Quarantine control of Hessian fly (Diptera: Cecidomyiidae) in exported hay: a new treatment for large-size, polypropylene fabric-wrapped bales and a 3-d fumigation for compressed standard bales

Hessian fly puparia (37,167), Mayetiola destructor (Say), did not survive a large-scale commercial test (three freight containers) of a new quarantine treatment using compression (32 kg/cm2) and hydrogen phosphide fumigation (61 g/28.2 m3) for large-size, polypropylene fabric-wrapped bales of hay exported to Japan. Mean +/- SEM temperatures in the large-size bales in different locations in the freight containers ranged from 18.0 +/- 0.9 to 26.0 +/- 1.3 degrees C during the 7-d test conducted in a heated building at 20.1 +/- 1.1 degrees C. Highest concentrations of hydrogen phosphide in most locations in the freight containers were observed after 3 d of fumigation and ranged from 366.7 +/- 96.1 to 425.0 +/- 162.7 ppm (mean +/- SEM) and throughout the 7 d of fumigation ranged from 253.6 +/- 59.9 to 407.1 +/- 76.5 ppm (mean +/- SEM). Hydrogen phosphide residues after fumigation and aeration were <10 ppb and were below the U.S. Environmental Protection Agency tolerance of 0.1 ppm in animal feeds. The results of the test fulfills regulatory agency testing requirements and confirms the efficacy of the treatment to control Hessian fly in large-size, polypropylene fabric-wrapped bales of hay. Hessian fly puparia (2,160) did not survive a large-scale commercial test of compression (105 kg/cm2) and a 3-d hydrogen phosphide (60 g/28.3 m3) fumigation for standard bales.     

Bale compression and hydrogen phosphide fumigation to control cereal leaf beetle (Coleoptera: Chrysomelidae) in exported rye straw

Control of larvae and adults of cereal leaf beetle, Oulema melanopus (L.), by bale compression and hydrogen phosphide fumigation was studied in rye straw, Secale cereale L., in Aurora, OR. Natural mortality of larvae after transport was 4.0 +/- 1.0% (mean +/- SEM). Compression (105 kg/cm2) of larvae in standard bales (122 cm long) of rye straw resulted in 100% mortality. Compression of adults in standard bales plus storage of the compressed bales (56 cm long) for 1 d in a freight container resulted in 100% mortality. A KD50 of 102 ppm hydrogen phosphide for 1 h was estimated from the probit regression line developed from dose-response data at 21 degrees C in basic laboratory tests. The LD50s and LD99s were 163 and 6,910 ppm for 2-h exposures and were 18 and 42 ppm for 6-h exposures at 21 degrees C, respectively. A tested close of 400 ppm for 24 h at 21 degrees C resulted in 100% mortality of the adults. Larvae (n = 10,560) and adults (n = 18,602) did not survive exposure to bale compression followed by hydrogen phosphide fumigation (60 g/28.3 m3) for 3 d in rye straw loaded in freight containers in large-scale tests. Copper plate corrosion values indicating the severity of exposure to hydrogen phosphide were 13 and 12, and mean temperatures of five locations in the freight container were 25 and 26 degrees C in large-scale tests with the larvae and adults, respectively. Hydrogen phosphide concentrations were > or = 400 ppm throughout the 3-d fumigation for larvae and during the first day of fumigation for adults. We propose that cereal leaf beetle can be controlled by a single treatment of bale compression followed by a 1-d storage period or by a fumigation in which 400 ppm hydrogen phosphide is maintained for 1 d at 21 degrees C or above. We confirmed that a multiple quarantine treatment of bale compression) followed by a 3-d fumigation will control cereal leafbeetle in exported rye straw.     

Biotypes of Hessian fly (Dipt., Cecidomyiidae) in Morocco

Hessian fly, Mayetiola destructor (Say), is the most important insect pest of wheat in Morocco, where host plant resistance has been used successfully for control. Our objective was to determine the frequency of Hessian fly virulence on H5, H13 and H22 resistance genes. Five Hessian fly populations from the principal cereal‐growing regions in Morocco were studied. The variability in percentage of susceptible plants across Hessian fly populations was highly significant (P < 0.01), indicating differences in virulence frequencies. Plants with the H13 gene had the lowest percentage of susceptible plants, 1.77 and 1.51%, when infested with Hessian flies from Fes and Marchouch, respectively. A low level of virulence to H22 was detected in Fes, Abda and Marchouch populations, 1.87, 1.54 and 1.99% susceptible plants, respectively. The level of virulence to H5 was low in all the five populations. The Beni Mellal population gave the highest percentage of susceptible plants carrying H13 and H22 genes, 6.43 and 7.28%, respectively. The size of live larvae on susceptible plants of the three cultivars carrying H5, H13 and H22 was similar to that of the susceptible check, indicating that a true virulence (biotype) is developing in Hessian fly populations in Morocco. Thus, continuous monitoring of the development of Hessian fly biotypes is essential for optimal deployment of resistance genes.     

A physically anchored genetic map and linkage to avirulence reveals recombination suppression over the proximal region of Hessian fly chromosome A2

Resistance in wheat (Triticum aestivum) to the Hessian fly (Mayetiola destructor), a major insect pest of wheat, is based on a gene-for-gene interaction. Close linkage (3 +/- 2 cM) was discovered between Hessian fly avirulence genes vH3 and vH5. Bulked segregant analysis revealed two DNA markers (28-178 and 23-201) within 10 cM of these loci and only 3 +/- 2 cM apart. However, 28-178 was located in the middle of the short arm of Hessian fly chromosome A2 whereas 23-201 was located in the middle of the long arm of chromosome A2, suggesting the presence of severe recombination suppression over its proximal region. To further test that possibility, an AFLP-based genetic map of the Hessian fly genome was constructed. Fluorescence in situ hybridization of 20 markers on the genetic map to the polytene chromosomes of the Hessian fly indicated good correspondence between the linkage groups and the four Hessian fly chromosomes. The physically anchored genetic map is the first of any gall midge species. The proximal region of mitotic chromosome A2 makes up 30% of its length but corresponded to <3% of the chromosome A2 genetic map.     

Impact of tillage practices on Hessian fly-susceptible and resistant spring wheat cultivars

Hessian fly, Mayetiola destructor (Say), is a residue-borne pest of spring wheat that can become important in reduced tillage production systems. The relative abundance of Hessian fly was examined on spring wheat cultivars grown under conventional tillage (CT) and no-tillage (NT) practices in northern Idaho from 2000 to 2002. Six cultivars were tested: Hessian fly-susceptible 'Penawawa' and'Westbred 936' and -resistant (H3 gene) 'Wawawai', 'Jefferson', 'Hank', and 'Westbred 926.' Hessian fly egg densities were not significantly different among treatments, indicating ovipositing females showed no preference for tillage treatment or cultivar. Mean number of Hessian fly puparia per plant was significantly greater in CT plots during the last sampling in 2000; however, in 2001, NT plots had significantly more puparia than CT plots. Tillage had no significant effect on mean Hessian fly per plant in 2002. Significantly more puparia were observed on susceptible compared with resistant cultivars in 2000 and 2002. In 2001, susceptible Penawawa had significantly more puparia than resistant cultivars, whereas puparial densities on susceptible Westbred 936 were higher than on resistant cultivars other than Wawawai. Yield and 100-seed weight were not affected by tillage treatment. Significant variation in yield among cultivars was observed only in 2000, when fly-resistant Hank yielded the highest. Hank had the highest 100-seed weight in 2000 and 2001, whereas Penawawa and Jefferson had the lowest 100-seed weights each year. Reduced tillage had no consistent effect on spring wheat yield or abundance of Hessian fly under the conditions of our trials, which evaluated small plots.     

Virulence of Hessian fly (Diptera: Cecidomyiidae) in the Fertile Crescent

The Hessian fly, Mayetiola destructor (Say), is an important insect pest of wheat (Triticum spp.) in North Africa, North America, southern Europe and northern Kazakhstan. Both wheat and this pest are believed to have originated from West Asia in the Fertile Crescent. The virulence of a Hessian fly population from Syria against a set of cultivars carrying different resistance genes, in addition to other effective sources with unknown genes, was determined in the field and laboratory at the International Center for Agricultural Research in the Dry Areas (ICARDA) during the 2005/2006 cropping season. Only two resistance genes (H25 and H26) were effective against the Syrian Hessian fly population, making it the most virulent worldwide. This high virulence supports the hypothesis that Hessian fly coevolved with wheat in the Fertile Crescent of West Asia. The ICARDA screening programme is using this Hessian fly population to identify new resistance genes to this pest.     

Effects of methyl bromide concentration, fumigation time, and fumigation temperature on Mediterranean and oriental fruit fly (Diptera: Tephritidae) egg and larval survival

The effects of methyl bromide (MB) concentration (16, 32,48, or 64 g/m3), fumigation temperature (15, 20, 25, or 30 degrees C), and fumigation time interactions on the survival of Mediterranean fruit fly, Ceratitis capitata (Wiedemann), and oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), eggs and first and third instars were recorded. Increasing the fumigation temperature from 15 to 20 degrees C or from 20 to 25 degrees C resulted in a significant reduction in fumigation time required for equivalent egg and larval mortalities at all studied MB concentrations; no further reductions in fumigation time resulted from increasing the temperature from 25 to 30 degrees C. Conversely, increasing temperature and time allowed for a reduction in MB concentration to obtain equivalent mortality. Thus, the optimum fumigation temperature for Mediterranean and oriental fruit fly eggs and larvae was 25 degrees C. Reducing MB concentrations required for phytosanitary fumigations would save time and expense, and reduce the amount of MB released into the atmosphere during aeration. Mediterranean fruit fly was as or more tolerant to MB than oriental fruit fly in MB tolerance for eggs and first instars. The egg stage was generally more tolerant to MB regardless of concentration. However, Mediterranean fruit fly eggs showed similar tolerance to first instars at 25 degrees C for the three highest concentrations and to third instars at 25 and 30 degrees C for the highest concentration, with no significant difference between them. Therefore, eggs alone can be used to obtain MB fumigation efficacy and quarantine security data at fumigation temperatures between 15 and 30 degrees C for Mediterranean and oriental fruit fly.     

The evolution of Hessian fly from the Old World to the New World: Evidence from molecular markers

Eighteen polymorphic microsatellite loci and 11 single‐nucleotide polymorphisms were genotyped in 1 095 individual Hessian fly specimens representing 23 populations from North America, southern Europe, and southwest Asia. The genotypes were used to assess genetic diversity and interrelationship of Hessian fly populations. While phylogenetic analysis indicates that the American populations most similar to Eurasian populations come from the east coast of the United States, genetic distance is least between (Alabama and California) and (Kazakhstan and Spain). Allelic diversity and frequency vary across North America, but they are not correlated with distance from the historically documented point of introduction in New York City or with temperature or precipitation. Instead, the greatest allelic diversity mostly occurs in areas with Mediterranean climates. The microsatellite data indicate a general deficiency for heterozygotes in Hessian fly. The North American population structure is consistent with multiple introductions, isolation by distance, and human‐abetted dispersal by bulk transport of puparia in infested straw or on harvesting equipment.     

Hot-water immersion quarantine treatment against Mediterranean fruit fly and Oriental fruit fly (Diptera: Tephritidae) eggs and larvae in litchi and longan fruit exported from Hawaii

Immersion of litchi fruit in 49 degrees C water for 20 min followed by hydrocooling in ambient (24 +/- 4 degrees C) temperature water for 20 min was tested as a quarantine treatment against potential infestations of Mediterranean fruit fly, Ceratitis capitata (Wiedemann); and oriental fruit fly, Bactrocera dorsalis Hendel, eggs or larvae in Hawaiian litchi, Litchi chinensis Sonnerat. The 49 degrees C hot-water immersion of litchi provided probit 9 (99.9968% mortality with >95% confidence) quarantine security against eggs and first instars. There were no survivors from 15,000 each feeding and nonfeeding Mediterranean fruit fly or oriental fruit fly third instars immersed in a computer-controlled water bath that simulated the litchi seed-surface temperature profile during the 49 degrees C hot-water immersion treatment. Litchi served as the model for longan, Dimocarpus longan Lour., a closely related fruit that is smaller and also has commercial potential for Hawaii. Modified fruit infestation and holding techniques used to obtain adequate estimated treated populations from poor host fruit, such as litchi and longan, are described. Data from these experiments were used to obtain approval of a hot-water immersion quarantine treatment against fruit flies for litchi and longan exported from Hawaii to the U.S. mainland.     

Evaluation of the efficacy of the methyl bromide fumigation schedule against Mexican fruit fly (Diptera: Tephritidae) in citrus fruit

Methyl bromide fumigation is widely used as a phytosanitary treatment. Mexican fruit fly, Anastrepha ludens (Loew) (Diptera: Tephritidae), is a quarantine pest of several fruit, including citrus (Citrus spp.), exported from Texas, Mexico, and Central America. Recently, live larvae have been found with supposedly correctly fumigated citrus fruit. This research investigates the efficacy of the previously approved U.S. Department of Agriculture-Animal and Plant Health Inspection Service treatment schedule: 40 g/m3 methyl bromide at 21-29.4 degrees C for 2 h. Tolerance ofA. ludens to methyl bromide in descending order when fumigated in grapefruit (Citrus X paradisi Macfad.) is third instar > second instar > first instar > egg. Two infestation techniques were compared: insertion into fruit of third instars reared in diet and oviposition by adult A. ludens into fruit and development to the third instar. Inserted larvae were statistically more likely to survive fumigation than oviposited larvae. When fruit were held at ambient temperature, 0.23 +/- 0.12% of larvae were still observed to be moving 4 d postfumigation. Temperatures between 21.9 and 27.2 degrees C were positively related to efficacy measured as larvae moving 24 h after fumigation, pupariation, and adult emergence. Coating grapefruit with Pearl Lustr 2-3 h before fumigation did not significantly affect the proportion of third instars moving 24 h after fumigation, pupariating, or emerging as adults. In conclusion, fumigation with 40 g/m3 methyl bromide for 2 h at fruit temperatures >26.7 degrees C is not found to be inefficacious for A. ludens. Although a few larvae may be found moving >24 h postfumigation, they do not pupariate.     

Susceptibility of low-chill blueberry cultivars to Mediterranean fruit fly, oriental fruit fly, and melon fly (Diptera: Tephritidae)

No-choice tests were conducted to determine whether fruit of southern highbush blueberry, Vaccinium corymbosum L., hybrids are hosts for three invasive tephritid fruit flies in Hawaii. Fruit of various blueberry cultivars was exposed to gravid female flies of Bactrocera dorsalis Hendel (oriental fruit fly), Ceratitis capitata (Wiedemann) (Mediterranean fruit fly), or Bactrocera cucurbitae Coquillet (melon fly) in screen cages outdoors for 6 h and then held on sand in the laboratory for 2 wk for pupal development and adult emergence. Each of the 15 blueberry cultivars tested were infested by oriental fruit fly and Mediterranean fruit fly, confirming that these fruit flies will oviposit on blueberry fruit and that blueberry is a suitable host for fly development. However, there was significant cultivar variation in susceptibility to fruit fly infestation. For oriental fruit fly, 'Sapphire' fruit produced an average of 1.42 puparia per g, twice as high as that of the next most susceptible cultivar 'Emerald' (0.70 puparia per g). 'Legacy', 'Biloxi', and 'Spring High' were least susceptible to infestation, producing only 0.20-0.25 oriental fruit fly puparia per g of fruit. For Mediterranean fruit fly, 'Blue Crisp' produced 0.50 puparia per g of fruit, whereas 'Sharpblue' produced only 0.03 puparia per g of fruit. Blueberry was a marginal host for melon fly. This information will aid in development of pest management recommendations for blueberry cultivars as planting of low-chill cultivars expands to areas with subtropical and tropical fruit flies. Planting of fruit fly resistant cultivars may result in lower infestation levels and less crop loss.     

Large-scale, on-site confirmatory, and varietal testing of a methyl bromide quarantine treatment to control codling moth (Lepidoptera: Tortricidae) in nectarines exported to Japan

In total, 30,491 codling moth, Cydia pomonella (L.), 1-d-old eggs on May Grand nectarines in two large-scale tests, and 17,410 eggs on Royal Giant nectarines in four on-site confirmatory tests were controlled with 100% mortality after fumigation with a methyl bromide quarantine treatment (48 g3 for 2 h at > or = 21 degrees C and 50% volume chamber load) on fruit in shipping containers for export to Japan. Ranges (mean +/- SEM) were for percentage sorption 34.7 +/- 6.2 to 46.5 +/- 2.5, and for concentration multiplied by time products 54.3 +/- 0.9 to 74.5 +/- 0.6 g.h/m3 in all tests. In large-scale tests with May Grand nectarines, inorganic bromide residues 48 h after fumigation ranged from 6.8 +/- 0.7 to 6.9 +/- 0.5 ppm, which were below the U.S. Environmental Protection Agency tolerance of 20 ppm; and, organic bromide residues were < 0.01 ppm after 1 d and < 0.001 ppm after 3 d in storage at 0-1 degree C. After completion of larger-scale and on-site confirmatory test requirements, fumigation of 10 nectarine cultivars in shipping containers for export to Japan was approved in 1995. Comparison of LD50s developed for methyl bromide on 1-d-old codling moth eggs on May Grand and Summer Grand nectarines in 1997 versus those developed for nine cultivars in the previous 11 yr showed no significant differences in codling moth response among the cultivars.     

Virulence in Hessian fly (Diptera: Cecidomyiidae) field collections from the southeastern United States to 21 resistance genes in wheat

Genetic resistance in wheat, Triticum aestivum L., is the most efficacious method for control of Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). However, because of the appearance of new genotypes (biotypes) in response to deployment of resistance, field collections of Hessian fly need to be evaluated on a regular basis to provide breeders and producers information on the efficacy of resistance (R) genes with respect to the genotype composition of Hessian fly in regional areas. We report here on the efficacy of 21 R genes in wheat to field collections of Hessian fly from the southeastern United States. Results documented that of the 21 R genes evaluated only five would provide effective protection of wheat from Hessian fly in the southeastern United States. These genes were H12, H18, H24, H25, and H26. Although not all of the 33 identified R genes were evaluated in the current study, these results indicate that identified genetic resistance to protect wheat from Hessian attack in the southeastern United States is a limited resource. Historically, R genes for Hessian fly resistance in wheat have been deployed as single gene releases. Although this strategy has been successful in the past, we recommend that in the future deployment of combinations of highly effective previously undeployed genes, such as H24 and H26, be considered. Our study also highlights the need to identify new and effective sources of resistance in wheat to Hessian fly if genetic resistance is to continue as a viable option for protection of wheat in the southeastern United States.     

Changes in phytohormones and fatty acids in wheat and rice seedlings in response to Hessian fly (Diptera: Cecidomyiidae) infestation

Phytohormones and fatty acids (FAs) play important roles in plant resistance to insects and pathogens. In this study, we investigated the similarities and differences in the accumulations of phytohormones and FAs in the resistant wheat (Triticum aestivum L.) 'Molly' and the nonhost rice (Oryza sativa L.) 'Niponbare' in responses to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), larval attacks. Using chemical ionization-gas-chromatography/mass spectrometry, we analyzed the concentrations of 13 phytohomones and FAs at the attack site of wheat and rice plants at 1, 6, 24, or 48 h after the initial attack. Hessian fly attack resulted in increases of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), palmitic acid (FA16:0), but a decrease of abscisic acid in both wheat and rice plants. In addition, the accumulation of jasmonic acid (JA) increased, whereas the accumulation of cinnamic acid (CA) decreased in wheat plants, but no changes were observed in the accumulation of JA, and the accumulation of CA increased in rice plants after Hessian fly attack. However, the accumulations of benzoic acid, strearic acid (FA18:0), and oleic acid (FA18:1) increased in rice plants, but no changes were observed in wheat plants after Hessian fly attack. Hessian fly-induced changes were more rapid in wheat plants in comparison with those in rice plants. Our study suggests that SA and OPDA may be involved in resistance of wheat and rice plants to Hessian fly and that the R gene-mediated resistance responses are more rapid than nonhost resistance responses.     

Insecticidal activity of wheat Hessian fly responsive proteins HFR-1 and HFR-3 towards a non-target wheat pest, cereal aphid (Sitobion avenae F.)

The interaction between Hessian fly (Mayetiola destructor) and wheat (Triticum aestivum) involves a gene-for-gene resistance mechanism. The incompatible interaction leading to resistance involves up-regulation of several Hfr (Hessian fly responsive) genes encoding proteins with potential insecticidal activity. The encoded proteins HFR-1, HFR-2 and HFR-3 all possess lectin-like domains. HFR-1 and HFR-3 were produced as recombinant proteins using Escherichia coli and Pichia pastoris, respectively as expression hosts. Purified recombinant proteins were assayed for insecticidal effects towards cereal aphid (Sitobion avenae), an insect to which wheat shows only tolerance. Both HFR-1 and HFR-3 were found to be insecticidal towards S. avenae when fed in artificial diet. Although HFR-3 has sequence similarity and similar chitin-binding activity to wheat germ agglutinin (WGA), the latter protein was almost non-toxic to S. avenae. HFR-3 binds strongly to aphid midguts after ingestion, whereas WGA binds but does not persist over a feed-chase period. Quantitative PCR showed that Hfr-3 mRNA does not increase in level after cereal aphid infestation. The results suggest that the lack of effective resistance to cereal aphid in wheat is not due to an absence of genes encoding suitable insecticidal proteins, but results from a failure to up-regulate gene expression in response to aphid attack.     

Combined heat and controlled atmosphere quarantine treatments for control of western cherry fruit fly in sweet cherries

Nonchemical quarantine treatments, using a combination of short duration high temperatures under low oxygen, elevated carbon dioxide atmospheric environment were developed to control western cherry fruit fly, Rhagoletis indifferens Curran, in sweet cherries, Prunus avium (L.). The two treatments developed use a chamber temperature of 45 degrees C for 45 min and a chamber temperature of 47 degreesd C for 25 min, both under a 1% oxygen, 15% carbon dioxide, -2 degrees C dew point environment. Both these treatments have been shown to provide control of all life stages of western cherry fruit fly while preserving commodity market quality. There was no definitive egg or larval stage, which was demonstrated to be the most tolerant to either controlled atmosphere temperature treatment system treatment. Efficacy tests for both treatments resulted in 100% mortality of >5000 western cherry fruit flies in each treatment. These treatments may provide, with further study, quarantine security in exported sweet cherries where western cherry fruit fly is a quarantine concern and fumigation with methyl bromide is not desired.     

Economic impact of Hessian fly (Diptera: Cecidomyiidae) on spring wheat in Oregon and additive yield losses with Fusarium crown rot and lesion nematode

Damage caused by Hessian fly, Mayetiola destructor (Say), was quantified in spring wheat, Triticum aestivum L., trials near Pendleton and Moro, OR, during 2001 and 2002. Five field experiments were established to examine genetic resistance to Fusarium crown rot, Fusarium pseudograminearum (O'Donnell & Aoki), and economic damage by lesion nematode, Pratylenchus neglectus (Rensch, 1924) (Filipjev Schuurmanns & Stekhoven, 1941) and Pratylenchus thornei (Sher & Allen, 1941). Hessian fly became the dominant factor affecting grain yield in four experiments. Genotypes carrying the H3-resistance gene had grain yields 66 and 68% higher than susceptible genotypes in cultivar trials during 2001 and 2002, respectively. Yield reductions were detected when Hessian fly infestation rates exceeded 50% plants during 2001 and 15% plants (8% tillers) during 2002. In two trials during 2001, in-furrow application of aldicarb (Temik) at planting improved yields of four Hessian fly-susceptible cultivars by 72 and 144% (up to 1,959 kg/ha) and yields of one Hessian fly-resistant cultivar by 2 and 3%. Resistant cultivars and aldicarb improved grain quality as much as two market grades during 2001. The value of increased grain production with Hessian fly-resistant cultivars in four field experiments ranged from dollar 112 to dollar 252/ha, excluding price incentives for improved market quality. Yield reduction due to combined damage from Hessian fly and either Fusarium crown rot or lesion nematode was additive. This report seems to be the first quantitative yield loss estimate for Hessian fly in spring wheat in the semiarid environment of the inland Pacific Northwest.     

Ultrastructural changes in the midguts of Hessian fly larvae feeding on resistant wheat

The focus of the present study was to compare ultrastructure in the midguts of larvae of the Hessian fly, Mayetiola destructor (Say), under different feeding regimens. Larvae were either fed on Hessian fly-resistant or -susceptible wheat, and each group was compared to starved larvae. Within 3 h of larval Hessian fly feeding on resistant wheat, midgut microvilli were disrupted, and after 6 h, microvilli were absent. The disruption in microvilli in larvae feeding on resistant wheat were similar to those reported for midgut microvilli of European corn borer, Ostrinia nubilasis (Hubner), larvae fed a diet containing wheat germ agglutinin. Results from the present ultrastructural study, coupled with previous studies documenting expression of genes encoding lectin and lectin-like proteins is rapidly up-regulated in resistant wheat to larval Hessian fly, are indications that the midgut is a target of plant resistance compounds. In addition, the midgut of the larval Hessian fly is apparently unique among other dipterans in that no peritrophic membrane was observed. Ultrastructural changes in the midgut are discussed from the prospective of their potential affects on the gut physiology of Hessian fly larvae and the mechanism of antibiosis in the resistance of wheat to Hessian fly attack.