Host Plant Nutritional Quality Research Articles

Nitrogen fertilizer promotes the rice pest Nilaparvata lugens via impaired natural enemy, Anagrus flaveolus, performance

The application of nitrogen (N) fertilizers to agricultural crops is a common practice globally and a crucial component in the current levels of productivity. Excessive N use, however, is costly and damaging to ecosystems. It is recognized that overuse of N fertilizer can promote pest herbivores by enhancing host plant nutritional quality, but less is known of the effects of N on natural enemies of pests via improved quality and availability of prey, and how these may cascade to indirect effects on pests. Here, we explored the effects of N fertilizer on a key egg parasitiod of Nilaparvata lugens (Stal) (Hemiptera: Delphacidae) (brown planthopper, BPH). Application of N to rice plants significantly prolonged the development time of Anagrus flaveolus (Hymenoptera: Mymaridae), increased wing size and decreased fecundity of adult females. Importantly, N application to rice plants significantly decreased the per capita parasitism of BPH eggs by A. flaveolus. Utilizing planthopper prey on N-treated rice plants led to reduced searching efficiency of A. flaveolus. Ultimately, the strength of biological control exerted by A. flaveolus was negatively affected by nitrogen application under field conditions. Parasitoids were able to discriminate between BPH infested rice plants with different levels of nitrogen using visual plant cues. We conclude that N fertilizer use can have profound effects on natural enemy efficiency which potentially increase the dependence on insecticides, another potentially hazardous input.

Spectral characterization of wheat functional trait responses to Hessian fly: Mechanisms for trait-based resistance.

Insect herbivores can manipulate host plants to inhibit defenses. Insects that induce plant galls are excellent examples of these interactions. The Hessian fly (HF, Mayetiola destructor) is a destructive pest of wheat (Triticum spp.) that occurs in nearly all wheat producing globally. Under compatible interactions (i.e., successful HF establishment), HF larvae alter host tissue physiology and morphology for their benefit, manifesting as the development of plant nutritive tissue that feeds the larva and ceases plant cell division and elongation. Under incompatible interactions (i.e., unsuccessful HF establishment), plants respond to larval feeding by killing the larva, permitting normal plant development. We used reflectance spectroscopy to characterize whole-plant functional trait responses during both compatible and incompatible interactions and related these findings with morphological and gene expression observations from earlier studies. Spectral models successfully characterized wheat foliar traits, with mean goodness of fit statistics of 0.84, 0.85, 0.94, and 0.69 and percent root mean square errors of 22, 10, 6, and 20%, respectively, for nitrogen and carbon concentrations, leaf mass per area, and total phenolic content. We found that larvae capable of generating compatible interactions successfully manipulated host plant chemical and morphological composition to create a more hospitable environment. Incompatible interactions resulted in lower host plant nutritional quality, thicker leaves, and higher phenolic levels. Spectral measurements successfully characterized wheat responses to compatible and incompatible interactions, providing an excellent example of the utility of Spectral phenotyping in quantifying responses of specific plant functional traits associated with insect resistance.

Sub-lethal viral exposure and growth on drought stressed host plants changes resource allocation patterns and life history costs in the Speckled Wood butterfly, Pararge aegeria.

This study investigated the interactive effects of growth on drought stressed host plants and pathogen challenge with the baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) on survival and fitness-related traits using the Speckled Wood butterfly, Pararge aegeria (L.). Exposure to AcMNPV significantly reduced survival to pupation. For surviving larvae, sub-lethal infection significantly decreased daily mass acquisition rates and pupal mass. Growth on drought stressed plants increased daily mass acquisition rates resulting in heavier pupae, and increased resource allocation to adult reproduction. The interaction between host plant drought and viral exposure resulted in different resource allocation strategies, and thus different growth trajectories, between larvae. This in turn resulted in significantly different allometric relationships between larval mass (at inoculation) and both development time and investment in flight muscles. For larvae with relatively lighter masses there was a cost of resisting infection when growth occurred on drought stressed host plants, both within the larval stage (i.e. longer larval development times) and in the adult stage (i.e. lower investment in flight muscle mass). This multi-factor study highlights several potential mechanisms by which the complex interplay between low host plant nutritional quality due to drought, and pathogen exposure, may differentially influence the performance of P. aegeria individuals across multiple life stages.

The green peach aphid Myzus persicae perform better on pre-infested Chinese cabbage Brassica pekinensis by enhancing host plant nutritional quality.

The green peach aphid, Myzus persicae Sulzer, is a notorious pest on vegetables, which often aggregates in high densities on crop leaves. In this study, we investigated whether M. persicae could suppress the resistance level of Chinese cabbage Brassica pekinensis. M. persicae performed better in terms of weight gain (~33% increase) and population growth (~110% increase) when feeding on previously infested (pre-infested) Chinese cabbage compared with those on non-infested plants. However, when given a choice, 64% of the aphids preferred to settle on non-infested leaves, while 29% of aphids chose pre-infested leaves that had a 2.9 times higher concentration of glucosinolates. Aphid feeding significantly enhanced the amino acid:sugar ratio of phloem sap and the absolute amino acid concentration in plant leaves. Aphid infestation significantly increased the expression levels of salicylic acid (SA) marker genes, while it had marginal effects on the expression of jasmonate marker genes. Exogenously applied SA or methyl jasmonate had no significant effects on M. persicae performance, although these chemicals increased glucosinolates concentration in plant leaves. M. persicae infestation increase amino acid:sugar ratio and activate plant defenses, but aphid performed better on pre-infested plants, suggesting that both nutrition and toxics should be considered in insect-plant interaction.

Tritrophic interactions between parasitoids and cereal aphids are mediated by nitrogen fertilizer

Host plant nutritional quality can directly and indirectly affect the third trophic levels. The aphid-parasitoid relationship provides an ideal system to investigate tritrophic interactions (as the parasitoids are completely dependent for their development upon their hosts) and assess the bottom up forces operating at different concentrations of nitrogen applications. The effects of varying nitrogen fertilizer on the performance of Aphidius colemani (V.) reared on Sitobion avenae (F.) and Aphidius rhopalosiphi (D.) reared on Rhopalosiphum padi (L.) were measured. Parasitism and percent emergence of parasitoids were positively affected by nitrogen fertilizer treatments while developmental duration (egg, larval, and pupal stages) was not affected by increasing nitrogen inputs. In males and females of both parasitoid species, adult longevity increased with the increasing nitrogen fertilizer. Hind tibia length and mummy weight of both parasitoid species increased with nitrogen fertilizer concentrations, as a result of larger aphids. This study showed that nitrogen application to the soil can have important consequences for aboveground multitrophic interactions.

Variation within and betweenFrankliniellaThrips Species in Host Plant Utilization

Anthophilous flower thrips in the genus Frankliniella (Thysanoptera: Thripidae) exploit ephemeral plant resources and therefore must be capable of successfully locating appropriate hosts on a repeated basis, yet little is known of interspecific and intraspecific variation in responses to host plant type and nutritional quality. Field trials were conducted over two seasons to determine if the abundance of males and females of three common Frankliniella species, F. occidentalis (Pergande), F. tritici (Fitch) and F. bispinosa (Morgan), their larvae, and a key predator, Orius insidiosus (Say) (Hemiptera: Anthocoridae) were affected by host plant type and plant nutritional quality. Two host plants, pepper, Capsicum annuum L. (Solanales: Solanaceae) and tomato, Solanum lycopersicum L. that vary in suitability for these species were examined, and their nutritional quality was manipulated by applying three levels of nitrogen fertilization (101 kg/ha, 202 kg/ha, 404 kg/ha). F. occidentalis females were more abundant in pepper than in tomato, but males did not show a differential response. Both sexes of F. tritici and F. bispinosa were more abundant in tomato than in pepper. Larval thrips were more abundant in pepper than in tomato. Likewise, O. insidiosus females and nymphs were more abundant in pepper than in tomato. Only F. occidentalis females showed a distinct response to nitrogen fertilization, with abundance increasing with fertilization. These results show that host plant utilization patterns vary among Frankliniella spp. and should not be generalized from results of the intensively studied F. occidentalis. Given the different pest status of these species and their differential abundance in pepper and tomato, it is critical that scouting programs include species identifications for proper management.

Bottom-up effects of host plant nutritional quality on Plutella xylostella (Lepidoptera: Plutellidae) and top-down effects of herbivore attack on plant compensatory ability

The plant stress and plant vigour hypotheses are competing paradigms pertaining to the preference and performance of herbivorous insects on their host plants. Tests of these hypotheses ideally require detailed information on aspects of soil nutrition, foliar nutrient levels and parameters of herbivore fitness, but such studies are uncommon. These hypotheses were tested using the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), reared on its host plant, Brassica napus (L.), grown in an experimental system of five nutrient regimes. Different levels of fertilizer treatments significantly affected the nutrient content of B. napus foliage and this in turn affected the preference and performance of P. xylostella. Ovipositing females discriminated among host plants grown in soils subjected to different fertilizer treatments and selected plants on which pre-imaginal survival was highest, development fastest and longevity of the next generation of adults the longest, even when food was scarce. Plants subjected to herbivory by P. xylostella responded by producing elevated levels of some nutrients (e.g., sulphur), but other nutrient levels declined in infested leaves (e.g., nitrogen). Regardless of the rate of fertilizer application, plants compensated for herbivory by increasing root mass compared to un-infested control plants; plants grown in soils receiving the optimum quantity of fertilizer developed the most robust root systems when infested. The plant stress and the plant vigour hypotheses are likely to be at the opposite ends of a continuum of responses between insects and their host plants. Our investigations indicate a complex set of interactions involving both bottom-up and top-down effects, which interact to affect host plant quality, oviposition site selection by female herbivores and the fitness of their offspring.

Influence of drought stress on Mexican rice borer (Lepidoptera: Crambidae) oviposition preference in sugarcane

Two sugarcane, Saccharum hybrids, varieties were grown in the greenhouse under well watered or drought conditions to examine the influences of stress on Mexican rice borer, Eoreuma loftini (Dyar), oviposition preference and selected nutritional components without impinging factors common to field conditions that alter responses. Our research revealed that, under controlled conditions, drought induced a wider range and greater uniformity of free amino acid (FAA) accumulations than have been previously reported. Drought stress resulted in increased dry leaf tissue and elevated concentrations of 7 of 9 detectable free essential (for insect health) amino acids in stalks, the chief food of Mexican rice borer larvae. Stressed sugarcane was preferred for oviposition, likely related to greater numbers of dry leaves and heightened host plant nutritional quality. Dry leaf tissue, which is not consumed, may be a cue for improving chances of larvae encountering nutrient-enhanced living tissue, and for concealing eggs in folds. Excised dry leaf tissue from the treatments was indistinguishable for oviposition preference; therefore, biochemical status of living tissue may provide oviposition cues. Varieties exhibited no major genotypic differences in FAA accumulations or oviposition preference.

Host plant nutritional quality affects the performance of the parasitoid Diadegma insulare

The well documented biochemical profile of Brassicaceae, oligophagy of the herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and host specialization of the parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) provide an ideal system for investigating tritrophic interactions mediated by nutritional quality of plants. We evaluated the bottom-up effects of five soil fertility regimes on nutritional quality of canola ( Brassica napus L.) and then on several fitness correlates of female and male D. insulare as mediated through P. xylostella. Variation in soil fertility influenced the nutritional quality of host plants and this in turn affected the performance of D. insulare. In general, D. insulare performed best on plants grown with 3.0 g fertilizer pot −1; these plants had 2.06-, 3.77-, and 1.02-fold more nitrogen, phosphorous and potassium, respectively than ones grown without any added fertilizer. P. xylostella escape from D. insulare was highest (32%) on plants grown at 1.0 g fertilizer, and this could be attributed to both physical and physiological defense mechanisms mediated by host plant nutritional quality. Plant stress and plant vigor are competing paradigms pertaining to the performance of herbivorous insects on their host plants. These hypotheses were originally proposed to predict responses of herbivores, but may also explain the effects of plant quality on koinobiont parasitoids, such as D. insulare.

Nitrogen fertilization rate affects feeding, larval performance, and oviposition preference of the beet armyworm, Spodoptera exigua, on cotton

AbstractNitrogen (N) is one of the most critical chemical elements for plant and animal growth, exerting a variety of bottom‐up effects. Development and oviposition of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), were studied in relation to varying N fertilization levels (42, 112, 196, and 280 p.p.m.) in cotton [Gossypium hirsutum L. (Malvaceae)]. Low N fertilization of cotton plants led to reduced plant biomass and a lower percentage of N in leaf blades and in leaf petioles. Development of S. exigua larvae fed with plants with reduced N applications (42 and 112 p.p.m.) was prolonged relative to treatments receiving higher N fertilization. Almost all larvae reared on artificial diets underwent only five instars before pupation. However, most larvae reared on cotton plants, irrespective of N levels, experienced a supernumerary sixth larval instar. Furthermore, significantly more larvae reared on lower N cotton plants underwent supernumerary development compared to larvae reared on higher N cotton plants. Life‐time feeding damage per larva ranged from 55 to 65 cm2, depending on the nutritional quality of the food plant, although the differences were not statistically significant. Larvae distinguished between cotton plants with various nutritional qualities and fed preferentially on higher N plants. Female moth oviposition choice was also affected by host plant nutritional quality: cotton plants with higher N levels were preferentially chosen by S. exigua females for oviposition. The mechanisms of these effects are unclear, but they can have important implications for population dynamics and pest status of beet armyworms in the field.

Selection on offspring size varies within and among families in relation to host nutritional quality

Life history theory often assumes a positive relationship between offspring size and fitness, although the strength and form of this relationship is expected to vary with environmental conditions. In arthropods, surprisingly few studies have examined the influence of larval environment on the offspring size–fitness relation. In phytophagous insects, the few studies that have examined variation in larval host plants have found a negative correlation between host plant nutritional quality and the strength of selection favoring larger offspring size, suggesting that this pattern might be general. I present experimental evidence for such a relationship in a population of the moth Rothschildia lebeau feeding on its three primary host plant species. Unlike previous studies, I consider the effect of offspring size on growth and survival at two levels, both among families and among siblings within families. Neonate caterpillar mass had a significant effect on growth and survival. The effect on growth, however, was weak, resulted primarily from variation among families, and did not differ among host plant diets. The effect on survival was stronger and varied among host plant diets, among families, and within families on different host plants in a manner that was generally consistent with the hypothesized negative correlation between host plant nutritional quality and the strength of selection favoring larger offspring size. Overall, results suggest that the consequences of variation in offspring size for survival within and among families are host plant-dependent in this system.

Host nutritive quality and host plant choice in two grass miners: primary roles for primary compounds?

The relationship between host plant choice and plant nutritional quality was investigated in two oligophagous grass miners Chromatomyia milii and C. nigra (Diptera, Agromyzidae). We tested whether host choice is determined by chemically mediated host suitability for offspring performance and/or adult performance. A second goal was to relate the observed variation among the different fitness parameters to quantitative and qualitative variation in foliar food quality. Choice experiments illustrated that both miners discriminated among grass species, and that C. milii has a smaller host range than C. nigra, as observed under natural conditions. Oviposition preference was correlated with adult feeding preference and related adult performance (longevity and fecundity) for both miners. Offspring performance measures (survival and pupal size) of at least C. nigra were more weakly related to host preference. Nearly all variation in adult performance of both miners was explained by foliar protein content, which had a positive effect on adult longevity and fecundity. Pupal size of both miners was positively related to foliar water and amino acid content and negatively related to lignin content. No clear relationship between host chemistry and offspring survival was observed. These observations show that fitness parameters are differentially related to host chemistry. Secondly, they suggest that chemically mediated host suitability for adult performance is an important determinant of host choice in this species. Finally, the results suggest a primary role for foliar protein content in host choice of the study species in general and in shaping the host range of C. milii in particular.

Effects of drought stressed cotton, Gossypium hirsutum L., on beet armyworm, Spodoptera exigua (Hübner), oviposition, and larval feeding preferences and growth.

The beet armyworm, Spodoptera exigua (Hübner), has been anecdotally reported to oviposit more on drought stressed than on nonstressed cotton plants. Using potted cotton plants in cages, this study demonstrated that beet armyworms deposited 3.3, 4.6, and 2.3 times more (P < or = 0.05) eggs on cotton plants that were grown on 1500, 1000, and 750 ml water/wk, respectively, than on cotton plants grown in well watered (4000 ml water/wk) soil. Third instars, however, showed no preference for stressed cotton foliage over nonstressed foliage. Third instar beet armyworms raised on well watered cotton plants were 1.5, 2.3, and 2.6 times heavier than those reared on cotton grown in the 1500, 1000, and 750 ml water/wk plants (P < or = 0.05), respectively. Physiochemical analyses showed that drought stressed leaves had significantly greater accumulations of free amino acids that are essential for insect growth and development. Soluble protein and soluble carbohydrates were also more abundant in stressed leaves compared to nonstressed leaves. Despite the apparent increase in nutritional quality in drought stressed plants, larval survival was reduced, probably because the limiting factor became water. Greater amounts of cotton leaf area were consumed from drought stressed leaves (P < or = 0.05) than from nonstressed leaves, probably because the larvae had to metabolize greater portions of assimilated energy to supplement body water with metabolic water derived from respiration. The association of greater host plant nutritional quality to oviposition preference, and conversely, to reduced survivorship, is discussed.

Impact of diet quality on demographic attributes in adult grasshoppers and the nitrogen limitation hypothesis

1. Various formulations of the nutrient stress hypothesis predict that insect herbivore populations will respond positively to increased nutrient quality of host plants, especially dietary nitrogen. Survival and reproduction by adult females of two grasshopper species [Acrididae;Melanoplus sanguinipes (Fabricius) and Phoetaliotes nebrascensis (Thomas)] were evaluated in response to defined diets that varied factorially in both total nitrogen (1–7%) and total soluble carbohydrate (4.3–26.7%). These grasshopper species coexist naturally but are typically shifted phenologically so that specific developmental stages normally encounter host plants of different nutritional quality under natural conditions.2. Demographic responses by adult females of both species varied according to diet quality, but not in the same fashion. Diet quality affected survival significantly in P. nebrascensis but not in M. sanguinipes. Survival in P. nebrascensis was greatest on diets containing the lowest nitrogen concentrations; carbohydrate level had no effect.3. Diet quality influenced reproduction significantly in both species. Egg production rate (eggs/day) in M. sanguinipes exhibited a negative linear response to increased carbohydrate, coupled with a significant quadratic response to nitrogen that reached a maximum at an intermediate level of about 4% total N. A significant quadratic response to total N for pod production rate (indicating the timing of reproduction) was also observed. Clutch size in M. sanguinipes exhibited a negative relationship with total carbohydrate in the diet, but no response to nitrogen. No interaction was observed between nitrogen and carbohydrate levels. For P. nebrascensis, response to diet quality was much weaker, with only a suggestive maximum at 4% total‐N for both egg production rate (eggs/day) and clutch size (eggs/pod) and a suggestive linear response for pod production rate as carbohydrate level increased. Female body weight did not contribute to any reproductive response as a covariate variable.4. Combined with a similar, previous analysis of demographic responses by the grass‐feeding grasshopper, Ageneotettix deorum, these results challenge the ability to draw generalizations about host plant nutritional quality and grasshopper demographic responses. These three grasshopper species respond quite differently to defined diets that vary in total nitrogen and carbohydrate levels. Thus, although host plant quality can contribute significantly to grasshopper population responses, a uniform explanation is not likely.

Phytochemical Changes during Recurrent Selection for Resistance to the European Corn Borer

Feeding performance of herbivorous insects is influenced by host plant nutritional quality which can be improved for insect resistance by artificial selection. This study was conducted to determine which biochemical constituents in maize (Zea mays L.) change during recurrent selection for resistance to first‐ and second‐generation European corn borer (ECB) [Ostrinia nubilalis (Hübner)]. Four cycles of selection (C0, C2, C4, and C5) from the BS9 population were field grown, artificially and naturally infested with ECB, and the following tissues sampled for biochemical analysis: immature and mature leaf blade, leaf sheath, rind, node, and pith. Tissue was analyzed for percent protein, DIMBOA [2,4‐dihydroxy‐7‐methoxy 2H‐l,4‐benzoxazin‐3 (4H)‐one], fiber, and cell wall‐bound phenolics, which included p‐coumaric acid (pCA), ferulic acid (FA), cyclobutane dimers (CBD), and diferulic acid (DFA). Leaf and stalk toughness were also determined and showed significant increases over cycles of selection. Protein content was lowest in stalk tissues with advanced cycles having lower levels, but leaf protein content did not differ significantly. DFA reached high levels in the rind (0.85 mg/g) and leaf sheath (1.35 mg/g) tissues, and increased significantly in immature leaf tissue (0.55–1.02 mg/g) over cycles of selection and may serve to fortify tender whorl tissue. Number of tunnels per stalk was negatively correlated with DFA content in the pith (r = − 77, P = 0.02). Microspectrophotometer determinations of epidermal cell wall absorbance for leaf blade and rind tissue showed increased absorbances (23 and 27%, respectively) in the spectral region characteristic of phenolic acids over cycles of selection. Phenolic acids, in particular DFA, have increased over cycles of selection to render maize tissue more resistant through fortification of cell walls, especially in leaf and rind epidermal tissue.

European Corn Borer Resistance and Cell Wall Composition of Three Maize Populations

Feeding activities of herbivorous insects are influenced by host plant nutritional quality. Improved insect resistance resulting from either natural or artificial selection may be due, in part, to changes in nutritive constituents of plants. The first objective of this study was to measure changes in detergent fiber, lignin, ash, and N concentrations in whorls, leaf‐sheaths, and stalks of the BS9 maize (Zea mays L.) population across five cycles of selection for resistance to the European corn borer (ECB) [Ostrinia nubilalis (Hübner)]. The second objective was to evaluate ECB resistance in the WFISIHI and WFISILO maize populations, which were developed for high and low concentrations, respectively, of indigestible plant constituents (acid detergent fiber, lignin, and silica) in the leaf sheath. Leafsheath composition for all five cycles of BS9 was measured in three environments in Iowa. Whorl, leaf‐sheath and stalk composition, as well as first‐ and second‐generation ECB resistance of populations WFISIHI, WFISILO and Cycles 0, 2, 4, and 5 of BS9 were evaluated in two environments in Wisconsin. Whorl composition was not related to changes in ECB resistance in anypopulation. In BS9, leaf‐sheath and stalk concentrations of neutral and acid detergent fiber, cellulose, and lignin increased linearly over selection cycles. In contrast, WFISIHI was as susceptible to second‐generation ECB as WFISILO, suggesting that the responses in BS9 may be due to linkage or unintentional selection. Populations BS9, WFISIHI, and WFISILO, however, were derived from diverse sources, and it is likely that mechanisms for resistance differ for the three populations.

Free amino acids in the phloem sap from oats and barley resistant to Rhopalosiphum padi

The concentration and composition of free amino acids were determined in the phloem sap of barley and oat cultivars, breeding lines and wild species previously selected for resistance against the aphid Rhopalosiphum padi (L). No relationship was found between degree of aphid resistance and the total concentration of free amino acids in plant sap. In all investigated accessions aspartic acid, glutamic acid, glutamine, and serine dominated the amino acid composition. The concentration of glutamic acid was significantly higher in resistant plants than in susceptible cultivars. Sap from most resistant accessions contained less asparagine than that of the controls. The content of γ-aminobutyric acid, glycine, histidine, methionine, and tryptophan was generally low in the tested plants. The oat accessions had significantly more methionine in their phloem sap than the barley accessions. These results are discussed in light of previous work on plant resistance to aphids and host plant nutritional quality.

Evaluation of Possible Nonlethal Side Effects of Permethrin Used in Predator Exclusion Experiments to Evaluate Amblyseius limonicus (Acari: Phytoseiidae) in Biological Control of Cassava Mites (Acari: Tetranychidae)

Permethrin has been recommended for use in predator exclusion studies of Amblyseius limonicus Garman & McGregor (formerly in Typhlodromalus), an important predator of tetranychid cassava (Manihot esculenta Crantz) pests: green mite, Mononychellus progresivus Doreste, and twospotted spider mite, Tetranychus urticae Koch. Among criteria for a chemical exclusion agent are selective toxicity and lack of side effects such as enhanced pest fecundity, stimluation of plant growth, or improvement of host plant nutritional quality. Laboratory and field tests provided no evidence for phytostimulation, change in cassava nutritional quality, or increased spider mite fecundity; however, abundance of nontarget arthropods was significantly reduced in permethrin-treated field plots. A possible repellency effect of permethrin on M. progresivus was suggested under laboratory conditions. Implications of repellency and lowered abundance of nontarget species for field use of permethrin as a predator exclusion agent are discussed.

Influence of Nitrogen on Population Parameters of Potato Insects: Abundance, Population Growth, and Within-plant Distribution of the Green Peach Aphid, Myzus persicae (Homoptera: Aphididae)

Population parameters of the green peach aphid (GPA), Myzus persicae (Sulzer), were compared on potatoes fertilized with different levels of nitrogen (N) (56, 84, 140, and 224 kg N/ha). Abundance of GPA was not significantly different among N treatments. The population growth rate of GPA increased with the level of N applied. However, the highest level of N (224 kg N/ha) reduced the rate of population growth. Populations grew more rapidly on potatoes fertilized with a split-applied intermediate N treatment (56 + 84 kg N/ha) than on potatoes fertilized with 56, 56 + 28, or 56 + 168 kg N/ha. Additionally, populations of GPA grew more rapidly when 224 kg N/ha was applied in one application than when applied in two applications. The rate of population growth was positively correlated with the concentration of free amino acids in leaves. The within-plant distribution of GPA was similar for all treatments. GPA were consistently more abundant on the lower leaves in each treatment. Population growth rates were greater on lower leaves than on other leaves in most N treatments. Population growth rates were negatively correlated with foliar concentrations of free amino acids within leaves of different vertical zones, but were positively correlated with foliar concentrations of nitrate N within leaves of different vertical zones. Also, population growth rates were positively correlated with the percentage of the total organic N pool composed of amino N within leaves of different vertical zones. Implications of variable host-plant nutritional quality in pest management are illustrated.