Pest & Disease Spotlight – Caterpillars
January 1, 2024

by Kate and Len Lucas

It will not have escaped your attention that a caterpillar looks nothing like the finished article of butterfly or moth. Such is a product of evolution. The transformation from the one to the other is the very reason why caterpillars cause so much damage in the garden.

Why so? The sole function of the caterpillar is to eat as much as it possibly can because the transformation to beauty can only happen by using a large amount of stored food reserves and those reserves are built up through a regime of gluttony. That enormous change from caterpillar to adult cannot take place overnight which is why the caterpillar “rests” as a chrysalis so it can completely rearrange it’s body tissues into the finished product.

And to complete the picture the sole purpose of the adult insect is to disperse the species and find a mate, all made easier by having wings.

So a caterpillar is an eating machine and arrives uninvited to dine on what we have provided. As if that wasn’t enough not all caterpillars turn into butterflies or moths but the underlying biology is the same in that they turn into something with wings. Some caterpillars in your garden will turn into sawflies, which of course aren’t flies at all but are closely related to ants, bees and wasps.

This difference is significant because most gardeners who grow cabbages, cauliflowers and nasturtiums will be familiar with the cabbage white butterfly who’s arrival on your plot means the caterpillars will soon follow.
The caterpillars of the large cabbage white are brightly coloured and easy to see once they get big enough so you can remove them by hand. Those of the small cabbage white are much smaller, the same colour as a cabbage and will sit on the mid-rib of the leaf and will seem to be invisible. So don’t be fooled if you get the large one then you will surely get the small one.

A sawfly, wanting to lay eggs and looking nothing like a butterfly, is small and insignificant as it flits around your borders and will arrive almost undetected until the caterpillars actually hatch and start their three course meal.

The caterpillars of butterflies, moths and sawflies look very much the same and you can usually tell a sawfly
because their body often curls into an “S” shape as they eat the leaves and they usually have round black dots all along their body. As far as we gardeners are concerned they are all the same.

We were unaware of how bad an attack of sawfly caterpillars can be until our very ornamental red leaved Berberis suddenly had no leaves because they had all been eaten. Similarly these pests attack solomon seal (which is why we don’t grow it) and our Aquilegia (we put up with it because we like the flowers). We don’t grow fruit but for those who do sawfly can be a nuisance.

Our garden doesn’t really suffer from classic caterpillar damage but sawfly is bad enough every year for us to do
something about it and because we know which plants will be attacked we do spray them with insecticide before any damage starts. This same insecticide can be used to treat any caterpillar.

Let’s not panic here, there will be the odd caterpillars, mostly green, in the garden almost all the year round and you can safely leave them alone. The birds like them even if your’e not keen.

Sometimes when turning over the soil in the borders you might even come across some very fat caterpillars, with loads of legs, even in the winter. These are called cutworms and are the larvae of noctuid moths. They feed on the roots of all sorts of plants but most of the time we ignore them and probably so can you because they don’t turn up in large enough numbers. They can some times be a problem if you grow vegetables.

And just in case you are beginning to think that the natural world has it in for you-relax you have a lot to look forward to.