Predicting Damage to Hop Cones by Tetranychus urticae (Acari: Tetranychidae)

Abstract

Twospotted spider mite (Tetranychus urticae Koch) is a cosmopolitan pest of numerous plants, including hop (Humulus lupulus L.). The most costly damage from the pest on hop results from infestation of cones, which are the harvested product, which can render crops unsalable if cones become discolored. We analyzed 14 yr of historical data from 312 individual experimental plots in western Oregon to identify risk factors associated with visual damage to hop cones from T. urticae. Logistic regression models were fit to estimate the probability of cone damage. The most predictive model was based on T. urticae-days during mid-July to harvest, which correctly predicted occurrence and nonoccurrence of cone damage in 91 and 93% of data sets, respectively, based on Youden’s index. A second model based on the ratio of T. urticae to predatory arthropods late in the season correctly predicted cone damage in 92% of data sets and nonoccurrence of damage in 77% of data sets. The model based on T. urticae abundance performed similarly when validated in 23 commercial hop yards, whereas the model based on the predator:prey ratio was relatively conservative and yielded false-positive predictions in 11 of the 23 yards. Antecedents of these risk factors were explored and quantified by structural equation modeling. A simple path diagram was constructed that conceptualizes T. urticae invasion of hop cones as dependent on prior density of the pest on leaves in early spring and summer, which in turn influences the development of predatory arthropods that mediate late-season density of the pest. In summary, the biological insights and models developed here provide guidance to pest managers on the likelihood of visual cone damage from T. urticae that can inform late-season management based on both abundance of the pest and its important predators. This is critically important because a formal economic threshold for T. urticae on hop does not exist and current management efforts may be mistimed to influence the pest when crop damage is most probable. More broadly, this research suggests that current management practices that target T. urticae early in the season may in fact predispose yards to later outbreaks of the pest.

HopBox: An image analysis pipeline to characterize hop cone morphology

“…Researchers have utilized various categorical scales to evaluate cone color and damage from pests, diseases, or abiotic factors. For example, Weihrauch (2005) and Woods et al (2021) used a 1-4 scale to evaluate the damage caused by twospotted spider mites, which ranged from 1 (no damage) to 4 (over 25% of cones aborted). Gent et al (2014) utilized an ordinal scale of 1-10 to quantify hop cone color, where 10 indicates the best possible color, and 1 the worst.…”

“…For example, Weihrauch (2005) and Woods et al. (2021) used a 1–4 scale to evaluate the damage caused by two‐spotted spider mites, which ranged from 1 (no damage) to 4 (over 25% of cones aborted). Gent et al.…”

Tetranychus urticae (Koch, 1836) em cultivo de lúpulo Humulus lupulus L. (Cannabaceae) no Rio Grande do Sul, Brasil - primeiro relato de ocorrência

“…No lúpulo, quando encontrado em altas densidades, ocasiona manchas irregulares, acarretando bronzeamento e desfolhamento. Além disso, é responsável por perdas ao se alimentar dos cones, causando descoloração, ressecamento e tornando-os quebradiços, podendo acarretar a perda total da produção (Gent et al 2009;Woods et al 2021). No Brasil, são poucos os estudos sobre pragas e doenças relacionadas ao cultivo do lúpulo, devido à sua recente introdução no país (Spósito et al 2019).…”

“…Weihrauch (2005) concluiu pela primeira vez em seu trabalho realizado na Alemanha no período próximo da colheita do lúpulo, que o TSSM pode demonstrar risco econômico tanto no rendimento, quanto no teor de ácido dos cones, quando o número ultrapassa 90 ácaros por folha. Woods et al (2021) corroboram com estes dados, pois concluíram em seu estudo, a previsão para os surtos de TSSM nos cones, antes mesmo das plantas colocarem suas flores, com base em dados algorítmicos criados anteriormente a partir de danos eventuais nos primeiros estágios das plantas.…”