Infestation by Crustacean Borers (Fig. 2)

Limnoria numbers were estimated by ruling the test blocks into one-inch squares and counting the number of burrows. These usually follow the grain, and are marked at the surface by a series of respiratory holes. A strong needle run along the line of these respiratory holes will generally expose the tunnel for its full length. The male and female live together at the head of the tunnel, female first, and the young are liberated fully developed from her brood pouch. They commence to burrow on their own account away from the parent tunnel, so that badly infested timber will show branching tunnel systems. As the tunnels eventually become three page 16 or four deep, or even more, it becomes impossible to do more than make a very approximate estimate of the number present. This point was reached in the present experiment in about nine months. The short-term blocks rarely showed "Limnoria damage," indicating that more than a month's immersion under local conditions on this particular type of test block is necessary before much damage is apparent. Counts on the long-term series show accelerated breeding in October, and again in March, 1950. In both instances, the Limnoria population trebled the previous month's total. The first sharp rise in population numbers coincided with rapid temperature rise in spring. The cumulative effect of warm summer water probably accounts for the second increase in autumn. A decrease in numbers in November is shown, and must be due to some unknown factor affecting breeding peculiar to that block. Apart from this one instance, population numbers increased throughout the experiment. The vertical block and the rougher surfaces were consistently more heavily attacked than the horizontal block. The presence of a fairly thick coat of mud on the upper surface of the horizontal block probably accounts for this, since it was observed that where mud had accumulated there was little "Limnoria damage."