Killing Burrowed Insects

There is natural concern that insects that have burrowed deeply into wooden objects will be slow to respond to an anoxic atmosphere, and that this could substantially increase the amount of time needed to reach 100% mortality. One approach to determining the additional treatment time is to measure the diffu- sion exchange rates of nitrogen, oxygen, and air through different types of wood. The porosity (air volume) of most woods when dry is about 45-50%. After a wooden block that has been standing in air is placed in a sealed chamber containing essentially pure nitrogen, the oxygen content within the chamber will rise as the oxygen diffuses out of the pores. The amount of time needed to bring the wood to anoxic conditions throughout can thereby be determined and then used to calculate the additional time required for treating infestation. Lambert and Maekawa evaluate the exchange rate through poplar, oak, and walnut in this manner (Lambert and Maekawa, forthcoming). Wooden blocks (3.8 X

3.8 x 50.4 cm, about 700 cm 3 ), both uncoated and coated along the sides with shellac, were placed in a 32 l acrylic chamber containing 5000 ppm oxygen in

nitrogen. The increase in oxygen was followed with a Teledyne trace-oxygen analyzer, which could detect changes to 1 ppm. The time required to get to equilibrium for the six samples, described in Table 2.4, ranged from 9 hours for uncoated poplar to 120 hours for coated walnut. However, the long exchange times are misleading because they were determined for blocks of fresh wood.

Insect boring and tunneling open up the structure of wood and dramatically increase the rate of exchange. The time needed to get the oxygen out of

Table 2.4

Time required to completely displace air with

W ood type

nitrogen in dried wood a (Lambert and Maekawa, forthcoming).

Sample condition

Poplar

O ak

W alnut

120 h a Wood block size: 3.8 x 3.8 X 50.4 cm; chamber size: 32 l; ambient temperature: 23 °C; starting 0 2 % in N 2 :0.1-0.2%;

equilibrium 0 2 % in N 2 : 0.40%.

Chapter 2 Chapter 2

In a more direct assault on this problem, Rust and Kennedy (1993) sealed powderpost beetles and western drywood termites inside blocks of wood and compared the time needed for their total demise with the time needed to kill unsealed insects. Beetles of the Lyctidae family, collectively referred to as true powderpost beetles, feed on hardwoods such as oak, walnut, and mahogany. Western drywood termites, one of the most destructive termites in the United States, establish themselves in almost any wood that is not decayed. Rust and Kennedy encased these insects in blocks of Douglas fir, measuring 8 x 8 x

14 cm, that had been cut in half and hollowed in the center to hold small, open- ended, glass cylinders containing the insects. To eliminate air leakage through the sectioning, a sheet of parafilm was sandwiched tightly between the two block halves. There were 1.3-2.5 cm of solid wood surrounding each vial. In control studies with exposed insects, it took 5 days to completely kill all life stages of the powderpost beetle. With the beetles placed inside the wood, 100% mortality was achieved in 5 and 6 days in separate tests. Nearly identical results were obtained with the western drywood termites with and without enclosure in

the Douglas fir blocks. Rust and Kennedy concluded that the deep burrowing of insects into wood does not significantly increase the time needed for their complete extermination by anoxia.

Valentín (1993) investigated the exposure times needed to eliminate all insect infestation in wooden pieces, books, and other items in a collection of museum objects. Her results are summarized in Table 2.5. The species studied are active

feeders in works of art made of pine, cedar, walnut, oak, mahogany, and chest- nut. Column A shows the actual kill times for insects in the infested objects; column B shows the kill times for control insects exposed to comparable environ- mental conditions. There was little or no difference in required kill time between insects in infested objects and their controls for six of the eleven treatments. At

the other extreme, a complete kill of the furniture beetle in polychrome sculp- tures and a piano ranged from 10 to 14 days, substantially longer than the

Killing Burrowed Insects