UC Davis

European earwigs (Forficula auricularia) are widespread omnivores that can be considered both a pest, natural enemy, or both in different agricultural systems. Growers and pest control advisors of California citrus suspect that earwigs damage young citrus fruit. However, while there has been extensive research on earwigs as predators in citrus, very little is known about herbivory by earwigs on citrus fruit. This dissertation is the first to comprehensively describe herbivory of earwigs on citrus fruit in California and develop management options for earwigs. Overall, this work reports on results from a series of different field experiments that manipulated densities of earwigs conducted at Lindcove Research and Extension Center (LREC) in Lindcove, Tulare County, CA.The first chapter of this dissertation focuses on elucidating characteristics of damage generated by earwigs on navel orange (Citrus sinensis (L.) Osbeck; Sapindales: Rutaceae) fruit. This study is the first to experimentally evaluate whether earwigs damage young citrus fruit, amount of damage caused, and morphological descriptions of the damage. This study further assesses whether the amount of damage earwigs generate differs across developmental stage, sex of adult earwigs, or time. We tracked damaged fruit to determine whether they are retained to harvest and the damage morphology of the damaged fruit at harvest. We suspected that earwig damage may be mistaken for fork-tailed bush katydids (Scudderia furcata Brunner von Wattenwyl; Orthoptera: Tettigoniidae), a widely recognized chewing herbivore in citrus. Thus, damage and abscission in response to damage caused by earwigs was compared to that caused by katydids. We found that earwig nymphs generated more damage to young citrus fruit than adults. Earwigs chewed deep holes on fruit from 0 to 3 weeks after petal fall but not after. Fruit damaged by earwigs and katydids were retained by citrus trees, and scars generated by katydids and earwigs were morphologically similar. Exposure to katydids but not earwigs increased fruit abscission. This study confirms that earwigs can be damaging to C. sinensis fruit, and information from this study can help citrus growers recognize earwig damage. The second chapter of this dissertation aims to describe fruit feeding across three common citrus species: navel oranges (C. sinensis), clementines (C. clementina hort. ex Tanaka), and ‘Tango’ mandarins (C. reticulata Blanco). Specifically, this study confined earwigs to citrus branch terminals containing fruit to determine differences in amount of fruit feeding, rates of earwig death, preferential abscission of damaged fruit, and healing of damaged fruit for each citrus species. The major finding from this study was that earwigs fed extensively on C. sinensis and C. clementina fruit, but feeding was less frequent and less severe on C. reticulata fruit. This study suggests that, while earwigs may be direct pests in navel oranges and clementines, earwigs may not be significant pests in C. reticulata. The third chapter of this dissertation tests trunk barriers, which takes advantage of the primarily non-flying behavior of earwigs, to manage earwigs by blocking their movement into tree canopies. Trunk barriers may be used to simultaneously manage earwigs along with non-flying arthropod pests. However, trunk barriers may have negative outcomes such as preventing the movement of natural enemies into tree canopies. Densities of pest and natural enemy arthropods across treatments of sticky barrier, insecticide (bifenthrin) barrier, and no barrier (control) were compared. It was discovered that trunk barriers effectively prevented the movement of key citrus pests, earwig, Fuller rose beetles (Naupactus godmani; Coleoptera: Curculionidae), and ants (Hymenoptera: Formicidae) into tree canopies. Spiders were not prevented from accessing the canopies, and the reduction of earwig densities caused by trunk barriers did not lead to increased densities of citrus pests that earwigs prey on, aphids (Hemiptera: Aphididae) and California red scale (Aonidiella aurantia; Hemiptera: Diaspididae). Harvested fruit from trees with sticky barriers had less scarring and were infested with Fuller rose beetle eggs less often compared to trees without barriers. Overall, sticky barriers seem to be an effective way to simultaneously manage earwigs along with other arthropod citrus pests.