One of the most vulnerable stages in an insect’s life cycle is the egg stage, or when larvae first hatch from eggs. What can adult insects do to ensure their young survive these sensitive stages?
Social insects such as ants, termites, and some bees and wasps are experts at protecting offspring. Their colonies are highly organised with a caste system made up of reproductives (fertile queens and males) and sterile workers. In a colonial structure such as this offspring are produced by the reproductives, and protected within the nest by workers. In other words the entire colony is focused on the well-being of the next generation.
Non-social insects have different strategies. Predatory wasps of the families Crabronidae, Sphecidae and Vespidae either construct mud nests or dig nest burrows. These wasps hunt for an insect or spider to sting and immobilise, then place it in the nest to lay an egg on. The hatching wasp larva will be safe and sound inside a well constructed nest (such as a mud nest), or a hidden nest (burrow), with plenty to eat. Check out this Sphex sp. (Sphecidae) wasp hauling a paralysed locust into a burrow she dug (below).
Herbivorous insects such as aphids rely on rapid breeding, and careful positioning of eggs and young on food plants. Many aphid species lay live young (rather than eggs), and they have short life cycles of as little as 14 days in warm weather. Because of these two factors aphid populations can increase in size very rapidly.
Hover flies (Syrphidae) can find aphids easily, and the flies lay their eggs right in among the aphids. Those eggs hatch into ferocious aphid-eating maggots. Each insect – aphid and hover fly – is attempting to lay their eggs in such a way as to increase the chances of their offspring surviving. In the image below a rose aphid nymph strikes a jaunty pose oblivious to the hover fly egg behind it.
Not all insects are so precise with their egg laying. The “rain moth” Trictena atripalpis has a more lackadaisical approach to caring for her offspring. The larvae of these moths are known as “bardee grubs” and they live underground in tunnels and feed on the roots of various eucalypt species especially red gums.
Rather than laying the eggs carefully at the base of a red gum the female simply flies back and forth ‘bombing’ her eggs in the general direction of the roots of gum trees. Her strategy is ‘safety in numbers’. This insect has the highest recorded egg laying capacity of all moths – she has the potential to lay over 40,000 eggs! The more eggs she lays during her short life the more chance there is that some hatching larvae will survive to find food and start a new generation.
In contrast to our egg bomber above, some herbivorous insects lay their eggs inside the larval food plant thus giving extra protection to hatching larvae. The pear and cherry slug (Caliroa cerasi) is a good example. Actually before I get into that – a clarification. Pear and cherry slugs aren’t slugs – they are the larvae of a type of sawfly.
Caliroa cerasi is an introduced pest that is found throughout Australia wherever fruiting and ornamental cherry, pear, plum and quince trees occur. The sawfly cuts a slit under the first layer of leaf tissue and deposits an egg inside, causing the leaf to blister (below). The egg is protected under this layer of leaf tissue, safe from predators. Eventually a larva hatches from the egg and emerges from the leaf blister.
Leaf mining insects take this one step further. Their larvae remain inside the leaf throughout their larval and pupal stages, only emerging from the protection of the leaf as adults. You can see a citrus leafminer (Phyllocnistis citrella) larva below.
Beyond that you have insects such as various beetle larvae (borers) which spend their larval and pupal stages inside the wood of trees. Gall-inducing insects provide similar protection for their young. Of course there are predators and parasites which try to find them – but that’s a story for another day!
Some female insects, not content with providing optimum conditions for their eggs, actually stand guard over their eggs, and may continue this protection after the eggs have hatched. Earwigs, such as the pest European earwig (Forficula auricularia), are well known for this.
Female earwigs lay eggs into a burrow they have helped dig and stand guard protectively over the eggs (image below). They also keep the eggs clean to prevent fungal or bacterial infection. The females then look after the nymphs after they hatch by protecting them and feeding them.
Maternal care in insects! Who would have thought? Earwigs are not alone. Other common insects which show similar behaviour are some eucalyptus sawflies (Pergidae), and the cotton harlequin bug (Tectocoris diophthalmus).
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