European earwig population density surveys and fruit scarring

These data were gathered in four successive surveys of ca. 90 blocks of commercially produced citrus growing in California's San Joaquin Valley.  Data also include two measures of fruit scarring, one that involved 'bin samples' (i.e., checks of already-harvested fruit in large harvest bins) and one involving checking maturing fruit that are still on the trees.

Methods
European earwig population density surveys

We manufactured wooden board traps to survey 93 commercial citrus blocks, including navel oranges, Citrus sinensis (n = 73 blocks), clementines, Citrus clementina (n = 16 blocks), tangelos, Citrus x tangelo (n = 2), and mixed varieties (n = 2) growing in eastern Tulare and Fresno Counties, CA.  Board traps were 25.4 x 25.4 cm squares of OSB (oriented strand board) sheathing, 1.82 cm thick; OSB is an inexpensive, engineered wood panel, made of small wood strands glued together with waterproof adhesives.  Boards were distributed across each block in six locations, at each of which three adjacent trees received one board trap each for a total of 18 boards/citrus block, except for a few very small blocks that received 9 boards/block.  Boards were deployed in the citrus blocks during February, 2022, placed under the edge of the citrus canopy, and subsequently visited during four time periods in the spring and late fall of 2022 and 2023 to count F. auricularia (4-8 April, 2022; 9 February – 2 March, 2023; 5 – 12 April, 2023; and 23 November – 1 December, 2023).  During each survey we counted the total number of F. auricularia adults, egg masses (no attempt was made to count the number of eggs per egg mass), and aggregations of newly hatched nymphs (first and second instars; again, no attempt was made to count the number of nymphs per aggregation).  Sampling dates were chosen to focus on times of the year when earwigs are present below ground, where they build nests, oviposit, and care for their young, or are actively foraging on the ground soon after nymphs emerge from their nests.  Citrus fruits are known to be vulnerable to F. auricularia herbivory immediately after fruit set (‘petal fall’), which typically occurs in late April to late May in this region.  A few sites were omitted from some surveys, because the sites were actively receiving pesticide applications and could not be visited safely.

Fruit damage estimates

In 2022 and 2023 we assessed the relationship between F. auricularia densities and fruit scarring.  Earwig density estimates were obtained from the survey of commercial citrus blocks conducted in April, just prior to the period of fruit vulnerability to earwig feeding damage.  We used two estimates of fruit scarring.  Our first estimate was derived from ‘bin samples.’   When fruit are picked, they are placed in large bins, which are eventually loaded onto trucks for transport to the packinghouse.  We inspected the visible portions (roughly the top half) of all fruit in the top layer of the bins, which typically involved ca. 200-300 fruit, and recorded the total number of fruit per bin bearing deep scabby scars.  In 2022 we had fruit scarring and earwig density estimates for 33 blocks, based on an average of 96.8 ± 108.0 bins checked per block); in 2023 we had fruit scarring and earwig density estimates for 30 blocks.  

We sampled fruit maturing on trees during September 2023, prior to the period when splitting of katydid-damaged fruit had been observed.  We chose a subset of N = 30 commercial citrus blocks that had been surveyed for F. auricularia densities, including all blocks that had high earwig densities and a subset of the many blocks that had few or no detected earwigs.  In each of these blocks we sampled 20 fruit from each of 25 trees located across the block (N = 500 fruit total per block) and recorded the presence/absence of deep, scabby scars as well as the approximate area of the scarred surface.  Scarring generated by insects with chewing mouthparts is distinctive and is readily distinguished from other sources of scars, including the citrus thrips Scirtothrips citri, the common garden snail Cornu aspersum, or mechanical processes (e.g., limb rub).