Identifying insecticide-resistant greenbugs (Homoptera: Aphididae) with diagnostic assay tests

Laboratory bioassays were used to develop a diagnostic assay test for identifying greenburg, Schizaphis graminum (Rondani), populations that are insecticide-resistant. Petri dish assays with chlorpyrifos showed greenbug mortality should be monitored after 2 h of exposure. One-hour exposure did not kill a high percentage of susceptible greenbugs, and a 3-h exposure killed too many resistant greenbugs. Ethanol and methanol were both good solvents for mixing with chlorpyrifos in the petri dish assay. From the laboratory bioassays, four diagnostic concentrations of chlorpyrifos (3, 10, 30, and 100 ppm) were evaluated in the field by Texas A&M University agricultural research and extension entomologists across the Texas High Plains. Results from the diagnostic assay tests were compared with gel-electrophoresis resistance tests to validate resistance detection. The diagnostic assay tests gave the same greenbug resistance identification as the gel-electrophoresis analysis in 21 of 22 field bioassays in 1994 and 35 of 39 field bioassays in 1995. Diagnostic concentrations of 30 and 100 ppm chlorpyrifos killed > or = 85 and > or = 90%, respectively, of greenbugs identified by gel-electrophoresis as susceptible and < 40% and < 55%, respectively, of resistant greenbugs. The diagnostic assay technique is a quick, reliable, and inexpensive method for detecting insecticide resistance in greenbug populations.     

Plant resistance components of two greenbug (Homoptera: Aphididae) resistant wheats

Several biotypes of the greenbug, Schizaphis graminum (Rondani), attack winter wheat, Triticum aestivum L., on the Southern Plains every year. Two wheat germplasm sources of resistance ('Largo' and 'GRS 1201') have been developed that provide protection against the three predominant greenbug biotypes (E, I, and K). Each source has agronomic and end-use quality advantages and disadvantages for the breeder to consider in choosing a greenbug-resistant breeding line. We compared these two germplasms to determine their levels of resistance against biotype E. Components of resistance (i.e., antibiosis, antixenosis, and tolerance) were measured on seedlings of GRS 1201, Largo, and 'TAM W-101' (a susceptible control). Several aphid and plant measurements (e.g., total number of aphids produced per plant, aphid selection preferences, and plant damage ratings) were recorded for each plant entry. Select data recorded for each resistance component were normalized and combined to derive a plant resistance index for each wheat entry. Results indicated that GRS 1201 had a higher level of combined resistance components than did Largo, followed by TAM W-101, the susceptible control. These data provide additional information for the breeder to consider in selecting a greenbug-resistant breeding line.     

Effect of greenbugs (Homoptera: Aphididae) on yield loss of winter wheat

The effect of greenbug, Schizaphis graminum (Rondani), feeding on the yield of four winter wheat cultivars commonly grown in Oklahoma was studied. Cultivars tested were 'Karl', a recent derivative 'Karl-92', and '2163', all greenbug-susceptible cultivars; and 'TAM-110', a cultivar with resistance to biotype E greenbugs. The objectives were to determine the effect of different greenbug densities during fall and spring on yield of winter wheat, and to develop mathematical models to quantify the effect of greenbugs on yield loss. The intensity of greenbug infestations achieved in plots by artificial infestation varied among years and growing seasons within a year, but was generally sufficient to cause a reduction in yield. Among yield components, the number of heads per square meter and the number of seeds per head were frequently negatively correlated with the accumulated number of greenbug-days per tiller. Seed weight was rarely affected by greenbug infestation. A regression model estimated yield loss for greenbug-susceptible cultivars at 0.51 kg/ha loss of yield per greenbug-day in years with near normal precipitation, and a loss of 1.17 kg/ha under severe drought conditions. The susceptible winter wheat cultivars exhibited similar yield loss in relation to the intensity of greenbug infestation, as indicated by a common slope parameter in the regression model. Results suggest that the model is robust for predicting yield loss for susceptible cultivars.     

Life history study of multiple clones of insecticide resistant and susceptible greenbug Schizaphis graminum (Homoptera: Aphididae)

Comparative differences and similarities in prereproductive time (d), progeny production in a time equal to d (Md), and intrinsic rate of increase (rm) were established for one susceptible (S) and three resistant (R) strains of the greenbug, Schizaphis graminum (Rondani), reared on sorghum hybrids Dekalb G550E and Cargill 607E. The R strains showed three patterns of elevated esterase activity. Four R1 clones, four R2 clones, one R3 clone, and four S clones were evaluated. The interaction of sorghum hybrid and greenbug strain did not significantly influence any of the parameters measured. However, R1 greenbugs exhibited a significantly longer prereproductive period than the other strains. In addition, the R1 strain had a significantly slower intrinsic rate of increase than the R2 or S greenbug strains, but did not differ significantly from the R3 strain. These results suggest that R1 greenbugs may be less fit than the other strains studied.     

Responses of Nasonovia ribisnigri (Homoptera: Aphididae) to susceptible and resistant lettuce

Nymphs and alates of aphid Nasonovia ribisnigri (Mosley) (Homoptera: Aphididae) were tested on 10 lettuce cultivars with N. ribisnigri resistance gene Nr and 18 cultivars without the resistance gene in various bioassays. Bioassays used whole plants, leaf discs, or leaf cages to determine susceptibility of commercial lettuce cultivars to N. ribisnigri infestation and to evaluate screening methods for breeding lettuce resistance to N. ribisnigri. Resistant and susceptible plants were separated in 3 d when using whole plant bioassays. Long-term (> or =7 d) no-choice tests using leaf cages or whole plants resulted in no survival of N. ribisnigri on resistant plants, indicating great promise of the Nr gene for management of N. ribisnigri. Effective screening was achieved in both no-choice tests where resistant or susceptible intact plants were tested separately in groups or individually and in choice tests where susceptible and resistant plants were intermixed. Leaf discs bioassays were not suitable for resistance screening. All lettuce cultivars without the resistance gene were suitable hosts for N. ribisnigri, indicating the great importance of this pest to lettuce production and the urgency in developing resistant lettuce cultivars to manage N. ribisnigri.     

Mechanisms of insecticide resistance in the aphid Nasonovia ribisnigri (Mosley) (Homoptera: Aphididae) from France.

Nasonovia ribisnigri, a main pest of salad crops, has developed resistance to various insecticides in southern France, including the carbamate pirimicarb and the cyclodiene endosulfan, two insecticides widely used to control this aphid. Here we have investigated the mechanisms of resistance to these two insecticides by studying cross-resistance, synergism, activity of detoxifying enzymes, and possible modifications of the target proteins. Resistance to pirimicarb was shown to be mainly due to a decreased sensitivity of the target acetylcholinesterase; this modification conferred also, resistance to propoxur but not to methomyl and the two tested organophosphates (acephate and paraoxon). Endosulfan resistance was associated with a moderate level of resistance to dieldrin, and resistance to both insecticides was due, in part, to increased detoxification by glutathione S-transferases (GST). The endosulfan resistant strain displayed the same amino acid at position 302 of the Rdl gene (GABA receptor) as susceptible aphids (e.g. Ala), indicating that the Ala to Ser (or to Gly) mutation observed among dieldrin resistant strains of other insect species was not present.     

Molecular analysis of sorghum resistance to the greenbug (Homoptera: Aphididae)

Chromosomal regions of sorghum, Sorghum bicolor (L.) Moench, conferring resistance to greenbug, Schizaphis graminum (Rondani), biotypes C, E, I, and K from four resistance sources were evaluated by restriction fragment-length polymorphism (RFLP) analysis. At least nine loci, dispersed on eight linkage groups, were implicated in affecting sorghum resistance to greenbug. The nine loci were named according to the genus of the host plant (Sorghum) and greenbug (Schizaphis graminum). Most resistance loci were additive or incompletely dominant. Several digenic interactions were identified, and in each case, these nonadditive interactions accounted for a greater portion of the resistance phenotype than did independently acting loci. One locus in three of the four sorghum crosses appeared responsible for a large portion of resistance to greenbug biotypes C and E. None of the loci identified were effective against all biotypes studied. Correspondingly, the RFLP results indicated resistance from disparate sorghums may be a consequence of allelic variation at particular loci. To prove this, it will be necessary to fine map and clone genes for resistance to greenbug from various sorghum sources.     

Amplification and methylation of an esterase gene associated with insecticide-resistance in greenbugs, Schizaphis graminum (Rondani) (Homoptera: Aphididae)

The greenbug aphid, Schizaphis graminum (Rondani) has developed resistance to organophosphorus insecticides by the over-production of esterases that have been classified as Type I and Type II. The first twenty N-terminal amino acids of the Type I esterase were determined and used to design an oligonucleotide, which in conjunction with an active site primer derived from conserved sequences of other insect esterases and two internal primers specific for esterases from another aphid species resulted in a 0.85 kb genomic DNA fragment from resistant greenbugs. This was extended by 5′ RACE which provided approximately 1.2 kb of the 5′ end of the esterase gene. The 5′ DNA sequence corresponded to 19 of the 20 known amino acids of the Type I esterase, with the last needing only a one base change (probably resulting from a PCR artifact). Furthermore, the sequence showed very close similarity to the amplified E4/FE4 esterase genes of Myzus persicae (Sulzer). A comparison of sequences suggested that the S. graminum gene has introns in the same positions as the first two introns of E4/FE4, with the second intron being considerably larger in S. graminum. Probing of Southern blots with the 0.85 kb esterase fragment showed that the gene encoding the Type I esterase is amplified 4- to 8-fold in resistant S. graminum and that the amplified sequences contain 5-methylcytosine at MspI/HpaII sites, again in agreement with previous findings for M. persicae genes.     

Responses of Schizaphis graminum (Homoptera: Aphididae) to leaf excision in resistant and susceptible sorghum

Greenbugs, Schizaphis graminum (Rondani), were reared on intact and excised leaves of varieties of sorghum which differed in their suitability as hosts for this aphid. Aphids grew poorly on intact leaves of three resistant varieties, but grew well on excised leaves of the same varieties. Leaf excision did not affect aphid growth on three susceptible varieties. By electronically monitoring the feeding behaviour of aphids on two resistant and one susceptible variety, significant differences were found in many parameters between aphids assayed on excised vs. intact leaves of only the resistant varieties. Aphids on excised leaves of the resistant varieties, and on excised or intact leaves of the susceptible variety, made fewer probes to the phloem, spending more time ingesting from phloem during each probe, compared to aphids on intact resistant plants. There was a higher level of free amino acids in excised leaves of all varieties, but aphid growth and feeding behaviour improved as a result of excision only on resistant varieties. This observation, coupled with the fact that intact plants of all varieties have similar amino acid levels, indicates that these nutrients are not of primary importance in sorghum suitability to the greenbug. Other explanations for the aphids' responses to excised leaves are discussed.     

Categories of resistance at four growth stages in three wheats resistant to the Russian wheat aphid (Homoptera: Aphididae)

Laboratory experiments were conducted to determine categories of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), in three wheats, Triticum aestivum L, (PI 372129, PI 243781, and PI 222668) at Zadoks growth stages 10, 20, 30, and 40. 'TAM 107' was used as the susceptible standard. Antixenosis was observed in PI 222668 and PI 372129. Antibiosis was expressed as reduced nymphipositional period, daily nymph production, and fecundity at the jointing (Zadoks 30) and boot (Zadoks 40) stages in PI 243781 and at tillering (Zadoks 20) in TAM 107. Antibiosis, expressed as reduced intrinsic rate of increase, was observed in PI 222668 at tillering (Zadoks 20). Tolerance to chlorosis and leaf rolling was expressed in the three resistant wheats at all growth stages tested. Tiller production, floret formation, spike length and wet weight were affected by Russian wheat aphid feeding after Zadoks 10. Reduction in spike length did not occur in PI 372129 and PI 243781.     

Response of resistant and susceptible barley to infestations of five Diuraphis noxia (Homoptera: Aphididae) biotypes

Since 2003, four new biotypes of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), RWA2-RWA5, have been discovered that have the ability to damage most of the wheat germplasm resistant to the original Russian wheat aphid population (RWA1). Barley germplasm lines with resistance to RWA1 have not yet been evaluated against the newest biotypes. Our study compared how biotypes RWA1-RWA5 affected the growth and leaf damage of RWA1-resistant germplasm (STARS 9301B, STARS 9577B), moderately resistant germplasm (MR-015), and susceptible varieties (Schuyler, Harrington, and Morex) under greenhouse conditions. Russian wheat aphid population levels also were determined 14 d after plant infestation. STARS 9301B exhibited strong resistance by showing only small differences in leaf damage and growth parameters from the feeding by the biotypes. STARS 9577B showed greater differences in damage by the Russian wheat aphid biotypes than STARS 9301B, yet, the ratings were still within the resistant category (e.g., chlorosis rating 2.3-4.9). Leaf chlorosis ratings for MR-015 ranged from 5.0 to 6.9 and fell within the moderately resistant to susceptible categories for all the biotypes. The greatest difference in leaf chlorosis occurred in Morex where RWA2 showed less virulence than the other biotypes. Feeding by the Russian wheat aphid biotypes produced only small differences in leaf rolling and plant growth within plant entries. Population levels of the Russian wheat aphid biotypes did not differ within barley entries (n = 610-971) at the termination of the study (14 d). From our research, we conclude that the new Russian wheat aphid biotypes pose no serious threat to the key sources of resistance in barley (STARS 9301B and 9577B).     

Categories of resistance at different growth stages in halt,a winter wheat resistant to the Russian wheat aphid (Homoptera: Aphididae)

This study was designed to categorize the resistance to the Russian wheat aphid, Diuraphis noxia (Mordvilko), resistant hard red winter wheat, Halt, as compared with susceptible wheat, TAM 107, at four different growth stages. Antixenosis was expressed in Halt at growth stage Zadoks 30. Antibiosis in Halt affected fecundity, number of aphids produced per reproductive day, maximum number of nymphs produced in one day, and intrinsic rate of increase. Fecundity was lower on Halt than TAM 107, and more nymphs were produced on both varieties at growth stage 20 than 10 and 40. Fewer nymphs were produced per reproductive day and on maximum production days by aphids reared on Halt than by those reared on TAM 107. The intrinsic rate of increase of Russian wheat aphids reared on Halt was lower than aphids reared on TAM 107. Differences in plant height and plant dry weight did not occur. Chlorosis ratings showed greater damage at the earlier stages in Halt and TAM 107 and significantly more damage in TAM 107 than Halt at growth stages 10, 20, and 30. Leaf rolling occurred on infested plants of TAM 107 at growth stages 10, 20, and 30, but not growth stage 40. Halt plants did not exhibit leaf rolling. The presence of a significant level of tolerance could make Halt compatible with other integrated pest management programs. However, care should be taken with cultivars containing evidence of antixenosis or antibiosis that could cause selective pressure on the Russian wheat aphid, potentially causing biotypes to be produced.     

Characterization of greenbug (Homoptera: Aphididae) resistance in synthetic hexaploid wheats

Twelve greenbug (Schizaphis graminum (Rondani)) biotype E-resistant synthetic hexaploid wheats synthesized by crossing Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal. were evaluated for the three known insect resistance categories, including antibiosis, anti-xenosis, and tolerance. Different methods were evaluated for calculating antibiosis and tolerance. Calculating intrinsic rate of population increase and measuring leaf chlorophyll content with a SPAD chlorophyll meter proved to be time- and labor-efficient for antibiosis and tolerance determination, respectively. The resistance in all synthetic hexaploids proved to be the result of a combination of antibiosis, antixenosis, and tolerance, which makes them valuable sources of greenbug resistance. To assist plant breeders in selecting the best germplasm for greenbug resistance, a plant resistance index was created that revealed differences among the synthetic hexaploid wheats.     

Efficacy of pyramiding greenbug (Homoptera: Aphididae) resistance genes in wheat

Durable resistance to greenbug, Schizaphis graminum (Rondani), in wheat is a goal of wheat improvement teams, and one that has been complicated by the regular occurrence of damaging biotypes. Simulation modeling studies suggest that pyramiding resistance genes, i.e., combining more than one resistance gene in a single cultivar or hybrid, may provide more durable resistance than sequential releases of single genes. We examined this theory by pyramiding resistance genes in wheat and testing a series of greenbug biotypes. Resistance genes Gb2, Gb3, and Gb6, and pyramided genes Gb2/Gb3, Gb2/Gb6, and Gb3/Gb6 were tested for effectiveness against biotypes E, F, G, H, and I. By comparing reactions of plants with pyramided genes to those with single resistance genes, we found that pyramiding provided no additional protection over that conferred by the single resistance genes. Based on the results of this test, we concluded that the sequential release of single resistance genes, combined with careful monitoring of greenbug population biotypes, is the most effective gene deployment strategy for greenbug resistance in wheat.     

Comparison of bioassay techniques for determining baseline susceptibilities to imidacloprid for green apple aphid (Homoptera: Aphididae)

The susceptibility of a clone of green apple aphid, Aphis pomi (De Geer), to the neonicotinyl insecticide imidacloprid was determined by direct and indirect bioassay techniques. Aphid numbers were assessed on potted apple seedlings treated with various concentrations of imidacloprid, adults were dipped in test solutions as per the Food and Agriculture Organization protocol, or nymphs and adults were reared on treated apple leaf disks. Effective concentrations required to kill half of the test population (EC50) varied depending on the bioassay technique, ranging from as low as 0.064 ppm for first instars reared for 3 d on treated leaf disks to 1.79 ppm for adult apterae dipped in solutions of imidacloprid and held for 24 h on clean leaf disks. When imidacloprid was directly applied to aphids, mortality continued to increase over 3 d, but the difference was not statistically significant between day 2 (1.36 ppm) and day 3 (1.19 ppm). Toxicity of neonicotinyls to aphids is expressed rather slowly and primarily after oral ingestion. The effect of imidacloprid on reproduction of green apple aphid was also assessed for adult apterae reared on treated leaf disks. Contrary to previous reports, our results demonstrated that imidacloprid does not have a direct negative effect on the reproductive physiology of this species. Negative effects can mostly be attributed to the antifeedant activity of this compound and the protracted time to death. The results of this study contribute to a better understanding of the most suitable techniques for assessing aphid mortality after exposure to these new insecticides and provides a baseline susceptibility to imidacloprid for green apple aphid.     

Amplification and methylation of an esterase gene associated with insecticide-resistance in greenbugs, Schizaphis graminum (Rondani) (Homoptera: Aphididae)

The greenbug aphid, Schizaphis graminum (Rondani) has developed resistance to organophosphorus insecticides by the over-production of esterases that have been classified as Type I and Type II. The first twenty N-terminal amino acids of the Type I esterase were determined and used to design an oligonucleotide, which in conjunction with an active site primer derived from conserved sequences of other insect esterases and two internal primers specific for esterases from another aphid species resulted in a 0.85 kb genomic DNA fragment from resistant greenbugs. This was extended by 5′ RACE which provided approximately 1.2 kb of the 5′ end of the esterase gene. The 5′ DNA sequence corresponded to 19 of the 20 known amino acids of the Type I esterase, with the last needing only a one base change (probably resulting from a PCR artifact). Furthermore, the sequence showed very close similarity to the amplified E4/FE4 esterase genes of Myzus persicae (Sulzer). A comparison of sequences suggested that the S. graminum gene has introns in the same positions as the first two introns of E4/FE4, with the second intron being considerably larger in S. graminum. Probing of Southern blots with the 0.85 kb esterase fragment showed that the gene encoding the Type I esterase is amplified 4- to 8-fold in resistant S. graminum and that the amplified sequences contain 5-methylcytosine at MspI/HpaII sites, again in agreement with previous findings for M. persicae genes.     

Categories of resistance to greenbug (Homoptera: Aphididae) biotype I in Aegilops tauschii germplasm

Categories of resistance to greenbug, Schizaphisgraminum (Rondani), biotype I, were determined in goatgrass, Aegilops tauschii (Coss.) Schmal., accession 1675 (resistant donor parent), 'Wichita' wheat, Triticum aestivum L., (susceptible parent), and an Ae. tauschii-derived resistant line, '97-85-3'. Antibiosis was assessed using the intrinsic rate of increase (rm) of greenbugs confined to each of the three genotypes. Neither parent nor the resistant progeny expressed antibiosis. Mean rm values for greenbug I on Wichita (0.0956), and Ae. tauschii (0.10543) were not significantly different. Mean rm values for Wichita and 97-85-3 were also not significantly different. Antixenosis was determined by allowing aphids a choice to feed on plants of each of the three genotypes. Ae. tauschii 1675 exhibited antixenosis, but this resistance was not inherited and expressed in '97-85-3'. In experiments comparing Wichita and Ae. tauschii 1675, greenbug I population distributions were not significantly different on Wichita at 24 h, but were shifted toward Wichita at 48 h. In the second antixenosis experiment, there were no significant differences in greenbug I population distributions on 97-85-3 or Wichita at 24 or 48 h. When all three lines were compared, there were no significant differences in greenbug biotype I populations at 24 or 48 h after infestation. Comparisons of proportional dry plant weight loss (DWT) and SPAD meter readings were used to determine tolerance to greenbug I feeding. Ae. tauschii 1675 and 97-85-3 were highly tolerant compared with Wichita. Infested and uninfested Ae. tauschii 1675 DWT was nonsignificant, and infested Wichita plants weighed significantly less than uninfested plants. When Wichita and 97-85-3 were contrasted, DWT of infested and uninfested Wichita plants were significantly different, but those of 97-85-3 were not. Mean percent leaf chlorophyll losses for the three genotypes, as measured by the SPAD chlorophyll meter, were as follows: Wichita = 65%; Ae. tauschii 1675 = 25%; and 97-85-3 = 39%. Percent leaf chlorophyll losses caused by greenbug feeding was significantly different in comparisons between Wichita and Ae. tauschii 1675, and comparisons between Wichita and 97-85-3, although feeding damage was not significantly different in comparisons between Ae. tauschii 1675 and 97-85-3. These data provided further evidence of the expression of tolerance to greenbug feeding in Ae. tauschii 1675 and 97-85-3.     

Spatial and temporal distribution of induced resistance to greenbug (Homoptera: Aphididae) herbivory in preconditioned resistant and susceptible near isogenic plants of wheat

Interactions between biotype E greenbugs, Schizaphis graminum (Rodani), and two near isogenic lines of the greenbug resistance gene Gb3 of wheat, Triticum aestivum L., were examined for 62 d after infestation. By comparing aphid performance and host responses on control and greenbug-preconditioned plants, we demonstrated that systemic resistance to greenbug herbivory was inducible in the resistant genotype with varying intensities and effectiveness in different parts of the plants. Preconditioning of susceptible plants resulted in modification of within-plant aphid distribution and reduction of cumulative greenbug densities, but it showed no effect on reducing greenbug feeding damage to host plant. Preconditioning of resistant plants altered greenbug population dynamics by reducing the size and buffering the fluctuation of the aphid population. Preconditioning in the first (oldest) leaf of the resistant plant had no phenotypically detectable effect in the stem and induced susceptibility locally in the first leaf within the first 2 d after infestation. The preconditioning-induced resistance reduced greenbug density, delayed aphid density peaks and extended the life of younger leaves in resistant plants. Expression of induced resistance was spatially and temporally dynamic within the plant, which occurred more rapidly, was longer in duration, and stronger in intensity in younger leaves. Host resistance gene-mediated induced resistance was effective in lowering greenbug performance and reducing damage from greenbug herbivory in host plants. Results from this study supported the optimal defense theory regarding within-plant defense allocation.     

Distribution and population development of Nasonovia ribisnigri (Homoptera: Aphididae) in iceberg lettuce

A field study was conducted to determine the distribution and development of aphid Nasonovia ribisnigri (Mosley) (Homoptera: Aphididae) populations in iceberg lettuce, Lactuca sativa L. 'Salinas'. Lettuce plants were transplanted and caged individually in the field and inoculated with apterous N. ribisnigri at 0, 1, 2, 3, and 4 wk after transplanting in spring and fall 2002. Plants were harvested 15-50 d after inoculations; numbers of alates and apterous N. ribisnigri were counted or estimated on each leaf for each plant. Inoculations during all 5 wk of plant development resulted in successful colonization of lettuce heads. Results indicated that head formation did not reduce the risk of colonization by N. ribisnigri to iceberg lettuce; plants were susceptible to colonization by N. ribisnigri throughout their development. For later inoculations, N. ribisnigri populations were relatively smaller, and aphids were found mostly within the heads. For earlier inoculations, N. ribisnigri populations were larger, and within-plant distributions shifted toward frame leaves. The shift of population distributions toward frame leaves correlated significantly with increases in N. ribisnigri population density. For most inoculations, more aphids were present on wrapper leaves than on other leaves. The proportion of alates did not vary significantly with population density. Population development of N ribisnigri also correlated significantly with heat unit accumulation. Yellow sticky cards were used to monitor alates in each cage. Catches of N. ribisnigri alates on yellow sticky cards were significantly correlated with total numbers of alates as well as with total population sizes on individual lettuce plants.