Aliens & Pests

The Forestry Commission has announced plans to replace non-native trees in England's forests with native species. The intent is to restore the forests to something resembling the forests of 100 years ago, and to make forests more attractive for wildife and recreation. Up to 20 million trees will be removed over a 20 year period. Reduction of conifer shade should allow native species to reproduce, though enrichment planting will also be funded.  Up to 30% of England's forests consist of ancient woodlands, stands over 400 years old, but which include lots of non-native conifers. The government had encouraged establishment of conifer plantations which today account for 15% of England's forests. Ironically, the government had paid for establishment of the conifer plantations that it is now paying to remove.

A seed is a wonderful place to live if you are a small insect. Protected by the seed coat and the fruit or cone, nourished by storage compounds like starch and fats, a developing insect has everything it needs. Many insects oviposit (plant eggs) directly into developing embryos. If an insect makes the wrong choice and oviposits into an unfertilized ovule, the developing insect may be out of luck. In conifers, unfertilized ovules become empty seeds, and the tree puts no resources into empty seeds.

Patrick von Aderkas of the University of Victoria and his colleagues found that some insects can avoid making the wrong choice by making the plant feed a larva even if the ovule was not fertilized. A tiny chalcid wasp, Megastigmus spermotrophus (Hymenoptera: Torymidae), deposits eggs in Douglas-fir ovules before they are fertilized. As the wasp larva develops, resources are directed to the seed just as if it had been fertilized. Instead of depending on the tree to supply resources to a fertilized see, Megastigmus manipulates seed development to its own advantages.

von Aderkas P et al 2005 Seed parasitism redirects ovule development in Douglas fir. Proc. Royal Society B Early Online Publishing