, about 4 inches, with a fainter stripe or none. On card #73, iNaturalist research-grade records of the European mantis peak in August and September, so a fresh, pale case found in October was probably laid this autumn.

Front desk note: a mantis egg case on the garden fence is a whole class of next spring’s hatchlings, quietly waiting for the bell. Leave it on the fence.

Eggs indoors

Indoors, loose eggs are uncommon. What you usually find instead are egg cases. The University of California IPM programme’s cockroach notes describe the American cockroach gluing a case about 3/8 inch long to a sheltered surface, brown at first and black within a day or two, holding about a dozen young. The German cockroach carries its smaller, light beige case until a day or two before it hatches, and each holds about 30 young. Cases in cupboards, behind appliances or under furniture mean the cockroaches are already living there, and the kitchen guide covers what to do next.

For tiny pale eggs along mattress seams, see our guide to the early signs of bed bugs, which deals only with the evidence on the bed.

Which ones to leave, and which to remove

The general rule is to find out first. Many egg clusters belong to predators that are on your side, and our own Field Guide shows how varied the starting points are: of the 181 cards, 101 are insects that go through complete metamorphosis (egg, larva, pupa, adult) and 43 go through incomplete metamorphosis, where the young hatch looking like small adults. A stink bug egg hatches into a tiny stink bug. A ladybird egg hatches into something that looks like nothing you would expect. The life cycles guide explains the difference.

If you cannot place the eggs, wait a few days and photograph what hatches, or photograph the adult insect nearby. The identifier works best on a clear shot of the insect itself, and it will say how sure it is.

Sources

• Penn State Extension: Spotted Lanternfly Management Guide
• Penn State Extension: Brown marmorated stink bug
• Ohio State University Extension (Ohioline): Lady Beetles (ENT-45)
• UC Agriculture and Natural Resources, Bug Squad: Of Monarch Eggs, Ladybug Eggs and Oleander Aphids
• UC Statewide IPM Program: Green lacewings
• UC Statewide IPM Program: Cockroaches (Pest Notes)
• Colorado State University Extension: Mantids of Colorado
• Seasonal pattern: iNaturalist research-grade observations, as shown on card #121, card #73 and card #27