Observations on the biology and life history of the walnut twig beetle, Pityophthorus juglandis Blackman, in black walnut and Arizona walnut

The walnut twig beetle (Pityophthorus juglandis), is a native bark beetle of the southwestern U.S. originally described from Arizona walnut (Juglans major). In recent years it has been recognized as the key vector of Geosmithia morbida that, in black walnut (Juglans nigra) and some other Juglans species produces a devastating disease known as 1000 Cankers Disease of Black Walnut. A major area of research to accomplish in the definition of a newly described disease is to understand the behavior and life cycle of the vector. Walnut twig beetle has multiple overlapping generations per year; however egg laying and brood development occurs primarily during the warm months. This beetle overwinters in pockets the beetles create in the major trunk and scaffold limbs of black walnut. Trapping records indicate the walnut twig beetle will fly any month of the year provided the weather is warm and sunny. Male beetles initiate the nuptial chamber in the cork cambium of the wood where they attract from 1 to 5 females who they mate with then proceed to cut egg galleries. The overall gallery appearance is star shaped. Eggs are laid on both sides of the gallery as is common in bark beetles. The larvae then feed with the grain of the wood in the bark. As generations proceed, new galleries are created near the wood cambium, underlying the original galleries. At no time are the beetles disassociated with the fungus Geosmithia morbida.