REVISION MALACOSOMA HUBNER IN NORTH AMERICA 121 



on Populus angustifolia were in the last instar, but the tents of calif ornicum 

 found nearby on Amelanchier utahensis were deserted and only a few 

 cocoons could be found. At the foot of the La Sal Mountains in San 

 Juan County, last instar i. discolor atum were found on Populus angustifolia, 

 but only a few fully mature larvae of californicum were found on Cerco- 

 carpus montanus nearby. 



This seasonal difference in development is further demonstrated by 

 what was found in Arizona. Sixteen miles north of Wikieup, Mohave 

 County, at an elevation of approximately 2700 feet, 3d, 4th, and last 

 instar larvae of incurvum incurvum were found on cottonwood. No cali- 

 fornicum were found nearby, but in the Aquarius Mountains to the 

 east at an elevation of approximately 4200 feet several colonies of 

 californicum were found, but very few larvae were present. Most of the 

 tents had been torn open by birds, and most of the larvae were gone. 

 Those that remained were mostly 3d and 4th instars, indicating that 

 the californicum larvae were not far behind in developm.ent, if any 

 (since the birds may have eaten only the larger larvae), even though 

 they were approxim.ately 1500 feet higher (Coll. Nos. 208 and 209). 



It is not known whether the earlier emergence of adult californicum 

 at these various localities is due to an earlier date of hatching or to a 

 faster rate of development, or some other factors, but the end result 

 is the same . . . m.ost of the californicum adults emerge before the 

 incurvum adults, thereby greatly limiting any gene exchange which 

 might take place if they emerged synchronously. Under laboratory 

 conditions they readily mate and produce normal cg^ masses which 

 appear to complete their embryological development satisfactorily. 

 It is not known whether any breakdown occurs in the Fj or F2 since it 

 was not possible to rear them for the reasons already given. 



Neither californicum nor incurvum are greatly restricted in their choice 

 of hosts, but they do show certain host preferences in some localities 

 that are worth comparing. M. incurvum discoloratwn was most often 

 found on Populus fremontii in Utah, but also was found on Salix sp., 

 Populus angustifolia, Populus alba, and fruit trees. In Arizona incurvum 

 incurvum was found on Populus fremontii, Prunus virens, Salix spp. and 

 fruit trees. M. californicum fragile was most commonly found on Prunus 

 fasciculata in southwestern Utah, but was also found on Amelanchier 

 utahensis, Purshia tridentata, and fruit trees. M. californicum was found on 

 Cercocarpus ledifolius, Cercocarpus montanus, Amelanchier utahensis, and fruit 

 trees in the areas in Utah where it was found at the same locality as 

 incurvum discoloratum. It also occurs on a wide variety of other shrubs 

 and trees in Arizona and other parts of its range in western North 

 America. 



