Studies on the walnut twig beetle (WTB), Pityophthorus juglandis, in relation to its association with Geosmithia morbida, its survival in felled logs, and its sensitivity to temperature extremes

Abstract

The walnut twig beetle (WTB, Pityophthorus juglandis) is the vector of Geosmithia morbida and a causal agent of thousand cankers disease (TCD) of black walnut (Juglans nigra). In addition to aggressive feeding by WTB, damage to walnut occurs from canker development around beetle galleries due to G. morbida. Cankers coalesce and girdle the branches and trunk eventually causing mortality. To better understand WTB, studies were initiated to: 1) determine if WTB is attracted to G. morbida, or other bark fungi, when given a choice; 2) determine whether clear plastic tarping or applications of bifenthrin, permethrin, and biodiesel to felled logs affected the persistence of the walnut twig beetle or altered the suitability of logs for future breeding by the insect; and 3) establish upper and lower lethal temperature (LT) limits of the beetle as well as determine supercooling point (SCP) and seasonal variation of SCP of walnut twig beetle. For the first study, in order to determine if WTB was attracted to G. morbida or other bark fungi, bioassays for both adult and larval WTBs were conducted. No differences in adult WTB numbers were detected between tubes that were filled with either potato dextrose agar (PDA) or PDA colonized by G. morbida. However, Xyleborinus saxeseni, an ambrosia beetle commonly associated with TCD during the later stages of decline w collected in higher numbers from tubes containing PDA compared to PDA colonized by G. morbida. Larvae preferentially migrated towards agar plugs colonized by G. morbida when tested against PDA. Similarly, larvae also migrated toward plugs colonized by Fusarium solani or Penicillium solitum when tested against PDA. However, when given a choice between two fungi, no preference in larval movement was observed. These results suggest that larvae are attracted to the bark fungi tested in general, but not specifically to G. morbida. In the second study (testing cultural and chemical controls to felled black walnut logs), beetle emergence from all logs was variable six months after trees were felled; no beetle emergence was recorded in a portion of both treated and untreated logs and overall detection of WTBs was poor. Nevertheless, only logs treated with bifenthrin were consistently devoid of beetles. Walnut twig beetles emerged from a small proportion of the treated and untreated logs for up to 21 months after sampling and even from some logs in which the beetles previously had not been detected suggesting colonization of felled logs. Geosmithia morbida was isolated from the beetle at each sampling date. Neither plastic sheeting nor the chemical applications of biodiesel or permethrin were an effective means of disinfesting black walnut logs of WTB. For the third study, based on logistic regression models, the lower median lethal temperature (LT50) and lower lethal temperature required to kill 99% of the population (LT99) for WTB adults was -16.7 °C and -22.97 °C respectively and for larvae was -16.9 °C and -25.19°C respectively. The upper LT50 and upper LT99 for WTB adults was 48 °C and 53 °C respectively and for larvae was 47 °C and 48 °C respectively. Mean monthly SCPs for WTB larvae and adults from January to June 2012, were between -16 °C to -18.5 °C. Several differences among monthly means were detected but those differences were no larger than 2°C.