150 THE MONTHLY BULLETIN. 



OAK PESTS-THE OAK TWIG GIRDLER. 



(Agrilus politus Say.) 

 Ordei — Coleoptera. Family — Buprestidae. 



By Lerot Childs^ Assistant Secretary State Commission of Horticulture. 



The live oaks {Quercus agrifolia) throughout the greater part of the 

 State are subject to the attacks of numerous insect pests and fungous 

 diseases which cause a great deal of damage not only to the actual health 

 of the trees, but distinctly mar the beauty of one of California's most 

 picturesque native trees. Neglect is especially apparent in the smaller 

 cities and towns where the oaks, the property of the cities, continue to 

 be breeding centers of many injurious insects which could be entirely 

 eliminated with no great expense. It is the aim of the author to publish 

 short accounts of the descriptions, life-history and control measures to 

 be applied in combatting these injurious insects, the first of which 

 will deal with the twig girdler, Agrilus politus. 



The injury inflicted by the oak twig girdler is very conspicuous 

 (Fig. 54), and the damage occasioned by its attack upon the smaller 

 twigs is more in the destruction of beauty than actual physical injury, 

 though in the case of a severe infestation, especially in trees where the 

 beetles ' presence has been one of long standing, the writer has seen oaks 

 whose vigor has been greatly reduced through the reduction of the 

 foliage surface and consequent limitation of food making possibilities 

 as a result of the curious girdling habits of this interesting insect. 



The presence of the small borer is easily detected on account of the 

 killing of the smaller twigs of the tree, the results of the girdling habits 

 the larva of this beetle possesses. 



Character of Injury. 



The movements of the feeding larva? are always of a definite character, 

 a continuation of which results in the formation of a spiral-shaped 

 burrow which has often been found to encircle the attacked twig from 

 four to seven times, death of the twig always advancing as the length 

 of the burrow is increased by the feeding grub. The feeding operations 

 occur in the soft growing tissues of the twig in the cambium layer at 

 the point that separates the bark from the woody portions of the stem. 

 Occasionally, in cases where the wood is exceptionally soft, the burrow 

 may extend into the wood a little but never to any great extent. This 

 point of attack is a very vital and sensitive part of the plant structure, 

 as not only the growth of the plant takes place there but the transpor- 

 tation of food properties and water occurs in the tissue that is affected 

 by the feeding larvre. It is therefore quite evident that as soon as the 

 insect encircles the twig the connectives joining the small twig and 

 leaves are cut off from the rest of the tree, and the result is sudden 

 death to that portion above the point of infestation. A tree attacked 

 by this pest is readily noticeable, owing to the presence of the many 

 small patches or tufts of dead leaves scattered irregularly over the 

 entire crown of the tree. 



The beetle larvae seldom operate in twigs of a greater diameter than 

 a half inch, and Avill be more often found in the smaller branches of 

 about the size of a lead pencil. 



