Biology, ecology, and management of walnut husk fly (Diptera: Tephritidae)

The walnut husk fly (Rhagoletis completa) is one of the main pests affecting common walnut in both Europe and America. This work examines the effects of abamectin on the development of walnut husk fly larvae by injecting the product Vertimec 1.8 EC (Syngenta) into the trunks of walnut trees in Hungary. In the case of properly injected trees, the infection rate was negligible and the abamectin content in the husk samples ranged between 1.54 and 3.00 ng/g; controls show a very high (> 90%) infestation rate and an abamectin content under the detection limit. Abamectin residue measured in walnut kernel did not exceed the maximum residue limit; moreover, the active ingredient content was below the detection limit (0.0003 mg/kg) in all the collected kernel samples. Our results confirm that trunk injection is a viable method for walnut pest control.

The Walnut Husk Fly, Rhagoletis completa Cresson (Diptera: Tephritidae), is native to North America (Midwestern US and north-eastern Mexico) and has invaded several European countries in the past decades by likely crossing the alpine divide separating most parts of Switzerland from Italy. Here, we determined its current distribution in Switzerland by sampling walnuts (Juglans regia L.) in ecologically and climatically distinct regions along potential invasion corridors. R. completa was found to be firmly established in most low altitude areas of Switzerland where walnuts thrive, but notably not a single parasitoid was recovered from any of the samples. Infested fruit was recovered in 42 of the 71 localities that were surveyed, with mean fruit infestation rate varying greatly among sites. The incidence of R. completa in Switzerland is closely related to meteorological mean spring temperature patterns influencing growing season length, but not to winter temperatures, reflecting survival potential during hibernation. Importantly, areas in which the fly is absent correspond with localities where the mean spring temperatures fall below 7°C. Historical data records show that the natural cold barrier around the Alpine divide in the central Swiss Alps corresponding to such minimal temperatures has shrunk significantly from a width of more than 40 km before 1990 to around 20 km after 2000. We hypothesize on possible invasion/expansion routes along alpine valleys, dwell on distribution patterns in relation to climate, and outline future research needs as the incursion of R. completa into Switzerland; and, more recently, other European countries, such as Germany, Austria, France and Slovenia, represent an example of alien species that settle first in the Mediterranean Basin and from there become invasive by crossing the Alps.

The success of invasive species appears to be a paradox: despite experiencing strong population bottlenecks, invasive species are able to successfully establish in new environments. We studied how the walnut husk fly, Rhagoletis completa, was able to successfully colonize California from the Midwestern United States, by examining genetic diversity and diapause variation of native and introduced fly populations. Climate plays an important role in the successful establishment of introduced insects, because insect diapause is highly dependent upon external climatic conditions. We examined if: (1) fly populations show signs of a population bottleneck, (2) native and introduced flies differ in diapause length when exposed to California and Midwestern climatic conditions, and (3) population genetic diversity is related to variation in diapause length. We assessed if fly diapause conformed more to a model of establishment by local adaptation or to a model of a highly plastic "general-purpose genotype". Our results indicate that only two populations close to the original introduced location showed signs of a population bottleneck, and native and introduced populations did not differ in genetic diversity. Genetic diversity increased in the northern introduced populations, suggesting that multiple introductions have occurred. Flies emerged about 2 weeks earlier under the Midwestern treatment than the California treatment, and introduced flies emerged about a week earlier than native flies. All flies emerged when walnuts are typically available in California. Although variance in diapause length differed between populations, it did not vary between populations or regions. Furthermore, genetic diversity was not associated with diapause variation. Therefore, multiple introductions and a "general-purpose genotype" appear to have facilitated the fly's invasion into California.

The walnut husk fly (Rhagoletis completa Cresson) is the most important pest of walnuts (Juglans regia L.). It causes economically significant crop losses (up to 80% yield loss) in many growing regions, including the United States and most European walnut-producing countries. This chapter describes the impact of pest infestation on yield quantity and quality along with the current geographic distribution of the pest. Its bionomy and infestation symptoms are described in detail. An overview of monitoring and control methods used is also provided, and new methods that may prove useful for walnut husk fly control are listed. Monitoring the occurrence of the pest is the most important part of controlling the walnut husk fly, as only with an effective monitoring system can insecticides be applied at the appropriate time. Emphasis is placed on biotic protection and the possible role of phenolic compounds in cultivar resistance to walnut husk fly. Other control methods (non-chemical, mechanical, and biological control) are also gaining importance in pest management as more and more active substances in pesticides are regulated or phased out each year. Mechanical control methods are more or less only suitable for walnuts grown in the protected areas.

Walnut husk fly (WHF), Rhagoletis completa Cresson, is a serious insect pest of walnuts in California. Control of WHF relies on repeated applications of broad-spectrum insecticides combined with a feeding stimulant to suppress adult flies before oviposition. The broad-spectrum insecticides suppress beneficial insects and mites that in turn suppress a number of secondary insect and mite pests. Thus an efficacious and environmentally benign WHF control needs to be developed to reduce or eliminate broad-spectrum insecticides. To this end, laboratory and field studies were conducted from 2002 to 2005 to evaluate spinosad as a replacement for broad-spectrum insecticides. In laboratory trials, spinosad at low rates was not as efficacious as malathion but was as efficacious as esfenvalerate and superior to pyrethrin. WHF mortality as a result of feeding on GF-120 was adversely affected by increased WHF density. The longevity of GF-120 appears to be very short under hot and dry California conditions. In field trials, GF-120 and Success 2SC provided significantly lower infestation as compared to the blank bait check and was equivalent to the grower standard of malathion or chlorpyrifos.

The walnut husk fly (Rhagoletis completa Cresson) was observed for the first time in the South-East of France in 2007. In highly infected orchards, this quarantine organism can damage up to 80% of the annual production. In 2008, a network of orchards was monitored and experiments were set up in order to; (1) follow spatial propagation and (2) assess husk fly management techniques. Six different substances were tested, focusing on non chemical products. Four of them are in accordance with the specifications of organic farming. Two commercial products composed of kaolin have been tested. Treatment consists of a suspension of clay in water, sprayed on the whole canopy. This method has been successfully used to control cherry and olive fruit flies. The clay deposited on the walnut husk forms a physical barrier which should prevent oviposition. Two plant protection products containing Spinosad have also been tested: one in the form of bait, the other sprayed as a classic insecticide. Moreover, five kinds of traps have been tested and the most efficient could be studied in order to validate a massive trapping programme in low fly density situations. The results concerning the efficacy of the different control methods and the spatial propagation of the walnut husk fly in 2008 are presented and discussed.

In Slovenia, walnut husk fly (WHF) was first recorded in 1997, in the SW part of the country. Nowadays, it is active all over the country except in the area close to the Slovenian - Hungarian border. Observations, performed during the year 2003, showed that in the Maribor area (NE), losses due to WHF damage exceeded 50%. Over the last three years, substantial efforts have been made to improve our knowledge of the WHF, and to find an efficient method of control. During the years 2007 and 2008, the seasonal dynamics of the WHF were followed at two locations. In 2007, flies were present from 16 July to 12 September. Using yellow sticky plates, three peaks of adult emergence (27 July, 7 August and 12 September) were recorded in Maribor, while in Razvanje, only one peak (21 August) was noted. In 2008, the adults appeared twelve days later than in the previous year, and were caught until 23 September on both locations. Two peaks of emergence have been observed in Maribor (31 July and 8 September), as well as in Razvanje (31 July and 14 August). The WHF was monitored using three types of yellow sticky plates. Considering their surface and the number of the caught flies, the plate Rebell ® amarillo (Andermatt Biocontrol AG) was the most efficient. It seems that topography of the walnut orchard (flat or slope), as well as weather conditions (presence or absence of wind) may influence the seasonal dynamics of the WHF. Long-term observations should be conducted in order to formulate efficient control measures.

The pest-free period for walnut husk fly, Rhagoletis completa Cresson, that occurs from the beginning of stone fruit harvest in spring through 1 July was verified by trapping for adults in walnut orchards and roadside trees at eight locations in Merced, Madera, Fresno, Tulare, and Kern counties in the San Joaquin Valley of California. Peaches, Prunus persica (L.) Batsch; nectarines, P. persica (L.) Batsch var. nucipersica; Japanese plums, P. salicina Lindl; and walnuts, Juglans regia, were evaluated for acceptability for walnut husk fly oviposition and immature development. Field choice tests resulted in the following mean number of ovipositional sites per fruit and mean number of pupae that developed per ovipostional site in each fruit species: peaches and nectarines, <1 site, <1 pupa; plums, 0 sites, 0 pupae; and walnuts, <1 site, 4.5 pupae. One- or 3-d laboratory choice tests showed that peaches and nectarines were more acceptable and plums were less acceptable than walnuts for oviposition, but survival to the pupal stage was significantly lower in the three stone fruit species (<1 mean pupa per ovipositional site) than in walnuts (seven mean pupae per ovipositional site). Laboratory no-choice tests showed that walnut husk fly will oviposit in the three stone fruit species and walnuts, but the mean number of pupae that developed per ovipositional site was <1 in peaches and nectarines, 0 in plums, and eight in walnuts. Our results demonstrate that peaches and nectarines are poor hosts for walnut husk fly development, plums are not an acceptable host, and walnuts are the natural host.

Two Smith–Lever IPM Grants provided the funds to develop and disseminate information in an educational video about Walnut Husk Fly (WHF). The video was produced as a new way of conveying information about the techniques available to control WHF. Helping clientele implement new information on treatment timing was also a goal. The video was shown to 1154 clientele at 16 educational meetings. Fifteen regional trials were conducted across California to demonstrate the techniques illustrated in the video. Three-hundred-and-twenty-four clientele took a WHF quiz before and after viewing the video. Test results improved by 83%. A questionnaire indicated the video was a better format than a slide presentation for learning new information. Clientele particularly liked the review section at the end of the video. Seventy-five clientele participating in a postharvest telephone interview indicated that the information in the video helped them control WHF. Sixty-two percent used the new techniques described and 65% felt that the information presented improved their treatment timing. As a result of using information presented in the video, both the average number of treatments and insect damage was reduced. This 2-year project trained clientele representing over 40,000 acres of walnuts. Each Cooperative Extension office and IPM Advisor was given a copy of the video. In addition, over 70 copies of the video have been sold, several to pesticide companies for training of their pest control advisors.

Physical characteristics of walnut husk in relation to infestation by walnut husk fly (<i>Rhagoletis completa</i>)

Hybridization can provide a window into how populations diverge to form new species. Here, we confirm hybridization between Rhagoletis completa Cresson, 1929 and Rhagoletis zoqui Bush, 1966, two species of walnut husk-infesting flies that geographically overlap in a narrow area of parapatry in Northeastern Mexico. Rhagoletis completa and R. zoqui are members of a species group (Rhagoletis suavis) that has been hypothesized to speciate in allopatry. Sexual selection has been argued to be a potentially important factor for generating pre-mating isolation among walnut husk flies, because of the differences in wing morphology and body coloration, and the existence of sexual dimorphism within species. However, there was no evidence for pre-mating isolation between R. completa and R. zoqui, based on choice and no-choice mating experiments conducted on adults of fly populations outside the contact zone. There was also no support for reduced fertility of hybrid matings or for F1 inviability; however, F1 hybrids appeared to have lower fertility and F2 offspring have reduced survivorship. Postzygotic isolation in later generation hybrids of mixed ancestry therefore appears to be the first intrinsic barrier to gene flow evolving between R. completa and R. zoqui. We discuss the implications of our results for allopatric speciation in walnut flies and the potential evolutionary fate of R. zoqui and R. completa if they were to come into broad geographic contact in the future. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

The walnut husk fly, Rhagoletis completa Cresson (Diptera: Tephritidae), has recently invaded Switzerland, Austria, Germany, and France, probably crossing the alpine divide after its initial introduction into Italy. Here, the susceptibility of 36 walnut [Juglans regia L. (Juglandaceae)] cultivars to attack by R. completa was studied in an experimental orchard in Switzerland. Walnut cultivars differed significantly in infestation rates; cultivars that produced large, heavy fruit harboured significantly more larvae than cultivars that produced smaller fruit. Pupal weight was significantly influenced by cultivar, but not by any of the physical properties that we measured. For individual fruit within a cultivar, pupal weight was weakly related to fruit weight and infestation level. Adult longevity was correlated with pupal weight and appeared to be favoured in flies that developed in large-fruit cultivars. The longevities of adults recovered from different cultivars differed significantly. The shortest longevity was recorded for flies recovered from Geisenheim 1049 (39.2 ± 2.80 days) and the longest for flies recovered from Sheinovo (68.8 ± 21.75 days). Differences in diapause length were also highly significant and varied between 167 ± 5.1 (Esterhazy III) and 257.4 ± 8.21 days (Mayette). These results suggest that (1) across and within cultivars, walnut husk flies prefer to infest (i.e., they develop better in) large, heavy fruit, and (2) offspring that develop in large fruit are likely to accrue fitness advantages over the offspring of females using smaller fruit. Our results provide the basis for subsequent studies on resource defence by males, as they enable a prediction of which type of fruit males should defend more vigorously.

Genetic and cytogenetic information is an essential basis for understanding the biology of insect pests, as well as for designing modern control strategies. The walnut husk fly, Rhagoletis completa (Cresson) (Diptera: Tephritidae), is an important pest of walnuts (Juglans spp.) in North America and has invaded Europe in the early 1990s. Studies on the genetics and cytogenetics of R. completa are scarce. The mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of this pest species are presented here. The mitotic metaphase complement consists of six pairs of chromosomes, the sex chromosomes being very small and similar in size. The analysis of the salivary gland polytene complement shows a total number of five long chromosomes (10 polytene arms) that correspond to the five autosomes of the mitotic nuclei and a heterochromatic mass corresponding to the sex chromosomes. The banding pattern as well as the most characteristic features and prominent landmarks of each polytene chromosome are presented and discussed.

Genetic Changes Coinciding with the Colonization of California by the Walnut Husk Fly, Rhagoletis completa

English walnut (Juglans regia) producers in California compete with many insect and disease pests to produce an acceptable crop. Traditional control strategies work reasonably well for most pests. However, environmental concerns, loss of certain pesticides and new or impending regulations threaten the use of many traditional techniques for control of many of the pests. Codling moth (Cydia pomonella), walnut husk fly (Rhagoletis completa), and walnut aphid (Chromaphis juglandicola) are the major insects that affect California walnut production. Control strategies that use integrated pest management programs, beneficial insects, mating disruption, insect growth regulators, improved monitoring techniques and precise treatment timing based on the insect's life cycle are leading edge techniques currently available for insect control in walnuts. Major diseases include walnut blight (Xanthomonas campestris pv. juglandis), crown gall (Agrobacterium tumefaciens) and crown and root rot (Phytophthora spp). Both copper resistant and copper sensitive strains of the walnut blight bacterium are best controlled with combinations of copper bactericides and maneb instead of copper materials alone. A new computer model, Xanthocast, used to forecast the need for walnut blight treatment is under evaluation. Crown gall is managed using a preplant biological control agent and a heat treatment to eradicate existing galls. Phytophthora crown and root rot is dealt with primarily by site selection, irrigation management and rootstock selection.