MONITORING WALNUT TWIG BEETLE ACTIVITY IN THE SOUTHERN SAN JOAQUIN VALLEY: OCTOBER 11-OCTOBER 12 Elizabeth J. Fichtner ABSTRACT Walnut twig beetle, Pityophthorus juglandis, is the vector of thousand cankers disease, an emerging disease on walnut caused by the fungus Geosmithia morbida. Thousand cankers disease was originally reported in declining black walnut in Colorado, but the disease was confirmed in California in 8 and has since been reported in several counties. Since 9, several incidents of thousand cankers disease in commercial walnut orchards in the southern San Joaquin Valley (SSJV) have been documented. In an effort to better understand the seasonality of activity of the walnut twig beetle, in the population of the beetles was monitored by trapping weekly in two Tulare County orchards. The survey relied exclusively on random trap catches of walnut twig beetle on yellow sticky traps. The availability of a new, patentpending, male-produced aggregation pheromone (Compound X) identified by US Forest Service researcher Dr. Steve Seybold, may enhance the probability of detecting walnut twig beetles. In 11 Compound X was utilized in conjunction with sticky traps, to assess walnut twig beetle activity in three orchards in the SSJV. In October 11, sticky traps were replaced with funnel traps, also adorned with Compound X, to capture beetles in an antifreeze solution. Funnel traps have proven less labor intense than the sticky traps, and allow for rapid preservation of trap catches for beetle enumeration. Our results demonstrate walnut twig beetle activity between the months of February 12 and early November 12, with no activity detected in December 11 and January 12. OBJECTIVE 1. Assess the seasonal activity of walnut twig beetle in walnut orchard systems in the southern San Joaquin Valley. SIGNIFICANT FINDINGS Minimal walnut twig beetle activity was detected across all three sites from mid-november 11 through early February 12. Reduced beetle flight activity during these winter months suggests that removal of thousand cankers disease-infected trees should be completed before February in the southern San Joaquin Valley as part of an orchard sanitation practice. PROCEDURES From October 11-present, walnut twig beetle activity was monitored in three walnut orchards including two commercial orchards in Tulare Co. (Farmersville, CA and Porterville, CA) and the walnut breeding block at Kearney Agricultural Center (Parlier, CA). California Walnut Board 33 Walnut Research Reports 12
Site 1: The Porterville, CA orchard was originally 'Serr' grafted onto black walnut rootstock; however, because all replants were on 'Paradox' rootstock, 7% of the trees are currently on 'Paradox'. In February 11, a random survey of the orchard conducted by Hishinuma and Fichtner documented at least 7 trees with walnut twig beetle galleries (Hishinuma and Fichtner). The neighboring orchard, 'Tulare' on 'Paradox' rootstock, also contained at least 7 trees colonized by walnut twig beetle in November ; however, all trees were removed and burned by mid- January 11. Site 2: The Farmersville, CA orchard was originally planted as 'Chico' on black walnut rootstock. The orchard now contains 9 black walnut trees (2.% of total) that grew from stump sprouts. All 9 black walnut trees exhibited galleries of walnut twig beetle. One tree (.2% of total) was a replant on 'Paradox' rootstock. Walnut twig beetle infested trees have not been removed from this orchard over time. Site 3: The site in Parlier, CA is a walnut breeding block at Kearney Agricultural Center. The block contains a diversity of walnut breeding lines, with each line grafted on both black walnut and 'Paradox' rootstocks. Trapping Strategy: Two poles containing traps were placed in each orchard, one pole on the orchard perimeter and one in the center of the orchard. Each pole contained two traps, one placed at approximately 4 ft above ground and one at approximately 8 ft above ground. Compound X was placed in a permeable plastic bottle associated with a funnel trap at each height. The twelve traps (3 locations x 2 heights x 2 locations) were changed weat 4 to 9 day intervals (generally weekly) from October 11 through the present. After trap catches were collected, samples were returned to the laboratory and frozen until processing. Beetle Quantification. Beetles were collected in antifreeze solution attached to the bottom funnel of each trap. The antifreeze solution was passed through a funnel filter on a weekly basis to capture the beetles. Filters containing the beetles were stored frozen, and periodically transported to the laboratory of Dr. Seybold (USDA Forest Service) for species-level verification and for determination of sex ratio of beetles trapped. The number of each sex caught during each trapping period was normalized to a 7-d period. RESULTS Site 1. Trap catch data from the Porterville site in Tulare County is available from October, 11 through October 2, 12 (Figure 1A). A total of 1,6 WTB were caught (46 males and 1, females) and the beetle was active in flight continuously during each week of the monitoring period from 3 February to 2 October, 12. At this site, WTB did not fly in response to the pheromone-baited traps from 11 November 11 until 3 February 12. Thus, there was no flight during the months of December and January. The highest mean weekly trap catch was recorded between 2 March and 9 March (approx. 2 females and 11 males per trap) with secondary peaks in late July, late September, and early October. The overall pattern of flight suggests a constant source of the beetle available to colonize trees between February and November. The ambrosia beetle, Xyleborinus saxeseni, was also trapped in relatively high numbers at the traps (172 female beetles total between mid-july and early October during the California Walnut Board 34 Walnut Research Reports 12
monitoring period, 11% of the trap catch of WTB). A second scolytid species, Hypothenemus (probably) eruditus, was also trapped in this study (14 individuals total between late April and mid-october). Site 2. Trap catch data from the Farmersville site in Tulare County is available from October, 11 through October 2, 12 (Figure 1B). A total of 1,64 WTB were caught (349 males and 7 females) and the beetle was active in flight continuously during each week of the monitoring period from 27 January to 2 October, 12. At this site, WTB did not fly in response to the pheromone-baited traps from 17 November until 3 December 11. After a small flight between 3 December and 6 January, the WTB did not fly again until the week of 27 January. Thus, there was no flight during the month of December and most of the month of January. The highest mean weekly trap catch was recorded between 9 March and March (approx. 14 females and males per trap) with a secondary peak in early September. The overall pattern of flight suggests a constant source of the beetle available to colonize trees between February and November. The ambrosia beetle, Xyleborinus saxeseni, was also trapped in relatively high numbers at the traps (4 female beetles total between mid-july and early October during the monitoring period, 14% of the trap catch of WTB). A second scolytid species, Hypothenemus (probably) eruditus, was also trapped in this study (11 individuals total between late April and mid-october). Site 3. Trap catch data from the Parlier site (Kearney Ag Center) in Fresno County is available from October, 11 through November 2, 12 (Figure 1C). A total of 1,143 WTB were caught (42 males and 741 females) and the beetle was active in flight continuously during each week of the monitoring period from May to 2 Nov., 12. At this site, WTB did not fly in response to the pheromone-baited traps from 17 November 11 until 9 January 12. There was only minor flight between early November and mid-may. The highest mean weekly trap catch was recorded between 8 June and June (approx. 22 females and males per trap). The overall pattern of flight at this site suggests a constant source of the beetle available to colonize trees between June and November. The ambrosia beetle, Xyleborinus saxeseni, was also trapped in relatively high numbers at the traps (44 female beetles total) with most caught between mid- July and early October during the monitoring period, approx. 39% of the trap catch of WTB). Two specimens were caught later in October; two were caught in mid- to late February, one was caught in late May, and one was caught in late June. A second scolytid species, Hypothenemus (probably) eruditus, was also trapped in this study (24 individuals total between mid-july and late-october). The relatively high catches of, H. eruditus, and woodboring beetles in the family Bostrichidae trapped primarily between mid-july and late September suggest that perhaps we had inadvertently used an antifreeze product containing ethanol during the middle of the summer. DISCUSSION Since our first finding of thousand cankers disease in Tulare Co. in October 9, both the documented incidences of disease and the known geographic distribution of the disease have increased gradually over time. The disease has been detected in commercial English walnut California Walnut Board 3 Walnut Research Reports 12
orchards in Tulare, Fresno, and Kings Cos. In the southern valley, the pathogen has been isolated from Tulare, Chico, and Chandler varieties, as well as from both black and Paradox rootstocks. Initially thousand cankers disease was observed only on stressed trees exhibiting prior infection by Phytophthora or Agrobacterium tumefaciens, or larval predation of roots by Ten Lined June Beetle. In, 11, and 12, however, walnut twig beetle activity and thousand cankers disease have been observed on trees with no evidence of prior stress or decline. Additionally, local surveys conducted in 12 suggest an extensive amount of beetle activity in several orchards, though most trees exhibiting beetle activity did not exhibit symptoms of thousand cankers disease. For example, in 12, a survey was conducted in the Tulare, CA orchard where the disease was first observed and confirmed in Tulare County. The survey results suggested that 69 of the 124 trees surveyed (6%) exhibited evidence of walnut twig beetle activity; however, only one tree exhibited the characteristic bleeding symptomatic of infection by G. morbida. In the southern San Joaquin Valley, we have observed walnut twig beetle activity in orchards with no history of thousand cankers disease. In the absence of symptomatic trees, the walnut twig beetle has been detected on sticky traps and in funnel traps, as well as in insect galleries on productive trees and on discarded wood in burn piles. Because bleeding cankers are not always associated with infection, the frequency of disease incidence is likely underestimated. Walnut twig beetle galleries have been associated with every documented case of thousand cankers disease. The number of documented incidences of thousand cankers disease in Tulare County continues to grow; however, many growers remove symptomatic trees from the orchard and retain only one for formal diagnosis. Consequently, as grower awareness of the disease increases, the number of documented new incidents of disease is expected to decrease. The overall goal of our walnut twig beetle monitoring program is to enhance understanding of the life cycle of the thousand cankers disease vector. In the two commercial walnut orchards (Porterville and Farmersville, CA), a constant source of beetles was available from February 12-November 12. At Kearney Ag Center, a non-commercial site, a constant source of walnut twig beetles was observed between June 12 and November 12. In winter months (mid-november through January), walnut twig beetle catches were either low or non-existent across sites. Given that growers most common question concerns the ideal timing of removal of infected trees, this data suggests that removal of infested wood from orchards should be completed by the beginning of February in the southern San Joaquin Valley. A comprehensive illustration of walnut twig beetle activity from statewide surveys will be compiled after another year of data collection, and will include comparisons between walnut growing regions within the state as well as the influence of trap location (.i.e., low vs. high and interior vs. orchard perimeter) on detection of walnut twig beetle populations. California Walnut Board 36 Walnut Research Reports 12
pheromone-baited traps ( N=4), Oct. 11 -Oct. 12, Tulare Co., CA 3 3 2 Site 1: Porterville 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 2 26 27 28 29 3 31 32 33 34 3 36 37 38 39 4 41 42 43 44 4 46 47 48 49 1 2 3 Figure 1A Midpoint of trapping interval, Oct., 11 -Oct., 12 pheromone-baited traps ( N=4), Oct. 11 -Oct. 12, Tulare Co., CA 2 Site 2: Farmersville, CA 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 2 26 27 28 29 3 31 32 33 34 3 36 37 38 39 4 41 42 43 44 4 46 47 48 49 1 2 3 Midpoint of trapping interval, Oct., 11 -Oct., 12 Figure 1B pheromone-baited traps ( N=4), Oct. 11 -Nov. 12, Fresno Co., CA Site 3: Kearney Ag Center 3 3 2 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 2 26 27 28 29 3 31 32 33 34 3 36 37 38 39 4 41 42 43 44 4 46 47 48 49 1 2 3 4 Figure 1C Midpoint of trapping interval, Oct., 11 -Nov., 12 California Walnut Board 37 Walnut Research Reports 12