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The oriental beetle, Anomala orientalis (Waterhouse) (Col., Scarabaeidae), is the most important root-feeding pest of blueberries and turfgrass in New Jersey, USA. Previous studies showed that mating disruption is a feasible option for oriental beetle management; however, assessing its efficiency can be challenging, and little is known on its long-term effects. Accordingly, we conducted studies to investigate low-dose pheromone lures equivalent to oriental beetle females (i.e. female mimics) as easy-to-use indicators of mating disruption success, determine the distance at which oriental beetle males respond to female-mimic lures and assess the long-term (3-year) effects of mating disruption on oriental beetle populations in entire blueberry fields. Our studies showed that rubber septa baited with 0.3 μg of the oriental beetle sex pheromone (Z)-7-tetradecen-2-one attract similar numbers of males as compared with virgin females and can thus be used as a female mimic. The range of attraction of this lure was found to be also similar to virgin females and <30 m. In blueberries, mating disruption provided 87% inhibition of oriental beetle populations (trap shutdown) over a 3-year period. Oriental beetle male captures in disrupted fields were threefold higher along the field edges than in the field interiors, indicating movement of males from nearby areas into the pheromone-treated fields. In addition, mating disruption reduced male attraction to female-mimic lures by 93% in all 3 years and reduced the number of larvae in sentinel potted plants in 1 of 2 years. These results show for the first time that mating disruption provides consistent long-term field-wide control of oriental beetle populations and that female-mimic pheromone lures can be used as a new tool to assess oriental beetle mating disruption success.
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- Materials and Methods
Highbush blueberries, Vaccinium corymbosum L., are a major component of the southern New Jersey (USA) agricultural economy where this crop is grown on 3000 ha with a total annual production of approx. 19.5 million kg, valued at US $ 80.8 million (USDA NASS 2013). The oriental beetle, Anomala orientalis (Waterhouse) (Col., Scarabaeidae), is the most abundant white grub pest in blueberry farms in New Jersey (Polavarapu 1996) and is considered by farmers as one of the most important insect pests of blueberries in the state (Rodriguez-Saona et al. 2009). In New Jersey, oriental beetle adults start to emerge in early June, reaching peak flight in mid- to late June (C. Rodriguez-Saona, personal observation). Females lay eggs in the soil at the base of bushes. The larvae go through three instars, with most larvae reaching the third instar by mid-September. Larvae remain in the soil during winter, resume feeding the following spring and enter the pre-pupal stage in late May. The root-feeding damage caused by larvae can result in complete destruction of the root system and the death of host plants, especially when larval populations are high (C. Rodriguez-Saona and D. Polk, personal observation). Infested bushes show reduced vigour and fewer berries. This insect is also considered a major pest of turfgrass and ornamentals (Vittum et al. 1999).
Limited options are currently available to control oriental beetle populations in blueberries. Although the neonicotinoid imidacloprid is the only insecticide registered in blueberries for larval control, there are several concerns associated with its use including limited efficacy against late-instar larvae (Koppenhöfer and Fuzy 2008) and its potential to disrupt pollination and biological control (e.g. Decourtye et al. 2003; Rogers and Potter 2003). Having a single control method for oriental beetle control also raises considerable resistance management issues. Because blueberries are grown in an ecologically sensitive region of New Jersey, known as the Pinelands or Pine Barrens that is characterized by porous acidic soils and a high water table, it is doubtful that other soil-applied insecticides will be registered. Chemical control methods do not target the adult stage because adults cause no economic damage to blueberries, their emergence period is long and occurs during harvest and they are difficult to target with insecticide sprays due to their cryptic behaviour (Facundo et al. 1999).
An alternative to insecticides for pest control is the use of mating disruption (Cardé 2007; Rodriguez-Saona and Stelinski 2009). Previous studies demonstrated the feasibility of using microencapsulated sprayable formulations of (Z)-7-tetradecen-2-one, the major component of the oriental beetle's sex pheromone, for oriental beetle mating disruption in blueberries (Polavarapu et al. 2002) and turfgrass (Koppenhöfer et al. 2005). Polavarapu et al. (2002) found a 90% reduction in trap male captures in 1-ha plots treated with the pheromone compared with untreated plots. However, because the oriental beetle pheromone is a ketone, use of sprayable microencapsulated formulations is restricted in food crops (Weatherston and Minks 1995). On the contrary, retrievable hand-applied plastic dispensers are exempt from tolerance restrictions. Using 50–75 hand-applied plastic dispensers per ha containing (Z)-7-tetradecen-2-one at 1 g a.i./dispenser, Sciarappa et al. (2005) showed lower trap captures, reduced mating rates and fewer larvae in 1-ha blueberry plots treated with pheromone dispensers compared with untreated plots. Subsequent studies showed that a low rate of 0.05 g a.i./dispenser at 50 dispensers per ha (total pheromone = 2.5 g/ha) was sufficient to provide effective oriental beetle mating disruption in blueberries and was comparable in cost to an imidacloprid treatment (Rodriguez-Saona et al. 2009).
Retrievable hand-applied plastic dispensers (ChemTica Internacional S.A., Costa Rica) for oriental beetle mating disruption became registered in 2013. Now that this technology is available to blueberry farmers, it is most likely that they will treat entire fields instead of field sections. Accordingly, in this study, we scaled-up the area under mating disruption by treating entire blueberry fields with hand-applied plastic pheromone dispensers. In previous studies, virgin oriental beetle females were used to assess the success of mating disruption in blueberries (e.g. Sciarappa et al. 2005; Rodriguez-Saona et al. 2009, 2010) and turfgrass (e.g., Koppenhöfer et al. 2008). Because the process of collecting and rearing oriental beetles to obtain virgin females is labour-intensive and unlikely to be used by farmers, another of our objectives was to identify a ‘female mimic’, that is, a pheromone-dispensing device and dose that attracts male oriental beetles in similar numbers as virgin oriental beetle females.
Thus, this study's main goals were to develop an easy-to-use method to assess mating disruption for oriental beetle and prove the long-term efficacy of mating disruption in suppressing oriental beetle populations. Specifically, (i) we tested low-dose pheromone lures equivalent to oriental beetle females for use as indicators of mating disruption success – a study carried out in blueberries and turfgrass, (ii) we determined the distance at which oriental beetle males respond to these low-dose pheromone lures–a study carried out in turfgrass and (iii) we conducted a 3-year study to determine the long-term effects of mating disruption on oriental beetle populations – a study carried out in blueberries.
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The results from our studies show that: (i) rubber septa baited with 0.3 μg of (Z)-7-tetradecen-2-one–the oriental beetle sex pheromone–can be used as a female mimic to evaluate mating disruption success of this insect pest; (ii) the range of attraction of this lure (i.e. estimated area where 99.9% of released males were attracted to the lure) is about 30 m; (iii) mating disruption can provide consistent long-term control of oriental beetle populations in blueberry fields; and (iv) mating disruption was more effective in the middle of fields than along the field edges.
This study tested and found for the first time a pheromone-dispensing device and dose that is similarly attractive to male oriental beetles as virgin females (i.e. a female mimic) and that could be used instead of virgin females by researchers and farmers to assess the efficacy of mating disruption for this insect. As indicated by Doye and Koch (2005), assessing the effectiveness of mating disruption can be challenging. In moths, mating disruption is typically assessed by measuring male captures in pheromone traps (trap shutdown), mating frequency of tethered virgin females and fruit injury at harvest (Cardé and Minks 1995). Because tethered females are openly exposed and thus easily consumed by predators, researchers often use cages or traps to enclose the females (Cardé et al. 1975; Schmidt and Seabrook 1979; Doye and Koch 2005; Stelinski et al. 2006; Briand et al. 2012). In oriental beetle, caged virgin females have previously been used to assess the success of mating disruption in both blueberries (Rodriguez-Saona et al. 2009, 2010) and turfgrass (Koppenhöfer et al. 2008). However, collecting and rearing oriental beetles to obtain virgin females is laborious. Our data indicate that the use of virgin oriental beetle females can be substituted with the use of 0.3-μg pheromone lures.
Our experiments could not clarify, however, whether cages or traps are more representative of the natural mate finding conditions. Although cages and traps provided similar results in blueberries, traps caught significantly higher numbers of males than cages in turfgrass. Cages might restrict mate finding to some extent due to the limited access to the female which may lead some males that find the caged female to give up before finding one of the cage openings. On the other hand, in turfgrass, traps might lead to greater than natural mate finding because the small cages containing the females are suspended near the top of the trap about 13 cm above the soil surface rather than in the grass where the females would naturally call. This should result in a less obstructed and easier to trace pheromone plume in the dense albeit low groundcover of a turfgrass area. In blueberries, the bottom of traps sat on top of, or just above, the soil–a practice commonly used to avoid predation by ants–which placed the lure at about 30 cm above the ground. Possibly this even higher placement in blueberries was detrimental to male capture thus reducing the advantage of freer access to the traps. Alternatively, in the blueberry fields which had very little ground cover (but rows of about 1.5 m high bushes), the higher placement of the lures had a smaller positive effect on male capture than in turfgrass. Direct observations might be needed to determine whether cages restrict male access to pheromone sources as compared with traps. Determining mating status after exposure of tethered but non-caged females might be the most natural (but also very labour-intensive) option; however, based on previous experiences (Koppenhöfer et al. 2005), such females cannot be left unprotected in the field because they can be predated by birds and ground predators.
In turfgrass, the range of attraction of the 0.3-μg pheromone lures was similar to that of virgin females, that is ≤30 m from the source. These results indicate that the lures need to be placed apart a certain distance depending on what level of recapture relative to the maximum possible recapture can be considered as significant interference. For example, the developed equation for the 0.3-μg lure predicted a maximum recovery of 11.7% at 0 m distance from the trap, with recovery declining to 1% (9% of maximum) at 11.71 m and 0.1% (0.9% of maximum) at 22.8 m. Hence, traps should probably be placed at least 20 m apart to minimize any interference among them. For comparison, the distance at which 0.1% of males were attracted was 44.2 m for 1.0 μg and 122.0 m for 3.0 μg (present study), and 210.7 m for 10 μg and 477.0 m for 30 μg (A.M. Koppenhöfer, unpublished data).
Rodriguez-Saona et al. (2010) also reported the range of attraction for oriental beetle females to be about 30 m in blueberries, suggesting that crop architecture might not affect how far the pheromone plume can travel. The lower recapture rates with virgin females and the 0.3-μg pheromone lures as compared with the 1- and 3-μg lures might be in part due to the fact that the former two treatments emit less pheromone. In the case of virgin females, it might also be because they call only for a few hours around dusk and their pheromone production declines with age (Facundo et al. 1994; Zhang et al. 1994). In contrast, red rubber septa loaded with 10, 100, 300 or 1000 μg provided a constant release over a 4-week period (Behle et al. 2008). Pheromone production may also have been affected by the unnatural female placement in cages.
Beyond replacing virgin females as an evaluation tool for mating disruption success, we also envisioned for the low-dose female mimic (0.3 μg lure) to replace the high dose (30–300 μg lure) used in previous studies (e.g. Koppenhöfer et al. 2005, 2008; Rodriguez-Saona et al. 2009, 2010). The assumption had been that a lower dose lure, similar to a virgin female, would be more sensitive by being less competitive with the actual pheromone dispensers which contain 50 mg of (Z)-7-tetradecen-2-one in blueberries. Thus, we had expected to observe greater reduction in male trap captures in disrupted fields with the lower dose lures than with the high-dose lures. However, percentage reduction in trap captures in disrupted fields compared with non-disrupted fields averaged across the 3 years was 92% (only using traps in the field interior; it was 82% for traps in the field exterior) for the 300-μg lures and 92% for virgin females, 94% for 0.3-μg lures and 93% for 1.0-μg lures. It may be that the high release rates from the 50-mg dispensers were so much greater that the difference between the 300-μg and the 0.3-μg lures was irrelevant. The comparison might have come to a different result in turfgrass using pelletized pheromone dispensers of smaller pheromone content (0.8–1.4 mg) but higher density (Koppenhöfer et al. 2008). Intuitively, a lure that attracts similar numbers of males from a similar distance as a virgin female should be a better indicator of how mate finding by males is affected by pheromone application. Considering that the distance at which 0.1% of males would be attracted is more than 20 times greater for a 30-μg lure (477 m) than 0.3-μg lure (22.8 m), it stands to argue that a 300-μg lure would be much more likely to be affected by edge effects as observed in this study and by interference with other traps.
In the present study, we also document for the first time the long-term effects of mating disruption on oriental beetle using commercially available hand-applied plastic dispensers deployed in entire blueberry fields. Although in the past four decades, mating disruption has proven effective for the control of several Lepidoptera pests, including pink bollworm (Pectinophora gossypiella (Saunders)), codling moth (Cydia pomonella (L.)), oriental fruit moth (Grapholita molesta (Busck)), obliquebanded leafroller (Choristoneura rosaceana (Harris)) and gypsy moth (Lymantria dispar L.) (Cardé 1990, 2007; Miller et al. 2006), only recently has it been explored for pest control in the Coleoptera. So far, mating disruption has been tested for seven species in the Coleoptera and shown to be effective for oriental beetle control in blueberries, turfgrass, cranberries and ornamental nurseries (Table 2). Two predictions were tested in our studies: first, that the efficacy of mating disruption changes in time; and second, that the efficacy of mating disruption is greater in the field interiors than along the field edges. An application rate of 50 dispensers per ha (2.5 g a.i. per ha) to blueberry fields provided 87% trap inhibition in all 3 years of this study. As opposed to our prediction, this level of trap inhibition did not vary significantly in time and was comparable to those reported in previous studies carried out in blueberries (90–95%; Polavarapu et al. 2002; Rodriguez-Saona et al. 2009), using smaller hectares under mating disruption, as well as in turfgrass (87–97%; Koppenhöfer et al. 2005, 2008) and cranberries (80–99%; Wenninger and Averill 2006), indicating that the efficacy of mating disruption for oriental beetle is not crop- or scale dependent. However, Rodriguez-Saona et al. (2010) showed that the density of dispensers has an effect on the efficacy of mating disruption in oriental beetle because under low-dose dispensers, the behavioural mechanism of disruption in this insect is via competitive attraction (Miller et al. 2006); thus, greater disruption is expected under higher number of dispensers. Here, we used a dispenser density shown to be effective and economically feasible (Rodriguez-Saona et al. 2009).
Table 2. Case studies of mating disruption in the Coleoptera
|Species (Family)||Pheromone||Commodity||Type of dispenser||Reference|
|Cylas brunneus F. (Apionidae)||Decyl- and dodecyl (E)-2-butenoate||Sweetpotato||PVC resin, controlled release formulation||Downham et al. 2001|
|Cylas puncticollis Boheman (Apionidae)||Decyl- and dodecyl (E)-2-butenoate||Sweetpotato||PVC resin, controlled release formulation||Downham et al. 2001|
|Melanotus okinawensis Ohira (Elateridae)||Dodecyl acetate||Sugarcane||Polyethylene tubes||Arakaki et al. 2008|
|Megaplatypus mutatus (Chapuis) (Curculionidae)||(+)-6-Methyl-5-hepten-2-ol, 6-methyl-5-hepten-2-one, 3-pentanol||Hazelnut and poplar||Glass vials with polyethylene semipermeable cap/Polyethylene bags||Funes et al. 2011|
|Prionus californicus Motschulsky (Cerambycidae)||(3R,5S)-3,5-dimethyl dodecanoic acid||Hop||Clear polyethylene sachets||Maki et al. 2011|
|Dasylepida ishigakiensis Niijima et Kinoshita (Scarabaeidae)||(R)-2-butanol||Sugarcane||Polyethylene tubes||Yasui et al. 2012|
|Anomala orientalis (Waterhouse) (Scarabaeidae)||(Z)-7-tetradecen-2-one||Blueberries||Microencapsulated sprayable formulation; Hand-applied plastic formulation; SPLAT-OrB||Polavarapu et al. 2002; Sciarappa et al. 2005; Rodriguez-Saona et al. 2009, 2010;|
| || ||Turfgrass||Microencapsulated sprayable formulation; Wax-based granule formulation||Koppenhöfer et al. 2005, 2008; Behle et al. 2008|
| || ||Cranberries||Retrievable wax discs||Wenninger and Averill 2006|
| || ||Ornamental nurseries||Microencapsulated sprayable formulation||Polavarapu et al. 2002|
Our data do support our second prediction that the efficacy of oriental beetle mating disruption varies within fields. Although oriental beetle male captures in pheromone-baited traps positioned at the perimeter and interior of blueberry fields were reduced in disrupted blueberry fields compared with non-disrupted fields, male captures in disrupted fields were threefold higher along the field edges than in the field interiors, and these findings were consistent over a 3-year period. These data indicate that to optimize field-wide oriental beetle mating disruption, higher dispenser density might be needed along field edges than in field interiors. Similarly, Knight (2007) reported reduced captures of male codling moth in sex pheromone traps located inside compared with those located at the border of apple orchards under mating disruption. Isaacs et al. (2012) reported greater levels of fruit infestation by the grape berry moth, Paralobesia viteana (Clemens), at the perimeter than in the interior of vineyards under pheromone mating disruption. These studies together with ours indicate movement of males from nearby areas into the pheromone-treated fields and suggest that the efficacy of mating disruption might be highest when deployed as an area-wide approach to account for possible pest migration (Calkins et al. 2000; McGhee et al. 2011; Tollerup et al. 2012).
The findings presented here have important implications for future implementation and assessment of mating disruption in oriental beetle. Area-wide application of dispensers for oriental beetle mating disruption may be the most effective approach to control this pest; however, its economic investment might be too high. Currently, blueberry growers treat individual fields with imidacloprid based on oriental beetle population levels in traps, and this same practice will likely be used with mating disruption. In addition, the labour costs associated with dispenser application will likely limit the area under oriental beetle mating disruption. The use of less labour-intensive methods of pheromone application such as wax matrices that can be mechanically applied is under investigation (Rodriguez-Saona et al. 2010). Moreover, we tested mating disruption for oriental beetle control in three consecutive years. Future studies need to compare the efficacy of continuous vs. interrupted applications of mating disruption. Finally, a remaining challenge is the assessment of crop injury. Measuring root injury by oriental beetle can only be carried out via crop destruction, which is not feasible in commercial blueberry farms. Instead, we have used sentinel plants baited with tethered females that often reflect the success of mating disruption (Rodriguez-Saona et al. 2009, 2010; this study). Alternatively, here, we provide a new easy-to-use and effective method, that is female mimics, to assess mating disruption for oriental beetle. The use of this more realistic low-rate trapping method would make the use of other methods such as caged virgin females or high-rate traps (trap shutdown), and ideally also sentinel plants, unnecessary.