INSECT DAMAGE TO SEEDS OF PACIFIC COAST CONIFERS. 5 



form. There are several species, one of which is very destructive, to 

 the seed of Douglas fir, white fir, and red fir. 



FIR-CONE MAGGOTS. 



Fir-cone maggots are the larvae of small gnats which have been 

 found hi the cones of white fir, red fir, and alpine fir. They mine 

 through the scales and seeds, causing great damage. The larvae do 

 not winter in the cones but burrow into the ground as soon as the cones 

 fall. They form small puparia within an inch or so of the surface, 

 and there they overwinter. 



ADAPTATION OF THE INSECTS TO THE INTERMITTENT CONE- 

 PRODUCING HABITS OF THE HOST TREES. 



There is a general life cycle for most of the cone-infesting insects 

 corresponding to the. period required by the host tree to develop the 

 seed crop. The adult insect, whether beetle, moth, fly, or seed 

 chalcidid, deposits the eggs in the spring or early summer while the 

 cones are small and undeveloped. With some species the attack is 

 such that the cone is killed ; with others the attack and feeding of the 

 larvae do not interfere with the growth of the cone, which matures 

 at the normal time, although much of the seed may be destroyed. 

 The feeding of the larvae ceases, however, when the cone matures, 

 usually during September. The insects then undergo a long dormant 

 period either as larva?, pupa?, or new adults. This dormant period 

 continues until there is another crop of cones in a proper condition for 

 attack; that is, the soft, immature cones which are found in the spring 

 or early summer. Some insects pass this dormant period in the pith 

 of the cones or in resinous masses among the scales. Other species 

 leave the cones and form the pupae in the ground or in debris on the 

 surface. 



The intermittent character of the seed production of conifers is a 

 well-established fact. 1 A few cones are produced every year, but a 

 good crop occurs at intervals of from two to five years. The years of 

 total failure are known as "off years." It is evident that if the 

 entire brood of any of these species of cone-infesting insects emerges 

 annually, it will sooner or later encounter an off year of the host tree. 

 This would mean the complete failure of the food supply for one 

 generation and would result in the almost complete extinction of the 

 species within the forest area affected by the crop failure. As a 

 matter of fact, observations show that this seldom happens. All 

 the individuals of a brood of overwintered insects do not emerge the 

 following spring. Many of them do emerge after the first winter, but 

 a large percentage of the brood, in some species 50 per cent or more, 



i U. S. Dept. Agr., Forest Service, Bui. 98, p. 13, Nov. 18, 1911. 



