As settlers moved west, in the U.S.A., during the second half of the nineteenth century, they eventually reached the State of Colorado. A harmless beetle that parasitised the wild Solanum rostratum, known as prickly potato, or the buffalo burr, moved into their potato crops and it became one of the worst crop parasites ever known. Like blight, it is a 'new encounter' parasite. Its Latin name is Leptinotarsa decemlineata, and it was given the English name of Colorado beetle, after its home state, and its centre of origin.
There are many areas, such as the United Kingdom, Africa, Central and South America, and the Pacific Northwest of North America, where the Colorado beetle does not occur. Anyone breeding potatoes in such areas obviously cannot accumulate horizontal resistance to this insect pest. And any new potato cultivars produced in such an area will have a commercial value limited to those beetle-free areas. Conversely, beetle-resistant potato cultivars will be useful in beetle-free areas, even if their resistance is unnecessary. These limitations should be borne in mind by anyone thinking of starting a potato-breeding club.
David Fisher (see Recommended Reading) is breeding potatoes with success, in the U.S.A., for horizontal resistance to Colorado beetle, and one of his colleagues has shown that the horizontal resistance is not due to a high glyco-alkaloid concentration. He also makes the useful observation that, because potatoes can lose up to one third of their leaf tissue without significant yield loss, it may be economically feasible to control the losses caused by Colorado beetle with levels of horizontal resistance that are far from maximal.
Some insect larvae, such as the Monarch butterfly, which feeds on poisonous milkweed, and the Colorado beetle, which feeds on poisonous potato leaves, are able to isolate these poisons in a special sac inside their bodies. This makes both the larvae and the adults poisonous to insect-eating birds. From their colouring, insect-eating birds recognise these poisonous insects and avoid them. Consequently, putting chickens in the potato patch will not provide a biological control of Colorado beetle. (Frogs and toads do not have this instinct, and they die if they eat these insects).
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