The Relationship Between Deer and the Yucca Moth

by M.J. Harvey - December 1999

You may not know this but getting seeds to form on a yucca plant in Canada is a minor triumph. I was thus really pleased with myself (‘chuffed’ in Yorkshire dialect), when transferring pollen with my tweezers from yucca ‘Ivory Tower’ to a similar but unnamed form resulted in four fat pods.

The reason that yuccas in northern gardens rarely set seed is the absence of a drab little moth called the yucca moth. The life cycle of this moth used to be featured in biology books because it demonstrated the balance of benefits in a close ecological specialization. The female yucca moth gathers pollen from yucca flowers and deposits it on the stigma of another yucca flower. At the same time she lays an egg of two on the yucca ovary. The net result is that the yucca sets seed and the moth grub lives safely inside the developing fruit eating some but not all the seeds. They both benefit.

I used to tell our first year biology students at Dalhousie University that everything we would tell them would be untrue. That we had to deal in generalisations and that there were exceptions to all rules – at least in Biology. (Only Physics and Theology deal in absolute truths.) Now I’m sure that not all of the 40 species of yuccas in the Americas depend on the yucca moth, but in my experience alternative pollinators such as bees are not active on Vancouver Island.

Pod development on yuccas is slow, so by November they were still somewhat green. Anxious to allow the seeds to plump up as much as possible I left the capsules on their stalk, meanwhile confidently adding yucca seeds to the seed list.

By mid November the leaves on the salmonberry are yellowing and the local deer turn to alternative plants for fodder. One particular doe which frequents our property had ignored the yuccas all summer (they are very fibrous). She now found yuccas absolutely delicious.

The very day on which the seed list arrived in my mailbox, irrevocably printed, I walked down the path and saw the gaunt yucca stalk utterly podless. Only a chewed fragment lay on the ground. And that my dear friends is why there are no yucca seeds available this year.

The next time I saw the doe – she was under the apple tree munching the last windfalls – I gave her a stiff dressing down. From about 10 metres away she looked at me with those big, soft eyes not even ceasing to chew her cud. I sometimes think she doesn’t understand a word I say.