Pirating Plant Defense

Cabbage Aphid

Cabbage Aphid

Plants in the cabbage family make glucosinolates (mustard oils) as a defense against herbivores. Glucosinolates are not toxic to the plant and are stored in separate compartments from the enzyme myrosinase. When plant tissue is damaged by chewing insects, the myrosinase and glucosinolates come together. The myrosinase converts the glucosinolates into toxic isothiocyanates.

Some species of insects adapted to cabbage family plants have the ability to detoxify the glucosinolates; some actively excrete them without activation and some store them in their body as defense. Brevicoryne brassicae, the cabbage aphid, feeds only on plants of the cabbage family that contain glucosinolates.

The aphid feeds by inserting its stylets into the plant phloem. The phloem contains glucosinolates but not myrosinase. The aphid does not metabolize glucosinolates but concentrates them in its body up to 15 times the concentration in the plant. The aphid has a special myrosinase that is confined to its muscle tissue so it is not in contact with glucosinolates. If the aphid is eaten by a predator, the myrosinase will be released & isothiocyanates are produced that poison the predator. Predators with weak detoxification systems, such as two spot lady beetle can die from eating these aphids. The cabbage aphid is one of many insects that can not only overcome plant defenses, but use those defenses to defend itself.

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
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