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Mother Knows Best - in insects at least

Lachlan Jones

The world is crawling with herbivorous insects. Moths and butterflies, beetles, flies, bugs, sawflies thrips… all these leaf munching, wood boring or sap sucking insects make up about a quarter of the described species on Earth (the non-herbivorous insects make up another quarter). And as you’d expect for such a diverse group, their body structures, life cycles and behaviours have adapted in various ways to almost every habitat on the planet.

But there is still much they have in common. For one, they are almost invariably fussy eaters – most insects only eat plants of a few closely related species. They also rarely show any parental care - the mothers simply place their eggs on plants before leaving. The juvenile stages lack wings and cannot move far, so being laid on a plant they can eat is vital.

Hence comes the evolutionary prediction known as the preference-performance or mother knows best hypothesis – that the mothers give their offspring the best start they can by laying the eggs on suitable host plants.

Since the idea was proposed in 1975 by Christer Wiklund, well over 100 studies have been published on various insect species testing if the plants they lay the most eggs on are indeed better for survival of the juvenile stages. There have been a few attempts over the years to amass the work that’s been done to see if there is any general conclusion to be drawn, and for most the answer was no. They were wrong.

And the reason they were wrong was primarily that no-one was checking if the plants and the insect in a study had the same native range.

You might expect an Australian insect to correctly pick the best of a choice of Australian plants, but probably not if given a choice of African plants. And as there are so many introduced species these days, crops in particular, this is a concerning source of bias. So that’s where I came in. I managed to find 178 studies on 161 insect species that had compared egg laying with offspring survival across a set of plant species. These could be split roughly in half depending on whether the host plants were native or not.

In the native group it seemed mothers really did know best – 83% of the time.

Interestingly, it didn’t seem to matter how specialised the insect was, generalists performed just as well. There was also no difference between beetles, moths, sawflies, flies or true bugs – adaptive egg laying behaviour dominated within all of them. By contrast, when the insects and plants did not share a native range only 57% of insects laid their eggs on the right host plant – better than chance perhaps but a considerably worse outcome than with the natives. Which is a reminder if we needed one of the disruptive effect that introduced plant species can have on native insects.

For more information, see the link to the article in Arthropod-Plant Interactions