European earwig and Portuguese millipede damage in wheat
Michael A. NASH
European earwig and Portuguese millipede damage in wheat
Information current as of 2 November 2015
© Government of South Australia 2015
PIRSA and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein as regards to its correctness, accuracy, reliability and currency or otherwise. PIRSA and its employees expressly disclaim all liability or responsibility to any person using the information or advice.
Dr Michael A. Nash
GPO Box 397, Adelaide SA 5001
T 08 8303 9537 F 08 8303 9542
Table of Contents
European earwig and Portuguese millipede damage in wheat 1
Michael A. NASH 1
Table of Contents 3
Results: Earwigs 5
Results: Millipedes 8
Management of invertebrate threats to production is often reactive to plant damage without correctly identifying the causal pest. The neglect of understanding species functionality in agro-ecosystems often leads land managers to apply broad spectrum controls, causing subsequent flares in secondary pests that are more tolerant to pesticides applied. The role of polyphagous species in farming systems, that can often switch feeding behaviours challenges growers. Ongoing reports of European earwigs (Forficula auricularia L.) (Dermaptera: Forficulidae) and Portuguese millipede, Ommatoiulus moreleti (Julida) highlight the difficulties Australian growers face in attributing damage to what could be considered beneficial invertebrate populations.
Abundance of earwigs is expected to increase in response to changes in management practices aimed at retaining more moisture (Nash and Hoffmann, 2012). Labidura sp. (Dermaptera: Labiduridae), are considered beneficial (Horne and Edward, 1995), however European earwigs have become an agricultural pest on every continent, except Antarctica (Sakai, 1987), and are often reported as pests in Australian cropping systems (Hoffmann, Weeks, et al., 2008). However, this simplistic view is misleading due to a number of factors.
Earwigs consume a wide variety of living and dead food, thus are considered omnivorous. This variety of diet can cause differences in opinion on the beneficial nature of species. For example, damage to grape bunches at early stages leads viticulturists to consider European earwigs as a pest; however the same species is now being considered a predator of Light Brown Apple Moth, the number one pest in Australian viticulture. Research suggests the principal control agent of Woolly aphid (Eriosoma lanigerum Hausmann) (Hemiptera: Aphididae) under apple IPM was identified as the European earwig (Nicholas, Spooner-Hart, et al., 2005). Another cosmopolitan species, Labidura riparia (Pallas) (Dermaptera: Labiduridae) was considered a household pest (Workman, 1963), but the general view is this species is beneficial particularly in attacking lepidopterous pests (Bishop and Blood, 1977, Price and Shepard, 1978). Labidura sp. is known to eat false wireworms (Tenebrionidae), prepupae and pupae of Heliocoverpa spp. (Donaldson and Ironside, 1982), aphids, cutworms and armyworms (Allsopp and Lloyd, 1982), Teleogryllus commodus nymphs and Phthorimaea operculella were found not to consume faba bean seedlings (Horne and Edward, 1995). One suggestion is immature earwigs feed on plant material (Simpson and Mayer, 1990).
Millipedes, both native (e.g. Order: Polydesmida) and exotic (e.g Order: Julida), contribute to the breakdown of organic matter (Baker, 1985, Black, 1997). Little is known about the contribution of exotic millipedes to plant litter breakdown in native or agricultural landscapes in Australia or about their interactions with native species (Norton B, under review).
The Portuguese millipede, Ommatoiulus moreleti (Julida), was initially regarded as a nuisance pest in houses in Adelaide. In recent decades its distribution expansion across grain production regions (Baker, Grevinga, et al., 2013), and the changes in farming practices that increase the organic matter food resource for this species and its resultant abundance, has led to increasing numbers of pest reports in establishing grain crops, particularly canola (Nash, DeGraaf, et al., 2014). Actual damage estimates are hard to come by due to the unpredictable nature of millipedes feeding on green plant material (Paoletti, Tsitsilas, et al., 2008).
Observations of pest activity in a wheat canopy, combined with experiments established to attribute and quantify damage were initiated in order to attribute actual plant losses due to either millipedes or earwigs. Likely causes of plant feeding are discussed. The second aim was to assess baits to reduce numbers in order to reduce Australian growers’ reliance on broad spectrum to control European earwigs and Portuguese millipedes.
A time lapse camera was used to observe earwig and millipede activity in a wheat canopy spring 2015, where they were suspected of causing damage.
In a field experiment (randomised block) novel bait products were applied to 100 m2 plots (4 by 7 grid, N=28) in an attempt to reduce pest numbers; with Lorsban applied as a foliar spray at 1.2 l/ha included as standard practice and an untreated control. Three methods were used to estimate pest numbers with one of each type placed in the centre of plots; a roll of cardboard pinned to the ground, a pitfall trap and a surface refuge. Cardboard rolls (herein referred to as “cardboard”) consisted of corrugated cardboard (ca 250 x 250 mm) rolled to form cylinders (ca 100 mm diameter) with longitudinal corrugations. Spaces between and within corrugations provided crevices for earwigs to shelter. Rolls were held in place using a wire loop inserted into the soil either side of the roll. Earwigs were counted once a week with rolls opened and all earwigs collected in a tray. After earwig numbers were recorded, earwigs were released onto the ground and the shelter trap reassembled (Burnip, Daly, et al., 2002). Pitfall traps (herein referred to as “pitfall”) consisted of a large plastic container (ca 500ml plastic tubs 100mm dia) embedded into the ground so the top lip is level with the soil surface (Nash, Thomson, et al., 2008), effectively trapping active invertebrates. Pitfalls were run without preservative. Surface refuge traps (herein referred to as “tile”) consisting of a 300mm by 300mm ceramic tile placed on the soil surface, as used to sample slugs (Nash, Thomson, et al., 2007). Spaces under tiles provide refuges for various invertebrates. Sampling occurred on 4 separate occasions (pre-treatment, 7, 14 & 21 days after application) at seven day intervals with specimens identified in the field, and vouchers returned to the lab for verification of species.
Monitoring in untreated plots indicates cardboard rolls captured the highest number of European earwigs, compared to pitfalls and tiles (Fig 1). However, the results from the cardboard rolls were more variable than pitfalls (as indicated in Fig 1 by longer error bars). Three weeks later earwig activity was a lot less, where as other species of earwigs’ activity remained the same. Four species of earwig were recorded from pitfall traps at the site, highlighting the need to use a variety of monitoring methods to understand fully what is occurring in your paddock and what are earwigs really doing. This is highlighted by the results with increased abundance in some treatments at 14 and 21 days after treatments (Fig 2b), which may be due to moisture in some pitfalls.
Overall treatments did not reduce earwig abundance, as there was a natural decline in populations (Fig 2). Data from surface refuges is not included as it was to variable. Data from cardboard rolls suggests seven days after treatments were applied there may have been a decrease in earwig abundance due to application of experimental product. There was no reduction in abundance in the farmer practice treatment, which was applied at an excessive rate (600 g Chlorpyrifos / ha) to elicit a response.
Figure 1. European earwig mean abundance in response to sampling methods placed in the centre of 10 by 10 m untreated plots (N=4). Error bars indicate standard deviation.
Figure 2. European earwig mean abundance in a/ cardboard rolls and b/ pitfalls in response to various treatments applied to 10 by 10 m plots (N=28). Errors bars represent standard deviation.
In associated experiments SARDI has been investigating how much snail bait is being eaten by Earwigs. The belief is earwigs are eating the bait before the snails, so the aim is to apply bait in the autumn that will kill both earwigs and snails.
Monitoring in untreated plots indicates cardboard rolls are not suitable to estimate millipede relative abundance as none were captured. The highest number of millipedes was recorded from pitfalls, compared to tiles (Fig 3). Relative abundance increased during October 2015, with both 1st year juvenile and 2 year old individuals sampled for all occasions. The increased abundance in pitfalls 23 Oct was thought in part due to moisture in some pitfalls, with individuals attracted to that moisture.
Overall treatments did not reduce millipede abundance, with numbers increasing even in treated the positive control plots treated with either Lorsban or Mesurol (Fig 4). Data from tiles suggests seven days after treatments were applied there may have been a decrease in earwig abundance due to application of experimental product, in line with positive controls. However, it was observed on 23 Oct millipedes were congregated under tiles where soil was still moist. High numbers of dead millipedes were observed in Mesurol treated plots at the end of the experiment (23 Oct, 21 days after application).
Figure 3. Portuguese millipede mean abundance in response to sampling methods placed in the centre of 10 by 10 m untreated plots (N=4). Error bars indicate standard deviation.
Figure 4. Portuguese millipede mean abundance in a/ pitfalls and b/ under tiles (300mm by 200mm) in response to various treatments applied to 10 by 10 m plots (N=28). Errors bars represent standard deviation.
Figure 5. 2015 Spring rainfall where earwigs and millipedes were active in wheat heads.
Results from methods used to estimate numbers, hence efficacy of treatments, were varied with no definitive indication that any treatment decreased either earwig or millipede numbers, when applied in an extremely dry spring to a flowering wheat crop (Z55-58). Yield estimates are yet to be obtained from field plots.
Means for cardboard roll had a higher variance than pitfalls. As earwigs are nocturnal, they readily hide in artificial refuges during the daytime; in this trial we used corrugated cardboard (Burnip, Daly, et al., 2002). In this trial we found data from ceramic tiles acting as surface refuges was also more variable than pitfalls and gave much lower numbers than cardboard rolls initially. However, in subsequent repeated sampling cardboard rolls and ceramic tiles gave comparable results (Fig 1). Cardboard rolls are functionally similar to the grooved boards used elsewhere by those studying earwigs (Helsen, Vaal, et al., 1998, Lamb and Wellington, 1974).
Pitfalls gave the least variable results and have been suggested by some Australian researchers as a suitable method to look at phenology and differences between tillage practices (Horne and Edward, 1995). The best time to sample for Labidura sp. is over summer when females carry eggs (Horne and Edward, 1995), however in this trial low numbers of Labidura sp. males and females were detected in spring. Differences in soil type (loam vs. sand), local or seasonal differences in climate, crop type and nutrition factors may all play a role in slightly differences in phenology of Labidura sp. reported (Simpson, 1991). The same may hold true for European earwigs.
The decline relative abundance of European earwigs in cardboard rolls (Fig 1) may have been due to extremely dry spring weather (Fig 5), with most of those captured in being nymphs. In comparison, abundance in some pitfalls at 14 and 21 days after treatment (Fig 2b) was thought to be due to moisture in those traps accumulating after rainfall (Fig 5). Moisture is critical to their survival (Jones and Bryant, 2012) and earwig development is quicker under moist conditions; up to 50% soil moisture (Simpson, 1991). Some variation in results is suspected to be due moisture responses, but trapping method also influenced confidence in results of treatment effects on European earwigs.
Portuguese millipede response to lack of moisture may have confounded results, with numbers accumulating under tiles and in pitfalls after limited rainfall events following an extremely dry period (Fig 5). Typically millipedes are about 60% moisture, and are more susceptible to losing water than insects. Millipedes have evolved a variety of behaviour and physiological mechanisms to reduce water loss. This includes walking to a wet area, minimizing loss that includes rolling up, elevation of internal osmotic pressure or addition of water by metabolism or transport from external environment (Hopkin and Read, 1992). Portuguese millipedes prefer high relative humidity and moderate temperatures and these preferences may explain summer aggregations (Baker, 1980). This may explain why increased numbers where recorded with the methods employed as it was noted both pitfalls and tiles also accumulated moisture. The movement of millipedes into the wheat canopy early morning in late September early October may be individual’s response to dry conditions and rehydrating in moist crop canopies, not to feed on the crop.
Based on overall trapping results, the use of pitfalls is recommended as the preferred method of monitoring for these pests in crops, but not when conditions are dry.
It is hoped investigations into these two pests will continue as the inconclusive nature of results from one trial, conducted under conditions (dry) likely to alter both target species behaviour, indicate the need for more research into control of both millipedes and earwigs in broad acre systems at different times of the year when moisture is not so limiting.
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