Wigging out, then wigging in: removing earwigs from stone fruit and augmenting them in pome fruit

Objective 1a – Testing different types of trap to improve earwig capture

Significant findings

• Standard cardboard traps captured the most earwigs
• Leaving the traps throughout the season increased the number of earwigs captured in one of the three field sites tested

Results and Discussion

In 2021, we found that standard cardboard traps captured more earwigs than 0.5L and 2L soda shelter traps at all locations (Fig. 4-6). While soda bottle shelters are commonly used in the UK, it is likely that enclosing the traps in plastic causes them to become too hot in arid central WA. In the UK, the plastic bottle primarily serves as rain protection, which is not necessary in central WA.

In Yakima, changing the trap only monthly or when damaged increased the number of earwigs captured (Fig. 4). These results might be related to the accumulation of aggregation pheromones or cuticular hydrocarbons (Quarrell et al. 2016) within the trap throughout the season, although there were no differences between "monthly" and "weekly" replacement treatments in Wenatchee (2021) or Medford (2021-2022)  (Fig. 5-6). It is possible that the traps become "fully saturated" with earwig aggregation pheromone prior to the one week monitoring period, resulting in the trends seen in Wenatchee and Medford. However, the increase in trap catch in Yakima indicates that at least in some locations, the presence of earwig pheromone affect trap catch. Researchers should be aware of this when designing protocols and maintain consistency in how often traps are changed.

Objective 1b – Mass trapping earwigs where they are pests: does it help reduce earwig populations and fruit damage?

Significant findings

• Mass removal of earwigs in stone fruit did not significantly reduce earwig populations
• Removal of earwigs slightly reduced the observed earwig damage on apricots in 2021 and on cherry in 2022
• Mass removing earwigs at a larger scale and/or for a longer period of time might be successful in decreasing fruit damage, but additional research will be needed to confirm this trend

Results and Discussion

We found that mass trapping and removing earwigs from stone fruit orchards did not significantly reduce earwig populations over time (Fig. 7).

There was a significant reduction in possible earwig damage in apricots in 2021, but not in 2022 (Fig. 8). However, in the apricot orchard in 2021, we were only able to sample fruits after harvest started. This may have affected results by causing us to sample a higher portion of damaged fruits, intentionally left on the tree by pickers. In cherries, we did not observe any significant difference in fruit damage between removal and control plots in 2021, but found weak evidence of damage reduction in 2022 (Fig. 8).

Therefore, mass removal of earwigs using corrugated cardboard traps was not consistently successful in reducing fruit damage for either crop. Future studies are needed to evaluate mass trapping for a longer term, across a larger area to truly determine if this tactic can reduce earwig damage. Although earwigs are poor dispersers, they can likely move into the edges of removal plots, resulting in no observed differences between treatments in our study. Repeating removal across multiple years may also slowly dampen the population, which we may have observed the beginnings of in the cherry orchard in 2022.

Objective 2a - Under what conditions can earwigs damage apples?

Significant findings

• Earwigs are much more likely to damage fruit that has already been damaged, exploiting breaks in the fruit skin
• Earwigs are very unlikely to cause yield loss to apples in the field

Results and Discussion

In both assays, earwig damage to 'Honeycrisp' was lower in the treatments with no previous damage (either artificial or natural stem end splits), regardless of whether  alternative food was provided (Fig. 9). When 'Honeycrisp' fruits were previously damaged, providing alternative food decreased the level of earwig damage. While the percent earwig damage in our study (even in the "no damage" treatments) would be considered very unacceptable at pack out, we created a "worst case scenario" by confining earwigs with fruit in plastic bags and still had fruit at the end of the study that were undamaged by earwigs. The risk to 'Honeycrisp' apples is minimal.

The grower carefully documented the damage to each 'Envy' apple prior to and after introducing earwigs (Fig. 10). Earwigs exploited previous damage to the skin of the fruit and were often found inside hail damage holes (Fig. 10).

The grower summarized the damage to each of the three apples per treatment and provided their summary notes for the last of evaluation (Day 12). Earwigs were most likely to enter the fruit through the calyx or prior damage that caused a break in the fruit skin (Fig. 11). Only one undamaged fruit had earwig-caused damage and this only occurred in the treatment where no alternative food was provided (Fig. 11, second row). In the 24 apples tested, there was only one apple where alternative food was provided and earwig feeding caused damage outside of pre-existing wound (Fig. 11, final row). The grower and the grant team independently concluded that earwigs are unlikely to cause primary damage to 'Envy'. In any future trials assessing earwig damage to apples, researchers should focus on the calyx end of the apple, which appears to be the main point of entry if earwigs cause primary damage. This will also help eliminate the risk of unintentionally counting stem end split damage as earwig damage (opposite end of the apple).

Objective 2b – Augmentation of earwigs in pome fruit: can we increase earwig populations and reduce pest pressure?

Significant findings

• Earwig counts significantly increased after releases, but only in the orchard where they were previously absent
• Earwig releases decreased populations of woolly apple aphid and pear psylla after two years, even in the orchards where they were previously absent
• To our knowledge, this is the first example of deliberately turning a pest into a natural enemy by changing its location

Results and Discussion

Earwig abundance prior to the releases varied between locations. Both apple and pear orchards in WA had high earwig abundance prior to releases, whereas the pear orchard in OR had no earwigs before releases in 2021. In the WA apple orchard, earwig abundance in treated plots, either with one mass release or continuous releases, did not differ from control in 2021 or 2022 (Fig. 12). We observed the same trend in the WA pear orchard (Fig. 12). In the OR pear orchard in 2021, the release treatments had significantly higher earwig abundance compared to control and also differed from each other (Fig. 12). Compared to the control, the mass release had 49× more earwigs and the continuous release had 4× more earwigs. In 2022, there was a low number of earwigs in the OR pear control plots prior to releases beginning that year. There was a significant effect of earwig release treatment in the OR pear orchard in 2022 (Fig. 12). All treatments were significantly different from each other, with continuous releases having overall higher earwig abundance, followed by the mass release, then the control.

Therefore, earwig releases increased earwig abundance within one season, but only in the OR pear orchard where they were absent prior to releases.

In terms of increasing earwig abundance, earwig releases might not be as effective in orchards where earwigs populations are high compared with orchards where they are virtually absent. Orpet et al. (2019) proposed use of earwig augmentation only in pome fruit orchards where earwig populations were low or undetectable because the pest control service provided by additional earwigs have minimal returns where they are already abundant (Nicholas et al. 2005; Dib et al. 2016). In our orchard with low earwig abundance (OR), releases resulted in earwig establishment; they began to be observed in the control plots only after releases began and were found in 2022 prior to the start of releases that year. Given that earwigs were not present in these plots prior to releases in 2021, this likely indicates that earwigs from released plots had begun to move into control plots. Overall numbers in the OR orchard were still very low even after two years of releases (seasonal average of release 1 per trap at most), suggesting that yearly releases might still be needed to achieve thresholds for pest control; it has been suggested greater than two earwigs per trap is correlated with second and third generation biocontrol of pear psylla (DuPont et al. 2023). Another argument in favor of yearly releases in orchards with low earwig abundance is that due to their poor dispersal behavior – moving only up to 30m per month (Moerkens et al. 2010) – earwigs are thought to recolonize areas slowly where they had been absent (Orpet et al. 2019a). 

Woolly apple aphid was more abundant than the other two aphid species (green apple and rosy apple aphids) in both years in the WA apple orchard. Woolly apple aphid populations were higher in 2021 than 2022. We observed significant differences in the number of woolly apple aphid colonies between treatments in 2022, but not in 2021 (Fig. 13). In 2022, the mass release treatment had the lowest number of woolly apple aphid colonies, differing significantly from the continuous release, and was marginally different from the control. Numerically, the mass release treatment had 61% fewer woolly apple aphids than the control. Green and rosy apple aphid counts did not differ between treatments in either year of the study (data not shown).

Our results agree with previous studies demonstrating that earwigs can reduce woolly apple aphid populations. Earwig efficacy in controlling woolly apple aphids seems to be highly dependent on factors such as plant architecture, and both current pest, earwig, and other natural enemies’ abundances (Carroll et al. 1985; Mueller et al. 1988; Mols 1996; Quarrell et al. 2017; Orpet et al. 2019). Our results showed that one mass release of earwigs earlier in the season seems to be preferable over multiple releases. This may indicate that earwigs can help prevent aphid population increases, but cannot control high populations (Piñol et al. 2009).

In our study, a possible explanation for no effect of earwigs in suppressing green and rosy apple aphids includes the low overall numbers of these pests in our WA apple orchard. Additionally, green and rosy apple aphids are tended by ants, reducing the efficacy of natural enemies, while woolly apple aphids are not (Minarro et al. 2010).

Our results only showed pear psylla reductions after releases in our WA pear orchard and only in 2022 with no significance in the WA pear orchard in 2021, or in OR (Fig. 14). One possible explanation for this could be that the OR pear orchard had consistently low pear psylla abundance, substantially reducing our ability to detect differences in pest levels. In 2022 in OR, although not significant, we did observe a similar trend in our results when compared to what we observed in WA in 2022; there were fewer pear psylla in the mass release treatment compared to the continuous release and the control. It is possible that in an orchard with no earwigs prior to releases (like our OR orchard) but with higher pest densities (like our WA orchard), differences in pear psylla abundance between treatments would be more dramatic.

For orchards with either high or low previous earwig abundance, one early-season mass-release seems to be preferable to continuous releases because we observed that a single release early in the season in the OR pear orchard had greater earwig establishment and less pear psylla compared with the continuous release treatment.

Objective 2d - Drone releases of earwigs

Significant findings

• Earwigs have higher recapture rates than many other organisms used in augmentative biological control
• SmartWater provides a long-lasting mark on earwigs that can be easily assessed in the field
• Drone releases of natural enemies require drones with "leak-proof" dispensing canisters and a tight-turn flight path to ensure adequate coverage of an orchard block, which was not accomplished in this trial

Results and Discussion

In the hand-release treatment, 24 and 11 marked earwigs were found 1 and 7 days post-release, respectively (mean of 6 and 2.75 per acre). This represents a recapture rate of 0.6% and 0.3%, respectively. No magenta-marked earwigs were found. In preliminary laboratory trials, the magenta and yellow mark both were detectable far past 1-week post-marking and the paint did not cause substantial mortality, so it is unlikely that these results were caused by differences between the marks. The programmed flight of the drone did not make tight turns throughout the orchard and consequently, missed the center of the block, where sampling occurred. To compare ground and drone releases more accurately, flights will need to ensure adequate coverage of the whole block; this is especially important given that earwigs are slow dispersers. Additionally, there were issues with earwigs rapidly crawling out of the dispensing device as they were being loaded (resulting in a decreased payload once the drone reached the target site). The study should be repeated with a drone that has a closable dispensing device and the earwigs should be chilled prior to loading to prevent this issue in future work.

The trial provided proof-of-concept that earwigs can be used as test organisms for drone-based natural enemy applications. Their rate of recapture is much higher than previous natural enemies tested (e.g., lacewings with 0.008% recapture, Orius insidiosus with 0.08% recapture) making it much easier to compare recovery rates between release methods. Additionally, the trial also showed that SmartWater can be used to mark earwigs and, due to their large size, the presence/absence of the mark can be easily assessed in the field.

Objective 3a - Acute short-term toxicity of orchard pesticides to earwigs

Significant findings

• The insecticides tested are not acutely toxic to earwigs as residues, with the exception of spinetoram
• Of the herbicides tested, paraquat and glufosinate residues were acutely toxic to earwigs

Results and Discussion

None of the insecticide residues tested increased earwig mortality relative to a water control after 48 h of exposure (Table 2). However, in the spinetoram treatment, many earwigs were observed to have died before and during the prey consumption experiment the next day (see Table 6 in Obj. 3b). Therefore, with the exception of spinetoram, the insecticides we tested appear to pose little mortality risk to earwigs if they are exposed to dry residues.

Table 2. Percent mortality of female earwigs confined on fresh, dried insecticide residue at 24 and 48 h (n=6 replicates of 5 earwigs).

At 24 h, paraquat was the only herbicide with significantly higher mortality than the control (Table 3). At 48 h, glufosinate also significantly differed from the control and mortality of earwigs exposed to paraquat increased (and remained different from the control and the other herbicides). Paraquat and glufosinate residues are appear to be highly toxic to earwigs.

Table 3. Percent mortality of female earwigs confined on fresh, dried herbicide residue at 24 and 48 h (n=6 replicates of 5 earwigs).

Objective 3b - Sublethal effects of pesticides on earwigs

Significant findings

• Insecticides: novaluron and bifenazate residue exposure caused increased movement in earwigs, whereas chlorantraniliprole, cyantraniliprole, and fenpyroximate caused decreased movement. Spinetoram, spinosad, and cyantraniliprole exposure caused reduced prey consumption.
• Herbicides: glufosinate and halosulfuron exposure caused increased movement. All herbicides reduced prey consumption, but the effect was greatest in the rimsulfuron treatment. (paraquat excluded from prey consumption effects study due to high mortality)
• Of the insecticides tested, spinetoram and cyantraniliprole appear to the most at risk to disrupt earwig biocontrol in the field, but spinosad may also be of concern.
• Of the herbicides tested, paraquat, glufosinate, and rimsulfuron are the most likely to disrupt earwig biocontrol, but all herbicides showed the potential for reducing earwig prey consumption. Glufosinate and paraquat are used as burndown herbicides and are increasing in use by growers due to consumer concerns about glyphosate. Our results (and other studies, Zilnik et al. 2023) indicate that the substitution of paraquat or glufosinate for glyphosate has a high risk for disrupting biocontrol in agroecosystems.

Results and Discussion

In general, earwigs spent very little of their time moving, which is expected for a nocturnal predator tested in lighted conditions. Overall, earwigs in the first assay (Table 4) appeared to be more active than those in the second assay (Table 5). Relative to the control, novaluron  and bifenazate had increased earwig activity, whereas chlorantraniliprole, cyantraniliprole, and fenpyroximate had decreased earwig activity; the other insecticide/miticides did not differ from the control (Table 4). 

Increased movement after pesticide exposure can indicates an irritant effect or a decreased level of motor coordination that results in rapid, but uncoordinated movements. Decreased movement can indicate that the insect is moribund and near death, but insects can also recover over time. Both increased and decreased movement would likely impact earwig the ability of earwigs to effectively hunt pests in the field.

Table 4. Percent of time spent moving (>0.4 cm/s, out of 30 min) by female earwigs after 48 h of exposure to insecticide residues. Treatments followed by the same letter are not statistically different (p<0.05, lsmeans).

Glufosinate and halosulfuron increased earwig movement relative to the control; no other herbicides differed from the control (Table 5).

Table 5. Percent of time spent moving (>0.4 cm/s, out of 30 min) by female earwigs after 48 h of exposure to herbicide residues. Treatments followed by the same letter are not statistically different (p<0.05, lsmeans).

At 30 min, spinetoram and spinosad exposed earwigs had reduced prey consumption relative to the control. Spinetoram results could not be included in the analysis at 60 min because at that point there were only 2 surviving individuals. At 60 min, cyantraniliprole and spinosad had reduced prey consumption relative to the control, with a greater reduction in spinosad compared to cyantraniliprole. 

Table 6. Percent of green peach aphid prey consumed (out of 10) by a female earwig after 48 h of exposure to insecticide residues at 30, 60, and 90 minutes after providing the aphids. Treatments followed by the same letter are not statistically different (p<0.05, lsmeans).

At 30 min, glufosinate, 2,4-D, halosulfuron, and oxyfluorfen had reduced prey consumption compared to the control, but not rimsulfuron or glyphosate. At 60 min only rimsulfuron did not differ from the control and at 90 min all herbicide treatments differed from the control. Halosulfuron caused the greatest reduction in prey consumption; glyphosate, rimsulfuron, and glufosinate had the least reduction in prey consumption, and oxyfluorfen and 2,4-D were intermediate.

Table 7. Percent of green peach aphid prey consumed (out of 10) by a female earwig after 48 h of exposure to insecticide residues at 30, 60, and 90 minutes after providing the aphids. Treatments followed by the same letter are not statistically different (p<0.05, lsmeans).