April, '11] NORTON: health of plants as related to insects 271 



severe, a slow death indicated by red or yellow, stunted foliage and 

 finally drying up of the leaf edges or whole leaf, due to reduction in 

 water supply and salts from the soil. In cabbage lack of water 

 stimulates the production of the bluish white bloom on the leaves 

 which protects the leaf against loss of water; so this symptom may 

 indicate root or stem injury by insects. Another means of protec- 

 tion against loss of water when roots are injured is for the plant to drop 

 some of the leaves; and so leaf fall in summer may be a result of insect 

 injury to the wood of stem or root. 



In case the live bark is injured so that the plant is partly girdled, the 

 downward flow of sap is interfered with and the parts below the 

 wound may suffer for food while a swelling may form above where 

 the nutrient material accumulates. The work of the bean vine 

 borer is an example. A very interesting and well-illustrated account 

 of interference with starch translocation from leaves, due to the work 

 of leaf miners, has recently been published. (Centralb. Bakt. II, 

 24: 158.) 



The destruction of the buds of plants by insects induces the pro- 

 duction of branches out of the normal position and number and thus 

 changes the habit of the plant, as in the case of plant bug injuries 

 to dahlias, when there is a mass of leaves, roots and stems but no flowers, 

 and the dwarfed greenish heads of wheat at the time they should be in 

 bloom, due to another insect. I might class with these the scrubby 

 growth of some pines, due to bud injury, and the crooked white pine 

 trunks due to weevil borings; also the bending of the bean stem fol- 

 lowing work of the vine borer, and the curling of many leaves due to 

 injury by insects on one side. Gummosis in peach and related trees 

 follows the work of insects as it does any injury. 



A serious trouble following defoliation by insects is the weakening 

 of trees by the production of new crops of leaves to take the place of 

 those lost. According to Britton two defohations of elms in one 

 season by the elm beetle will kill the trees. 



Diseases Due to the Presence of Insects or Their Products 



Under this head would come the multitude of galls formed by many 

 different kinds of insects. But cecidology is a science in itself and 

 having been studied more than some other branches of my subject 

 I shall not go into it further. Whether insect presence stimulates the 

 plant to excessive growth in gall formation I shall not attempt to 

 say. A common example is woolly aphis galls, and erineum effect. 

 Nematode root galls may be mentioned though hardly to be included 

 among insect work. 



