INSECT ENEMIES OF EASTERN FORESTS 67 
than a year. Although no reliable estimates are available, it is be- 
heved that the annual damage caused by marine borers runs into sev- 
eral million dollars. Recent information indicates that attack is 
restricted to the breeding season, which lasts from 8 to 10 weeks. 
Infestation is dependent on such factors in the water as salinity, 
temperature, pollution, hydrogen-ion values, dissolved oxygen, and 
sulfureted hydrogen. Infestation is increased in warm waters, where 
marine life develops luxuriantly. It is retarded where streams are 
polluted, resulting in a lack of dissolved oxygen and a concentration 
of hydrogen sulfide. With the molluscs, activity ceases when the 
water temperatures drop to just above the freezing point, when the 
borers remain dormant, according to Clapp (93). This condition is 
not so evident with the crustaceans. 
The two groups of marine borers are distinct in structure and 
method of attack. The molluscs are relatives of the clams, whereas 
the crustaceans are related to the lobsters. The former embed them- 
selves in the wood and do greater damage than the latter, which are 
superficial borers. Their attacks are limited to areas near the wood 
surface, principally between the low-tide and mid-tide levels of piling, 
where they are aided by the eroding action of water and debris in 
hastening the destruction of timbers. It is reported that under 
favorable conditions, a 14-inch pile can be destroyed in a few months 
by the molluscan shipworms, whereas it requires at least a year for 
the crustacean borers to do this. 
THE SHIPWORMS 
The molluscan borers are represented by three important genera, 
the wormlike Zeredo and Bankia (commonly known as shipworms) 
and the clamlike Martesta (Hunt and Garratt, 249). The free- 
swimming young shipworms usually attack timbers near the mud line, 
using their rasplike shells to penetrate the outer wood at right angles 
to the grain, and later bore longitudinally throughout the submerged 
portion of the timber (fig. 6, A.). As they bore, the body elongates 
and the burrows are lined with a calcareous deposit. The small open- 
ing to the surface enables the shipworm to extrude two siphons, one to 
admit the water and much of the food, the other to expel it. It is 
believed that in addition to small aquatic organisms, wood itself is 
partly digested and serves as food for these worms. The opening 
to the exterior can be closed by means of a pair of plates to exclude un- 
favorable water or harmful organisms. In heavy attacks the burrows 
are often from 1% to 14 inch in diameter and a few inches long. Under 
favorable conditions they may attain a diameter up to 1 inch and a 
length of 1 to 4 feet. Because their entrance holes are usually located 
near the mud line and are only 1% inch or less in diameter, the exten- 
sive destruction within the piling often is not realized until the timber 
gives way under stress. 
The clamlike-appearing molluscs, the I/artesia, like the shipworms, 
have free-swimming young which attack the timbers, making entrance 
holes up to 1% inch in diameter. Once embedded in the wood, they 
excavate it sufficiently to accommodate their bodies, which are usually 
not over 21% inches long and 1 inch in diameter when mature. They 
are capable of causing considerable damage to untreated timbers. 
