Pieris brassicae (Lepidoptera: Pieridae)

The Pieridae are a pretty family of butterflies (the sulphurs and the whites). I’ve written about them in a previous article; this post concentrates on a single species, one of the few insect species in the world important enough to deserve its own book: Pieris brassicae (the book is Feltwell (1982)). It’s a species native to most of Eurasia, but with high invasive potential. The butterfly is colloquially called the Large Cabbage White, and the caterpillar is well-known as an agricultural pest of cabbages; this post is about the caterpillar.

Pieris brassicae

The species is frequently trivoltine, going through three generations per year: the first adults emerge in early spring, and caterpillars emerge in two bouts from spring to late summer, or even autumn in exceptional cases.

Pieris brassicae caterpillars feed only on glucosinolate-releasing plants, as characteristic of the Brassicaceae, the family that includes cabbages and mustards (Hopkins et al., 2009). Because of the repellant nature of glucosinolates, this relationship has classically been viewed as curious, and was the subject of the very first research project dedicated solely to examining the chemistry of an insect’s relationship with its host plant (Verschaeffelt, 1910).

When a cabbage leaf is damaged, the glucosinolate is broken down by the enzyme myrosinase, and several gases are released, including mustard oils that act as deterrents for most herbivores. Despite this, P. brassicae caterpillars will always prefer feeding on these plants, and they even grow larger than when fed on non-toxic leaves (Smallegang et al., 2007). Their secret is that they’ve evolved a protein in their gut that detoxifies the harmful breakdown products of glucosinolates (Wheat et al., 2007).

Pieris brassicae congragation

In the wild, most of these plants do not tend to be large or long-lived enough to support the 150-strong broods of Pieris brassicae (Le Masurier, 1994). Suitable plants include wild cabbages, and where they grow, they will often harbour large populations of the caterpillars (Moyes et al., 2000).

In agricultural lands, this limitation is removed, leading to the species becoming a serious pest (Kular & Kumar, 2011). Besides the obvious consequence of massive herbivory from the caterpillars, just egg-laying has a negative effect on the plants. Little et al. (2007) noted that the plant’s gene expression changes in the same way that it changes when infected with a pathogen. Localised cell death occurs, and there is higher production of reactive oxygen species, an action that promotes the expression of defensive genes (Suzuki et al., 2011). The eggs also induce the accumulation of salicylic acid, which is a hormone that suppresses the plant’s defenses (Bruessow et al., 2010).

These reactions are part of a coevolutionary race between the caterpillar and the plant: as the caterpillar evolves better ways to feed, the plant evolves better ways to defend itself, so the caterpillar evolves ways to counter those defences, ad infinitum. The lengths to which this has gone to can be seen in the indirect defences of the plants, described next.

Herbivory induces even more defensive reactions, as the plant reacts to several chemicals produced by the caterpillar, most notably the lytic enzyme ß-glucosidase in the caterpillar’s regurgitant (Mattiacci et al., 1995).

This enzyme causes the plant to start producing terpenoid volatiles that act as indirect defense: they attract females of a curious little wasp, Cotesia glomerata (Geervliet, 1997). This species is a rather devious parasitoid that seeks out young caterpillars feeding on the top leaves – they are more nutritious, meaning the caterpillar will be a better food source for the young (Coleman et al., 1997). The female will lay 20-30 eggs inside the caterpillar, where they will hatch. Parasitised caterpillars can host more than 70 wasp larvae inside them. When they are ready to pupate, the wasp larvae get out of the host caterpillar, who then proceeds to spin a web around the wasp pupae and aggressively defend them against any disturbance. In other words, these are another example of parasites that affect their hosts’ behaviour. C. glomerata can infect as much as 80% of the larvae on a plant (Baker, 1970).

P. brassicae has another nefarious parasite, the hyperparasitoid wasp Trichogramma brassicae. A hyperparasitoid is a parasite that infects eggs. But this one has quite a roundabout way of reaching the eggs. When butterflies mate, the male has several tricks to ensure the female will not mate with another male. One trick is to fill the female’s mating receptacle with apyrene, non-fertilising sperm, so no active sperm from other males can fit (Cook & Wedell, 1999). Another trick is to transfer pheromones that act as anti-aphrodisiacs (Andersson et al., 2003). This not only a dick move for other males, one anti-aphrodisiac, benzyl cyanide, also attracts female T. brassicae (Fatouros et al., 2005). They hitch a ride on the now-pregnant female butterfly, and as soon as she lays her eggs, the wasp gets off and injects her own eggs into the butterfly’s eggs.

Besides these parasites, the caterpillars often find themselves in the digestive tract of birds. They are fairly conspicuous and have no effective defence, which may explain why the adults lay so many eggs.

Pieris brassicae

The photos scattered through the article show the typical appearance of the caterpillar. They are taken by my lovely monandrous mate whose photography website I designed, Mia Delfini.

It’s fairly easy to culture the larva at home. Keep them on Brussels sprouts at temperatures between 20-25°C, and 16 hours of light to 8 hours of dark.

Fun fact: you can cut all nervous connections to and from the brain, even transplant it into the abdomen, and the caterpillar will still be able to measure the photoperiod, because the pigment responsible is not in the eyes, but in the brain (Claret, 1966). If you really have the necessary equipment, keep the wavelength of the light at 400-530nm, the wavelengths that the species is most sensitive to (Claret, 1972).

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