Introduced Plants Secret Weapon: Non-native Pests

Japanese beetles on milkweed
This nasty pest came to us courtesy of an innocent and beautiful Japanese iris
© Beatriz Moisset

Introduced plants have a dark side that we often ignore. I am not talking about plants that become invasive, but about some of the well behaved ones, the ones that never leave your garden or the park where they were first planted. Some plants bring a living cargo to this land and this cargo may turn out to be worse than we can imagine.

I will give a couple of examples, an iris plant brought to New Jersey about a hundred years ago and viburnum bushes also brought that long ago.

Hardly anybody remembers that innocent iris, but the grubs hiding among its roots are a well known pest that has been spreading steadily since its arrival. I am talking about the Japanese beetle.

Viburnum leaf beetle on arrowwood © Beatriz Moisset

The several introduced viburnums are handsome and well liked by gardeners. They are not known to cause any troubles. But a small leaf beetle is another story. This beetle was barely noticed for about eighty years. Entomologists acknowledged their presence a few times. It was associated with its host plant, the ornamental viburnums found in nurseries and gardens. Only in recent years it became a problem. We don’t know what the right combination of factors permitted these leaf beetles to jump to native viburnums and to prosper on them, on arrowwood in particular. But, once they discovered the native viburnums there was no stopping them. The nonnative viburnum leaf beetles have been spreading southward and westward and leaving behind ugly skeletons of the once healthy viburnums.

These two cases illustrate a couple of important points: 1) Introduced plants don’t need to be invasive to cause serious ecological damage. 2) Introduced pests can become invasive right away or they may take a long, long time. I often hear this argument: Oh! This plant isn’t invasive. Why shouldn’t I grow it in my garden? These examples should inspire caution toward introduced plants, even innocent looking ones.

Hemlock woolly adelgid, not just ugly; it kills trees
© Beatriz Moisset

If this isn’t enough cause for concern, let us take a look at a few other cases of pests or pathogens brought in by horticultural stock:

• The American chestnut once represented 20% of the forest canopy in Eastern North America. It was pushed to the edge of extinction by the chestnut blight brought in along with Chinese chestnut trees.
  
• Elms were afflicted with Dutch elm disease in a similar way.
  
• The hemlock woolly adelgid, that feeds on the sap of hemlocks, spread with hemlock varieties transported to other territories.
  
• Milkweeds are seriously affected by the oleander aphid. The plant that brought this pest is a handsome plant, oleander. This garden plant only grows in warmer climates in southern United States. But the bright yellow aphid has taken a ride on many species of milkweeds and it is widespread throughout the U.S. and Canada. No wonder monarch lovers loath it. Yet, some of them don’t hesitate to grow the non-native tropical milkweed and the butterfly bush.

Oleander aphid

Introduced with the plant, oleander

© Beatriz Moisset

In summary: “most nonnative insects and pathogens were introduced unintentionally as contaminants on nursery stock” (U.S. Congress. Office of Technology Assessment 1993). Introduced plants may be lovely and look quite innocent. If they don’t behave in an invasive manner we may be lulled into thinking that they are safe to grow in our gardens. However, we must remember the formidable hidden weapons some of them carry and think twice before planting them.

© 2015. Beatriz Moisset. First published in Native Plants and Wildlife Gardens.