Plant vigor metrics determine spatio-temporal distribution dynamics of Oulema melanopus (Coleoptera: Chrysomelidae) and its larval parasitoid, Tetrastichus julis (Hymenoptera: Eulophidae).

The cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), is a new invasive insect pest of oat, wheat, and barley in western Canada. Biological control with its principal larval parasitoid, Tetrastichus julis Walker (Hymenoptera: Eulophidae), is the primary management strategy. However, to implement control successfully, a thorough understanding of the spatio-temporal dynamics of the interactions between these two species is important. We examined the nature of spatial associations and distribution dynamics of O. melanopus and T. julis with reference to host plant nutrients and plant vigor traits using Spatial Analysis by Distance Indices. A grid design was used to understand spatial associations between O. melanopus and T. julis. Distributions of O. melanopus and T. julis indicated the presence of significant patches and gaps. Plant nutrient availability and plant vigor varied across the grid in all study years. On a spatial scale, O. melanopus and T. julis represented a tightly coupled system demonstrating the strong density-dependent nature of parasitoid dispersal. Among the factors examined, plant vigor traits significantly influenced field distributions of both O. melanopus and T. julis. Areas across grids with high plant density, greater plant height, and high availability of plant leaves indicated higher establishment of O. melanopus larvae, consequently exhibiting bottom-up effects on T. julis distributions. Maintenance of uniform plant vigor can be a critical aspect in mitigating yield losses from O. melanopus infestation.

New threshold temperatures for the development of a North American diamondback moth (Lepidoptera: Plutellidae) population and its larval parasitoid, Diadegma insulare (Hymenoptera: Ichneumonidae).

The currently accepted lower threshold temperature for the development of diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), the world's most destructive insect pest of cruciferous crops, is around 6.0°C, and there is no known upper threshold temperature. Neither are there established threshold temperatures for diamondback moth's major natural enemy, Diadegma insulare (Hymenoptera: Ichneumonidae). Laboratory studies were undertaken to determine the survival and development of a North American diamondback moth population and its parasitoid D. insulare at 20 constant temperatures ranging from 2.0 to 38.0°C. Diamondback moth completed development from second instar to adult within a temperature range of 4.0-37°C, and D. insulare completed its life cycle from egg to adult within a temperature range of 4.0-33°C. The developmental data were fitted into one linear and four nonlinear models. Using goodness-of-fit and the ability to estimate parameters of biological significance as selection criteria, the Wang model was the most acceptable among the nonlinear models to describe the relationship between temperature and development of both species. According to this model, the lower and upper threshold temperatures for diamondback moth were 2.1 and 38.0°C, respectively, and for D. insulare they were 2.1 and 34.0°C, respectively. Based on the Degree Day model, diamondback moth required 143 d above the lower threshold of 4.23°C to complete the life cycle, while D. insulare required 286 d above the lower threshold of 2.57°C. This study suggests that temperatures during the crop-growing seasons in North America are not limiting factors for development of either diamondback moth or D. insulare.

Antixenosis resistance to Oulema melanopus (Coleoptera: Chrysomelidae) in central Asian wheat germplasm.

ABSTRACT The cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), which is Eurasian in origin, is an emerging pest of agriculturally important cereal crops in western Canada, including wheat, oat, and barley. Among these crops, wheat (Triticum aestivum L.) has the highest number of resistant genotypes to O. melanopus infestation. Here we explored six genotypes from central Asia with putative resistance to the beetle in comparison with a widely grown Canadian wheat variety. We noted the highest oviposition on the susceptible control genotype, CDC GO, and the lowest on one of the central Asian genotypes (NN-100) in both choice and no-choice tests. Two of the other Asian genotypes (NN-78 and NN-105) were also less attractive for oviposition than the CDC GO check. Feeding trials also indicated less damage on some of the Asian genotypes (NN-100, NN-105, and NN-78) relative to CDC GO, but other Asian genotypes (NN-41, NN-45, and NN-27) were highly attractive for feeding and oviposition and did not exhibit stronger resistance to O. melanopus infestation than moderately resistant central Asian genotypes (NN-103, NN-78, or NN-105). There were no significant differences in the amount of feeding by overwintered and teneral adults. The results indicated that some wheat genotypes developed in central Asia have categories of nonpreference for feeding and oviposition worth considering in further development of host-plant resistance and integrated management of this pest.

New Miticides for Integrated Pest Management of Varroa destructor (Acari: Varroidae) in Honey Bee Colonies on the Canadian Prairies.

Varroa destructor Anderson and Trueman 2000 (Acari: Varroidae) is an ectoparasitic mite of the honey bee, Apis mellifera L. (Hymenoptera: Apidae). Honey bee colonies require extensive management to prevent mortality caused by varroa mites and the viruses they vector. New miticides (Thymovar and HopGuard) to manage varroa mites were evaluated during the spring and fall treatment windows of the Canadian prairies to determine their effectiveness as part of an integrated management strategy. Thymovar and HopGuard were evaluated alongside the currently used industry standards: Apivar and formic acid. Results demonstrated that Apivar and formic acid remain effective V. destructor management options under spring and fall conditions. Applications of Thymovar during spring were associated with a reduction in brood area, and therefore should be limited to the fall season. The miticide HopGuard was not effective in managing V. destructor, and alteration of the current delivery system is necessary. This study demonstrates the potential for new effective treatment options to supplement currently used V. destructor integrated pest management systems.

Factors Influencing Male Plutella xylostella (Lepidoptera: Plutellidae) Capture Rates in Sex Pheromone-Baited Traps on Canola in Western Canada.

Optimization of male moth trapping rates in sex pheromone-baited traps plays a key role in managing Plutella xylostella (L.). We investigated various ways to increase the attractiveness of pheromone-baited traps to P. xylostella in canola agroecosystems in AB, Canada. Factors tested included pheromone blend and dose, addition of a green leaf volatile to the pheromone at different times during the season, lure type, trap color, and height. The industry standard dose of 100 µg of pheromone (four-component blend) per lure (ConTech Enterprises Inc., Delta, British Columbia [BC], Canada) captured the most moths in the two lure types tested. Traps baited with pheromone released from gray rubber septa captured more males than those baited with red rubber septa. Traps baited with lures in which Z11-16: Ac is the main component attracted significantly more moths than those in which Z11-16: Ald is the main component. The addition of the green leaf volatile, (Z)-3-hexenyl acetate, to pheromone at a range of doses, did not increase moth capture at any point during the canola growing season. Unpainted white traps captured significantly more male moths than pheromone-baited traps that were painted yellow. Trap height had no significant effect on moth capture. Recommendations for monitoring P. xylostella in canola agroecosystems of western Canada include using a pheromone blend with Z11-16: Ac as the main component released from gray rubber septa at a dose of 100 µg.

Crop sequence effects on root maggot (Diptera: Anthomyiidae: Delia spp.) infestations in canola.

Strong market demand for canola, Brassica napus L., has prompted some western Canadian producers to increase the frequency of this crop in rotations with other crop species, but the impact of this practice on canola insect pests has not been determined. Here, we investigate 12 cropping sequences involving canola over a 3-yr period (2008-2010 inclusive) at five locations across western Canada. Cropping sequences varied from continuous production of two herbicide-tolerant canola varieties, to production in two of 3 yr, to canola production in one of the 3 yr. Treatments analyzed were the frequency and timing of canola within the rotational sequence. Damage by larvae of root maggots (Diptera: Anthomyiidae: Delia spp.) to canola taproots increased as the study progressed, particularly in 2010 after canola had been grown continuously for 3 yr. Yield declined with continuous canola production, and differences were greatest in 2010. At mean canola crop prices for 2010, the yield reduction from continuous production amounted to economic losses of approximately Can$282-$377/ha. Crop quality, in terms of oil and protein concentrations of harvested seed, was affected more by crop variety than cropping sequence. Crop sequence effects for root maggot damage, yield, and seed quality were relatively stable in the presence of environmental (location) variation. Results of our study suggest that continuous canola production could be unsustainable over the long-term even though market forces currently provide incentive for this practice.

Ground beetle (Coleoptera: Carabidae) diversity, activity density, and community structure in a diversified agroecosystem.

Diversity and abundance of ground beetles (Coleoptera: Carabidae) can be enhanced in vegetable and field intercropping systems, but the complexity of polycultures precludes the application of generalized assumptions of effects for novel intercropping combinations. In a field experiment conducted at Lacombe and Ellerslie, Alberta, Canada, in 2005 and 2006, we investigated the effects of intercropping canola (Brassica napus L.) with wheat (Triticum aestivum L.) on the diversity and community structure of carabid beetles, and on the activity density responses of individual carabid species. Shannon-Wiener diversity index scores and species evenness increased significantly as the proportion of wheat comprising total crop plant populations increased in one site-year of the study, indicating a positive response to enhanced crop plant species evenness in the intercrops, and in that same site-year, ground beetle communities in intercrops shifted to more closely approximate those in wheat monocultures as the percentage of wheat in the intercrops increased. Individual carabid species activity densities showed differing responses to intercropping, although activity densities of some potential root maggot (Delia spp.) (Diptera: Anthomyiidae) predators were greater in intercrops with high proportions of wheat than in canola monocultures. The activity density of Pterostichus melanarius (Illiger), the most abundant species collected, tended to be greater in canola monocultures than high-wheat intercrops or wheat monocultures. We conclude that intercrops of canola and wheat have the potential to enhance populations of some carabid species, therefore possibly exerting increased pressure on some canola insect pests.

Developmental responses of the diamondback moth parasitoid Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae) to temperature and host plant species.

Effects of constant rearing temperature and the plant species fed upon by its hosts were investigated for several developmental parameters of Diadegma semiclausum (Hellén), an important parasitoid of the diamondback moth, Plutella xylostella (L.). Temperature had highly significant effects on all developmental parameters measured, and effects were usually both linear and quadratic with increasing temperature. Host plant species, comprising Brassica napus L., Brassica rapa L. ssp. pekinensis and Brassica oleracea L. var. capitata, also affected development of the parasitoid, and significant interactions were observed between plant species and rearing temperature for all developmental parameters measured. Development of D. semiclausum occurred successfully on all host plant species tested for the temperature range of 10 to 25°C. However, when its P. xylostella hosts consumed leaf tissue of B. napus, no specimens survived to pupate at 30°C, whilst pupation and adult eclosion occurred at 30°C on B. rapa ssp. pekinensis and B. oleracea var. capitata. At high ambient temperatures, such as those characteristic of tropical or subtropical regions (especially at low elevations) or regions that undergo temperature increases due to climate change, P. xylostella is predicted to occur at a higher range of temperatures than its biocontrol agent, D. semiclausum. Effects of high temperatures are expected to be more profound on the parasitoid for some host plants than others, with greater developmental limitations for the parasitoid on B. napus than on B. rapa or B. oleracea.

Response of Pisum sativum (Fabales: Fabaceae) to Sitona lineatus (Coleoptera: Curculionidae) infestation: effect of adult weevil density on damage, larval population, and yield loss.

Sitona lineatus L. (Coleoptera: Curculionidae) is an invasive pest in North America and its geographical range is currently expanding across the Canadian prairies. Adults and larvae of S. lineatus feed upon the foliage and root nodules, respectively, of field pea, Pisum sativum L. (Fabales: Fabaceae), and may contribute to economic losses when population densities are high. Integrated pest management (IPM) programs that incorporate economic thresholds should be used to manage S. lineatus populations in a sustainable manner. The impact of nitrogen fertilizer on field pea yield and the relationships between adult weevil density and above- and below-ground damage and yield were investigated in southern Alberta, Canada using exclusion cages on field pea plots. In each cage, 32 field pea plants were exposed to weevil densities ranging from zero to one adult weevil per plant. Nitrogen-fertilized plants yielded 16% more than unfertilized plants. Nitrogen-fertilized plants had fewer root nodules than unfertilized plants, but fertilizer had no effect on foliar feeding by S. lineatus. Adult density affected foliar feeding damage, with increases in above-ground damage associated with increases in S. lineatus density. Adult density did not affect root nodule damage, larval density, foliar biomass or seed weight. Overall, these results indicate that terminal leaf damage may be used to estimate adult weevil density but cannot be used to predict larval density or yield loss. Further research is required to better understand the impact of larval damage on yield and determine if economic thresholds can be developed using data from large-scale production systems.

Plant nutrients and the spatiotemporal distribution dynamics of Ceutorhynchus obstrictus (Coleoptera: Curculionidae) and its parasitoids.

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), is an alien invasive pest of canola, Brassica napus L., in North America. Field populations of both adults and larvae are known to be aggregated in their distributions, but the causal mechanism for this clustering is not understood. We investigated the hypothesis that spatial distributions of C. obstrictus adults, larvae, and their parasitoids were directly or indirectly related to host plant quality as indicated by leaf tissue nutrient contents. During the years of 2005 and 2007, these insect populations were sampled in a grid pattern in three commercial fields of B. napus located near Lethbridge, Alberta, Canada. Leaf samples were collected from plants in each grid plot and analyzed for nutrient contents. Spatial patterns and associations among the collected variables were analyzed using Spatial Analysis by Distance IndicEs software. We observed significant spatial associations among C. obstrictus adults, larvae, and parasitoids. The distributions of C. obstrictus adults and larvae were correlated with several plant nutrients, most notably nitrogen and sulfur. Evidence was found for a possible nitrogen-sulfur interaction in which ovipositioning C. obstrictus females appeared to prefer plants with high levels of sulfur and low levels of nitrogen. Spatial tracking of C. obstrictus larvae by its parasitoids was evident in only one field, and this appeared related to comparatively low parasitoid populations in two of the three fields. The associations between C. obstrictus and plant nutrients may have possible applications in fertility management, trap cropping, and precision insecticide application technology.

Effects of canola-wheat intercrops on Delia spp. (Diptera: Anthomyiidae) oviposition, larval feeding damage, and adult abundance.

Reductions in oviposition and subsequent damage by root maggots (Diptera: Anthomyiidae, Delia spp.) to brassicaceous crops in the presence of nonhost plants has been demonstrated, but such investigations have not been conducted using intercrops of species commonly grown in the large-scale agricultural production systems of western Canada. A field experiment was conducted at three sites in Alberta, Canada, in 2005 and 2006 to determine interactions between root maggots and the various proportions of canola (Brassica napus L.) making up the total crop plant populations in intercrops with wheat (Triticum aestivum L.). The effect of a neonicotinoid seed treatment also was investigated. Root maggot damage to canola taproots decreased with increasing proportions of wheat in the intercrops. The presence of wheat in the intercrops had little effect on root maggot adult abundance in any single site-by-year combination or when data were combined over all sites and years, with different Delia species and sexes responding differently. Similarly, per plant root maggot egg populations were unaffected by intercropping, although egg populations were reduced on a per unit land area basis in intercrops compared with monocultures. Insecticidal seed treatment did not affect root maggot egg populations or canola root damage. Variable abundances and phenologies of the principal root maggot species infesting canola at different sites and years may influence their responses to canola-wheat intercrops. Intercropping canola and wheat may provide an opportunity for reducing crop damage from root maggot attack without compromising environmental sustainability.

Differences in Phyllotreta cruciferae and Phyllotreta striolata (Coleoptera: Chrysomelidae) responses to neonicotinoid seed treatments.

Insecticidal seed treatments are used commonly throughout the Northern Great Plains of North America to systemically protect seedlings of canola (Brassica napus L. and Brassica rapa L.) from attack by the flea beetles Phyllotreta cruciferae (Goeze) and Phyllotreta striolata (F.) (Coleoptera: Chrysomelidae). Here, we investigated differential responses by the two flea beetle species to the neonicotinoid seed treatments thiamethoxam (Helix and Helix XTra) and clothianidin (Prosper 400) in greenhouse experiments. P. cruciferae experienced higher mortality and fed less when exposed to these compounds than did P. striolata. Beetles of the overwintered and the summer generations responded differently when feeding on seedlings that developed with insecticidal seed treatments, with mortality higher for P. cruciferae in May than in August. When the two flea beetle species were held together at equal densities and allowed to feed on seedlings affected by the seed treatments, mortality of P. cruciferae significantly exceeded that of P. striolata. Differences in efficacies of these compounds for these beetles have ramifications for management strategies in regions where these insects occur sympatrically. Competitive release of P. striolata was previously reported to occur when P. cruciferae was excluded from brassicaceous crops; consequently, the consistent use of these seed treatments over millions of hectares of canola cropland may be a factor that contributes to a shift in prevalence of flea beetle pest species from P. cruciferae toward P. striolata.

Emergence and seasonal activity of the entomophagous rove beetle Aleochara bilineata (Coleoptera: Staphylinidae) in canola in Western Canada.

Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae) is an important natural enemy of root maggots (Delia spp.) (Diptera: Anthomyiidae), which are serious pests of brassicaceous crops in North America and Europe. Adults of A. bilineata feed on eggs and larvae of root maggots, and A. bilineata larvae parasitize Delia spp. pupae. Emergence and seasonal activity patterns of A. bilineata were investigated during 2003-2005 in canola (Brassica rapa L. and Brassica napus L.) in central Alberta, Canada, in relation to degree-day (DD) accumulations and Julian date. Captures of A. bilineata adults from pitfall traps within emergence cages situated over canola stubble from the previous year indicated that approximately 428, 493, and 455 DD (soil base 5.57 degrees C) and 187, 189, and 180 Julian days were required for 50% emergence in 2003, 2004, and 2005, respectively (3-yr mean = 185.1 +/- 2.8 Julian days [SEM]). Captures of A. bilineata adults from pitfall traps placed in current canola crops determined that 50% levels of activity density required 379 DD and 180 Julian days in 2004. A logistic model that described the relationship of degree-days and Julian days with emergence of adult beetles was appraised, and good correspondence was evident between predicted and observed cumulative emergence patterns. Emergence and seasonal activity periods of A. bilineata in canola were well synchronized with occurrence of preimaginal life stages of its principal hosts, Delia radicum (L.) and Delia platura Meigen, with beetle emergence beginning shortly after the onset of root maggot oviposition.

Resistance of some cultivated Brassicaceae to infestations by Plutella xylostella (Lepidoptera: Plutellidae).

Selecting insect-resistant plant varieties is a key component of integrated management programs of oligophagous pests such as diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), but rigorous research on important life history parameters of P. xylostella in relation to host plant resistance is rare. We evaluated six conventional brassicaceous species, namely, Brassica napus L. 'Q2', B. rapa L., B. juncea (L.) Czern., B. carinata L., B. oleracea L., and Sinapis alba L., and two herbicide-tolerant cultivars, namely, B. napus 'Liberty' and B. napus 'Conquest' for their resistance against P. xylostella. Brassicaceae species and cultivars varied considerably in their susceptibilities as hosts for P. xylostella. Sinapis alba and B. rapa plants were highly preferred by ovipositing females and trichome density on adaxial and abaxial leaf surfaces had nonsignificant effects on P. xylostella oviposition. Larval survival was similar on the genotypes we tested, but host plants significantly affected larval and pupal developmental time, herbivory, pupal weight, silk weight, adult body weight, forewing area and longevity (without food) of both male and female P. xylostella. Larval and pupal development of females was fastest on B. juncea and S. alba, respectively. Specimens reared on B. napus Liberty and B. oleracea, respectively, produced the lightest female and male pupae. Defoliation by both female and male larvae was highest on B. rapa, whereas least herbivory occurred on S. alba. Females reared on S. alba were heavier and lived longer in the absence of food than their counterparts raised on other tested host plants. Brassica oleracea could not compensate for larval feeding to the level of the other species we evaluated. B. napus Conquest, B. napus Q2, B. carinata, B. rapa, and S. alba produced, respectively, 1.6-, 1.8-, 1.8-, 3.9-, and 5.5-fold heavier root systems when infested than their uninfested counterparts, suggesting that these species were better able to tolerate P. xylostella infestations.

Effects of fall and spring seeding date and other agronomic factors on infestations of root maggots, Delia spp. (Diptera: Anthomyiidae), in canola.

Several agronomic benefits can result from fall seeding of canola (Brassica spp.), but extensive research data are lacking on the potential impact of this practice on infestations of root maggots (Delia spp.) (Diptera: Anthomyiidae), which are major pests of the crop in western Canada. Field experiments making up 13 location by year combinations were conducted in central Alberta, Canada, from 1998 to 2001 to determine the effect of fall versus spring seeding of canola on root maggot damage. Depending on the experiment, interactions with seeding rate, seed treatment, timing of weed removal, and canola species (cultivar) also were investigated. Root maggot damage declined with an increase in seeding rate for plots seeded in May but not in fall or April. Susceptibility to infestation was greater for plants of Brassica rapa L. than Brassica napus L., but seed treatment had no effect on damage by these pests. Combined analysis using data from all experiment by location by year combinations indicated that seeding date had no significant effect on root maggot damage. The extended emergence of Delia spp. adults, which spans the appearance of crop stages vulnerable to oviposition regardless of seeding date, prevented reduced root maggot attack. Covariance analysis demonstrated the importance of increasing seeding rate for reducing root maggot infestations, a practice that can be especially beneficial for May-seeded canola when growing conditions limit the ability of plants to compensate for root maggot damage. Results determined with the small plot studies described here should be validated in larger plots or on a commercial field scale, but both the combined and covariance analyses indicate that seeding canola in fall does not predispose plants to greater damage by larval root maggots than seeding in spring.

Oviposition by Plutella xylostella (Lepidoptera: Plutellidae) and effects of phylloplane waxiness.

Three approaches were used to investigate effects of host plant epicuticular waxes on oviposition site selection by Plutella xylostella (L.). In the first approach, oviposition on canola (Brassica napus L.) that had epicuticular wax reduced by application of a carbamate herbicide (S-ethyl dipropylthiocarbamate) was compared with oviposition on untreated control plants. A second approach compared oviposition on sibling strains of B. napus with different wax blooms (glossy and waxy), and a third approach compared oviposition by P. xylostella on parafilm that had been applied to glossy and waxy B. napus strains for transfer of leaf components. Significantly more eggs were deposited on herbicide-treated plants (with reduced epicuticular wax) than on untreated controls. Similarly, more eggs were deposited on glossy than on waxy sibling strains of B. napus. In parafilm assays significantly more eggs were deposited on treated than on untreated parafilm. Several mechanisms could explain the differences in attractiveness of surfaces with varying wax content as oviposition sites for P. xylostella, including visual, chemical, and tactile differences between substrates. These mechanisms are discussed.

New self-marking device for dispersal studies of black flies (Diptera: Simuliidae).

An effective and inexpensive device is described for marking newly emerged black flies with fluorescent dust. Survival of marked adults of Simulium venustum s.l. and Stegopterna mutata s.l. did not differ significantly from unmarked individuals over a 5-day period. Marked individuals were attracted to human hosts in landing biting tests, and the portions of the self-marking trap lying just above the water surface did not appear to significantly hamper black fly emergence success.

Toxicity and residual action of the photoactivated compound, cyano-alpha-terthienyl, and its efficacy for reducing pre-imaginal populations of mosquitoes.

The photoactivated compound, cyano-alpha-terthienyl (cyano-alpha-T), was highly toxic to pre-imagines of the mosquitoes Culex restuans, Cx. tarsalis and Culiseta inornata when synergized with piperonyl butoxide (PBO). Lethal concentrations for 50% mortality, determined during an outdoor trial using caged fourth-instar Culex spp. larvae, were 19.4, 15.4 and 12.9 g/ha at 24, 48 and 72 h after treatment, respectively. No residual activity of cyano-alpha-T was observed beyond 24 h following treatment. In artificial pool tests, greatest population reductions were achieved using dosages of 20 and 40 g/ha; statistically significant reductions were not observed following applications of 5 g/ha. Cyano-alpha-T plus PBO was more effective for reducing mosquito populations than alpha-terthienyl (alpha-T) plus PBO at comparable dosages, although it exhibited slightly lower insecticidal activity at a dosage of 20 g/ha than a formulation of Bacillus thuringiensis var. israelensis (Vectobac 12 AS, 0.12 ml/m2). Greatest effectiveness of cyano-alpha-T plus PBO was observed in pools with low organic content relative to pools high in organic content.

The impact of methoxychlor treatment of the saskatchewan river system on artificial substrate populations of aquatic insects.

The impact of methoxychlor exposure on aquatic insects inhabiting artificial substrates was monitored at three downstream sites relative to an upstream untreated site of the North Saskatchewan River. Treatment impact was studied for selected species of Simuliidae (Diptera), Perlodidae (Plecoptera), Hydropsychidae (Trichoptera), Baetidae and Heptageniidae (Ephemeroptera). At sites subjected to methoxychlor exposure which were 21, 38, and 107 km from injection, population changes varied depending on species and distance from the injection point. Although populations of some species were not significantly affected by treatment at any downstream site (P > 0.05), others were significantly reduced at one or more of the sites (P < 0.05- P < 0.01). Nymphs of Stenonema terminatum (Walsh) and Baetis tricaudatus Dodds (Ephemeroptera) apparently recolonized after dislodgement due to methoxychlor exposure. Species are categorized on the basis of their responses to methoxychlor treatment. Factors which probably caused different treatment impacts among species are discussed.