THE INSECT FAUNA OF THE GENUS CRATAEGUS 1049 



It will be noticed that there is a direct correspondence between the time 

 of feeding of the insects and the time of growth of the trees. The greatest 

 number of species feed during May and June, when the trees make their 

 greatest growth. The number decreases slightly during July and August, 

 at the time when droughts frequently check tree growth, and then it 

 increases slightly in September, at the time when fall rains often cause a 

 new growth. This relationship between the period of growth and the 

 time of feeding seems to be one of Nature's adjustments for maint ining 

 balance. 



The fact that a large majority of the insects feed on other host plants 

 also, lessens the danger of destruction of the hawthorns and is another of 

 Nature's provisions for maintaining balance. There are, of course, many 

 other factors that tend to lessen the insect injury to the trees, such as 

 the interrelations of the insects with their parasites and preyers, but so 

 little is known about them that the writer makes no attempt to discuss 

 them. 



A host of bees, flies, and beetles visit the blossoms in quest of pollen and 

 nectar. The winter buds in some species of hawthorn become coated with 

 a sticky exudation, which attracts insects emerging in late winter, such 

 as the stone flies and the chironomids. These transient members of the 

 Crataegus fauna have been omitted from consideration in this paper. 

 A list of insects that visit the blossoms is given by Knuth (1908). 



In the preparation of the catalog of hawthorn insects it became noticeable 

 that some of the species which have more than one host plant have chosen 

 only closely related hosts, such as the apple, the pear, or the medlar, while 

 many others have chosen their hosts from plants that grow in the same 

 communities regardless of close botanical relationship. A study of these 

 combinations of hosts and the habitats in which they grow has led the 

 writer to believe that the hawthorns are members of at least five different 

 plant communities, which may be described as follows: 



1. Open woods. In woodlands where the growth habit of the taller trees permits sunlight 

 to reach the ground so that an undergrowth may develop, such as that in a forest of oak, 

 hickory, and elm, Crataegas is commonly found along with Corylus, Rhamnus, Carpinus, 

 Prunus spinosa, and the like. 



2. Deforested areas. Where a shrubby growth has sprung up after the destruction of a 

 forest, numerous thorny forms such as Crataegus, Rubus, Berberis, and Prunus spinosa 

 are frequently found. 



3. Grazing lands. Hillsides or valleys where the soil is uncultivated and cattle are pastured 

 are frequently dotted with Crataegus, Rosa, and crab apple, which because of their thorns 

 can continue to thrive and outgrow the danger of being eaten by the cattle. 



4. Stream banks. Just back of the willows and alders on moist alluvial soil beside streams, 

 Crataegus grows to its greatest size and is associated with birch, willow, alder, and poplar. 



5. Fence rows. Where shrubs are allowed to grow up along the fences, Prunus virginiana, 

 Crataegus, wild plum, and wild cherry are frequently found closely associated. 



In each of these five communities insects will be found which feed on 

 the various plants of the community. For example, Psylla mali Schmid. 



