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BULLETIN 351, U. S. DEPARTMENT OF AGRICULTURE. 



The Leafward Migration as a Factor in the Spread of the Terrapin Scale. 



The leafward migration is a strong element in the spread of the 



scale over the branches of infested trees, but it is not directly effective 



in spreading it from tree to tree unless the trees are in actual contact. 



Indirectly it is* one of the strongest factors in the spread of the 



scale. The young larvse are not readily displaced by wind, but 



they sometimes drop purposely 

 from dead twigs, especially when 

 they have reached the tips with- 

 out finding foliage. Such larvae 

 may fall upon foliage lower down 

 or drift in air currents to foliage 

 on adj acent trees. Most of them, 

 however, perish on the ground. 

 During windy days particles of 

 bark and loosened leaves are car- 

 ried by the wind. That wind is 

 a prominent factor in the local 

 spread is indicated by the fact 

 that infestations travel through 

 orchards in the direction of the 

 prevailing wind. Thunderstorms 

 sometimes come so suddenly that 

 the young migrants are washed 

 from the twigs before they have 

 reached the leaves. This seldom 

 happens, because the young do 

 not ordinarily emerge when the 

 humidity is high. The migrants, 

 when displaced by rain, will float 

 for some distance, especially if ac- 

 companied by particles of bark 

 or other debris. 



The spread, except as indicat- 

 ed, requires the aid of some trans- 

 porting agent. The migrating 

 larvae cling readily to hairs, to feathers, and to other small ob- 

 jects. While the author has never taken insects with the larvae 

 attached, he has placed specimens of Brochymena upon branches 

 covered with migrating young, with the result that the larva 1 

 were soon clinging to their legs. Feathers touched lightly to 

 the same branches were clasped by the moving young. A pair 

 of cloth gloves placed for 10 minutes upon a branch had 20 

 larvae upon them when removed. This last observation indi- 



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Ftg. 6. — Tracing of four young terrapin scales during 

 the leafward migration. Reduced 8 times. Tem- 

 perature, 87° F. Average rate per hour, 29.095 cm. 

 (Original.) 



