Females Of Two-spotted Spider Mite Research Articles

Biological aspects of the two-spotted spider mite on strawberry plants under silicon application

ABSTRACT Silicon is an inducer of plant resistance to arthropod pests, being a promising strategy for integrated management. The aim of this study was to evaluate the effect of silicon on biological, reproductive and population aspects of parental and F1 generations of the two-spotted spider mite on strawberry plants. Potassium silicate, nanosilica and water were applied to the plants. Two-spotted spider mite females were confined to strawberry leaf disks for oviposition and, after hatching, larvae were observed until the emergence of adults. Once adults had been obtained, couples were formed in order to evaluate pre-oviposition, oviposition, longevity and fertility, with an estimated net reproduction rate, intrinsic rate of increase, finite rate of increase and the necessary time for doubling the generation. Silicon prolonged the duration of some immature stages of the mites in parental and F1 generations, did not affect the duration of the whole biological cycle, though. The periods of pre-oviposition, oviposition and longevity of the parental generation and the longevity and oviposition of F1 generation of the two-spotted spider mite were negatively affected by potassium silicate and nanosilica. The population parameters of the parental generation of the mites indicated that nanosilica is able to lead to a long-run decrease of this pest population.

Shelf life of essential oil of Acantholippia deserticola (Phil. ex F. Phil.) Moldenke and its acaricidal effect on two-spotted mite [Tetranychus urticae Koch (Acari: Tetranychidae)] under laboratory conditions

The acaricidal effect of two formulations of essential oil of "rika-rika" [Acantholippia deserticola Moldenke (Verbenaceae)] of 3 and 6 years of shelf life were evaluated on females of two-spotted spider mite [Tetranychus urticae (Acari: Tetranychidae)] re gistering the repellency and effects on the oviposition. The bean leaf discs [Phaseolus vulgaris Linneo (Fabaceae)] were covered with the formulation using the immersion technique under laboratory conditions (23.31 ± 2.82 49.70 ± 1.42% RH and 12:12 L:D photoperiod). The mortality of T. urticae was evaluated at 24, 48 and 72 hours post-application. The variables evaluated were: percentage of mortality by residual contact, percentage of repellency and number of eggs laid. Only the formulation at 1% (v/v) with 3 years shelf life reached a mortality of 50% after 24 hours of evaluation. The concentrations at 0.3% and 0.7% (v/v) with 3 years shelf life reached this value after 72 hours. None of the concentrations with 6 years shelf life reached values above 50% of mortality. A maximum mortality of 85.71% was registered in the concentration at 1% (v/v) with 3-year shelf life. The 3-year shelf life formulation showed repellent activity for the three periods evaluated (24, 48 and 72 hours), with values above 90% after 72 hours post-application. The ovisposture test revealed that the concentrations of 3 years shelf life had an inhibitory effect on the oviposition of the two-spotted spider. These results suggest a considerable acaricidal activity of the essential oil of A. deserticola with a 3-year shelf life on adult females of T. urticae.

Preference ofTetranychus urticae(Acari: Tetranychidae) for Kidney Beans Pre-Infested byFrankliniella occidentalis(Thysanoptera: Thripidae) and Possible Roles of Induced Bean Volatiles

Abstract We tested the selectivity of the adult female two-spotted spider mite (TSSM), Tetranychus urticae, for kidney beans subjected to various treatments, which were infested by 2nd instar larvae of western flower thrips (WFT), Frankliniella occidentalis, at different densities (5, 10, or 15 WFT/plant) and time (2, 4, or 6 days). The volatiles released from beans subjected to various treatments were evaluated by gas chromatograph-mass spectrometry. We found that TSSM was more likely to select bean plants infested for 2 days when the density was 5 WFT/plant. TSSM preferred bean plants infested for 4 days when the density was 10 or 15 WFT/plant. Irrespective of the WFT density, TSSM had the lowest preference for healthy beans and beans infested for 6 days. When the WFT infestation duration was the same, the percentage of TSSM was greatest on beans infested with 10 WFT, and there were no significant differences among the other three bean plant treatments. The results indicated that TSSM tended to prefer b...

Manifestation of resistance to insectoacaricides in inbred lines of the two-spotted spider mite under disruptive selection

The paper reports on the disruptive selection performed on two-spotted spider mite females in 20 inbred generations for resistance or sensitivity to four insectoacaricides belonging to different chemical groups: dimethoate, bifenthrin, abamectin, and bromopropylate. Due to the haplodiploid reproduction mode in mites, on average about 30% of nonresistant females were present in each generation of genotype differentiation in all acaricide-resistant lines. In single females from the lines under comparison, the electrophoretic separation of the enzymes revealed the presence of carboxylesterase fractions. Mites were tested with diagnostic concentrations of dimethoate, bifenthrin, and piridaben. The synergistic effects of the monooxygenase inhibitor piperonyl butoxide on mites were compared. It was concluded that the induction of carboxylesterase and monooxygenase activities is a universal adaptive response of the arthropod organism that underlies resistance to all acutely toxic insectoacaricides.

Olfactory response of the omnivorous mirid bug Nesidiocoris tenuis to eggplants infested by prey: Specificity in prey developmental stages and prey species

We studied olfactory responses of Nesidiocoris tenuis Reuter (Heteroptera: Miridae), an omnivorous mirid bug, to volatiles from eggplants infested by different developmental stages of tobacco cutworms, Spodoptera litura Fabricius (Lepidoptera: Noctuidae) and of female two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae). We first conducted predation experiments and found that N. tenuis preyed on first and second stadium larvae, but not on third and fourth stadium S. litura larvae. N. tenuis also preyed upon T. urticae females that had been reported to be less suitable prey species. When offered first stadium S. litura larvae and T. urticae females simultaneously, N. tenuis, preferred to prey upon S. litura larvae. We then tested the olfactory responses of N. tenuis to volatiles from eggplants using a Y-tube olfactometer. A significant preference for volatiles from uninfested eggplants over clean air was found. Volatiles of eggplants infested by either first, second or fourth stadium S. litura larvae were preferred over volatiles from uninfested eggplants. The olfactory preferences of N. tenuis to S. litura-infested eggplants were: eggplants infested by 10 first stadium larvae=eggplants infested by 10second stadium larvae>eggplants infested by 1 fourth stadium larva. Volatiles from eggplants infested by 200 T. urticae females also attracted N. tenuis, but the volatiles were not as attractive as those from eggplants infested by 10 first stadium larvae. Chemical analyses showed that volatiles from eggplants infested by different developmental stages of S. litura larvae had qualitative differences. First stadium larvae induced higher amounts of volatiles than second and fourth stadium larvae, despite the first stadium S. litura larvae doing the least damage. T. urticae female-induced volatiles were both qualitatively and quantitatively different from those induced by first stadium S. litura larvae. The specific olfactory responses of N. tenuis to different prey species/developmental stages were discussed in terms of prey availability and biological control.

CARACTERIZAÇÃO QUÍMICA E EFEITO DO ÓLEO ESSENCIAL DE ERVA-DE-SANTA-MARIA SOBRE O ÁCARO-RAJADO DE MORANGUEIRO

O ácaro-rajado, Tetranychus urticae Koch (1836) (Acari: Tetranychidae), é a principal praga na cultura do morangueiro. O objetivo deste trabalho foi a caracterização química do óleo essencial de Chenopodium ambrosioides L. e a avaliação de seus efeitos sobre o ácaro-rajado. Os compostos do óleo essencial foram identificados pelasanálises de CG-DIC e CG-EM. A mortalidade e a oviposição foram avaliadas após a pulverização do óleo (2,5; 5,0; 7,5 e 10,0% v v-1) sobre fêmeas do ácaro-rajado. O rendimento do óleo essencial foi de 0,3%, e os compostos identificados foram (Z)-ascaridol (87%), (E)-ascaridol (5,04%), p-cimeno (4,83%), a- terpineno (1,24%) e piperitona (0,7%). Os maiores valores para mortalidade e redução de oviposição observados foram 85% e 69%, respectivamente. O óleo essencial de C. ambrosioidesé uma opção promissora para o manejo do ácaro-rajado.

Interactions between two-spotted spider mite (Tetranychus urticae Koch) and western flower thrips (Frankliniella occidentalis Pergande) on cucumber plants treated with selected biostimulatorsInterakcje między przędziorkiem chmielowcem (Tetranychus urticae Koch) i wciornastkiem zachodnim (Frankliniella occidentalis Pergande) na roślinach ogórka traktowanych wybranymi biostymulatorami

Summary Studies were conducted in the Department of Applied Entomology at Warsaw University of Life Sciences, in laboratory conditions. The aim of this study was to determine mutual effect of western flower thrips (Frankliniella occidentalis Pergande) and two-spotted spider mite (Tetranychus urticae Koch) on selected biology paramete rs on cucumber plants trited with biostimulators Asahi SL and Siapton 10 L. It was found the significant decrease of fecundity of western flower thrips females on cucumber leaves infested by two-spotted spider mite, specialy in the case of strong infestation. An application of biostimulators had a additional negative influence on western flower thrips fecundity and time of larvae hatching. The fecundity of two-spotted spider mite females, feeding on the cucumber leaves injured by thrips was lower as compared to healthy leaves. An application of biostimulators increased number of eggs laid by females on the leaves taken from sprayed plants as compared to control, in both infested and not infested by western flower thrips.

<p><strong>Spirotetramat toxicity to immatures and sublethal effects on fecundity of female adults of </strong><strong><em>Tetranychus urticae </em></strong><strong>Koch*</strong></p>

Acute toxicity of spirotetramat to immatures and its sublethal effects on adult females of two-spotted spider mite, Tetranychus urticae Koch, were investigated in laboratory bioassays, at 27 ± 2°C, 50–80% RH and 16/8 L/D photoperiod.Acute toxicity was tested by successive treatments of eggs, larvae, protonymphs and deutonymphs on bean leaf discs, and mortality was evaluated based on the number of mites reaching the adult stage. Concentration-mortality data were subjected to probit analysis and the following LC50 data were computed: 0.10 mg/l (larvae), 0.17 mg/L (protonymphs) and 0.15 mg/L (deutonymphs). The acaricidal effect after the treatment of eggs was not the result of spirotetramat toxicity to this developmental stage, but rather of its residual activity on hatched larvae (LC50=0.62 mg/L). Sublethal effects were evaluated after the treatment of pre-ovipositing adult females on leaf discs using the concentrations of 2 mg/L, 20 mg/L and 200 mg/L.After 18–20 hours of exposure on treated discs, females showing no visible symptoms of poisoning were transferred to untreated leaf discs and re-transferred to new discs at 48 h intervals over ten days. Based on the number of eggs laid and survival rate of females, gross fecundity and net fecundity were calculated. At the end of the tenth day, the survival rate was 0.72 in untreated and 0.40, 0.27 and 0.05 in females treated at the concentrations mentioned above. Compared with the control mites, gross fecundity was reduced by 9% (2 mg/L), 29% (20 mg/L) and 93% (200 mg/L), while net fecundity decreased by 40%(2 mg/L), 67%(20 mg/L) and 98%(200 mg/L). The results indicate that spirotetramat exerts similar effects on spider mites as do the acaricides spirodiclofen and spiromesifen.

Diapause-inducing signals prolong nymphal development in the two-spotted spider miteTetranychus urticae

Female two-spotted spider mite Tetranychus urticae are grown under different photoperiods and the photoperiodic regulation of diapause is examined. The photoperiodic response curve for diapause induction was of the long day–short day type, with critical day lengths (CDLs) of 2 and 12.5 h; diapause was induced between these CDLs. The preimaginal period is significantly longer in diapausing females than in non-diapausing females; moreover, a significant positive correlation is detected between diapause incidence and deutonymphal period. Diapause incidence is high when long-night photoperiods are applied against a background of continuous darkness in the stages including the deutonymph; this stage appears to be the most sensitive to photoperiod. These observations suggest that diapause-inducing conditions inhibit nymphal development, particularly in the deutonymphal stage when photoperiodic time measurement for determination of reproduction or diapause is carried out.

The abundance of Orius niger (Wolf.) and O. minutus (L.) in potato fields and their life table parameters when fed on two prey species

Two-spotted spider mite (TSSM) and onion thrips are serious pests of potatoes in the Ardabil region (Iran). In the present study, anthocorid species were identified in potato fields of this region during 2006 and 2007. The results of the abundance study indicate that Orius niger (Wolff) and O. minutus (Linnaeus) are major predators of these pests in potato fields. The life table parameters of these predators were compared when they were fed 2nd instar larvae of onion thrips or female TSSM on potato leaves. In these experiments, O. niger had a lower nymphal mortality, longer oviposition period, higher net reproductive rate (R 0), and higher intrinsic rate of natural increase (r m ) when fed thrips instead of mites. O. minutus feeding on mites compare to thrips had a lower nymphal mortality, longer oviposition period, higher net reproductive rate, and higher intrinsic rate of natural increase. Based on these results, it can be concluded that these predators could be useful as biological agents in potato fields.

Effects of spirodiclofen on the reproductive potential of two-spotted spider mite (Acari: Tetranychidae) ovipositing females

A laboratory bioassay was conducted to evaluate the effects of spirodiclofen on the survival and reproduction of young and mated females of two-spotted spider mite (Tetranychus urticae Koch). The females were sprayed with a series of acaricide concentrations (96, 48, 24, 12, and 6 mg/l) 24-30 h after adult emergence, i.e., at the age most likely to exhibit dispersal behavior and close to their reproductive maximum. The proportions of T. urticae females that survived treatment without symptoms of poisoning were concentration-dependent, ranging between 0.41 and 0.88 (0.96 in the control). With the exception of females that survived 6 mg/l, fecundity of the treated female mites was strongly affected during the exposure, compared to the control. The mean daily fecundity (EL) and mean daily fertility (EH) of surviving females, transferred daily to new leaf disks over the following five days, significantly decreased as spirodiclofen concentrations increased. In treatments with 6 mg/l and 12 mg/l, only the latter concentration significantly reduced both EL and EH, compared to the control. In females that survived 24 mg/l and 48 mg/l, these life history parameters were reduced by over 90%, while treatment with 96 mg/l completely terminated egg-laying. The treated females lived for a significantly shorter time than untreated ones, with the exception of females that survived 6 mg/l. Compared to the control females, gross fecundity (GL) and gross fertility (GH) of the treated females were strongly reduced on the first and second day; from the third day onward, females treated with the lowest concentrations achieved marked recovery, their GL and GH going even above the values in the control. However, net fecundity (NL) and net fertility (NH) of all treated females decreased considerably throughout the trial, indicating that survival rates of these females were lower, compared to the control. Calculated as total sums of gross and net daily schedules within five days, fecundity and fertility significantly decreased as spirodiclofen concentration increased. The two lowest concentrations failed to achieve a significant reduction of GL, while GH, NL, and NH were significantly lower than control values starting with the females treated with 6 mg/L. A high percentage of unhatched eggs, especially during the initial two days after treatment (35-100%), further contributed to the significant reduction in fertility of the females treated with spirodiclofen. All concentrations of spirodiclofen significantly reduced the instantaneous rate of increase. Regression analysis showed a linear population decline with increased acaricide concentrations (y = 1.13 - 0.24x; R2 = 0.91, p < 0.05).

Evaluation of the predatory mite Phytoseiulus macropilis (Acari: Phytoseiidae) as a biological control agent of the two-spotted spider mite on strawberry plants under greenhouse conditions

The predatory mite Phytoseiulus macropilis is a potential biological control agent of the two-spotted spider mite (TSSM) Tetranychus urticae on strawberry plants. Its ability to control TSSM was recently assessed under laboratory conditions, but its ability to locate and control TSSM under greenhouse conditions has not been tested so far. We evaluated whether P. macropilis is able to control TSSM on strawberry plants and to locate strawberry plants infested with TSSM under greenhouse conditions. Additionally, we tested, in an olfactometer, whether odours play a role in prey-finding by P. macropilis. The predatory mite P. macropilis required about 20 days to achieve reduction of the TSSM population on strawberry plants initially infested with 100 TSSM females per plant. TSSM-infested plants attract an average of 27.5 +/- 1.0% of the predators recaptured per plant and uninfested plants attracted only 5.8 +/- 1.0% per plant. The predatory mites were able to suppress TSSM populations on a single strawberry plant and were able to use odours from TSSM-infested strawberry plants to locate prey in both olfactometer and arena experiments. Hence, it is concluded that P. macropilis can locate and reduce TSSM population on strawberry plants under greenhouse conditions.

“Sleeping with the enemy”—predator-induced diapause in a mite

Diapause in arthropods is a physiological state of dormancy that is generally thought to promote survival during harsh seasons and dispersal, but it may also serve to avoid predation in space and time. Here, we show that predation-related odours induce diapause in female adult spider mites. We argue that this response allows them to move into an area where they are free of enemies, yet forced to survive without food. Spider mites are specialised leaf feeders, but--in late summer--they experience severe predation on leaves. Hence, they face a dilemma: to stay on the leaf and risk being eaten or to move away from the leaf and risk death from starvation and thirst. Female two-spotted spider mites solve this dilemma by dramatically changing their physiology when exposed to predation-associated cues. This allows them to disperse away from leaves and to survive in winter refuges in the bark of trees or in the soil. We conclude that the mere presence of predation-associated cues causes some herbivorous mites to seek refuge, thereby retarding the growth rate of the population as a whole: a trait-mediated indirect effect that may have consequences for the stability of predator-prey systems and for ecosystem structure.

Effects of extending the light phase on diapause induction in a Japanese population of the two-spotted spider mite, Tetranychus urticae

Artificial lighting is a merit of a 'plant factory', which might be utilized to suppress an increase in pest population. We investigated the effects of extending the light phase on diapause induction in the two-spotted spider mite (TSSM), Tetranychus urticae. TSSM were reared at 18 degrees C under light phases ranging from 2 to 64 h combined with a constant dark phase of 16 h in aluminum bottles, with white light emitting diodes attached inside to minimize fluctuations in air temperature between the light and dark phases. Diapause was induced in adult TSSM females when the light phase was 24 h or shorter, and diapause induction was inhibited when the light phase extended over 32 h. The development of deutonymphs was delayed under a diapause-inducing photoperiod. Diapause inducing photoperiods may suppress an increase in the TSSM population, by slowing down development and reproduction.

Overhead and Drip-tube Irrigation Affect Twospotted Spider Mites and their Biological Control by a Predatory Mite on Impatiens

The effects of overhead and drip tube irrigation on twospotted spider mite (TSMs) (Tetranychus urticae Koch) and predatory mite (PMs) (Phytoseiulus persimilis Athias-Henriot) populations, as well as the biological control of TSMs by PMs, were investigated on Impatiens wallerana Hook. f. `Impulse Orange'. To determine the effects of the two irrigation methods on TSM populations, plants were inoculated with female TSMs 6 weeks after seeding. Plants were then irrigated twice every three days, and TSM counts were taken 3 weeks later. To assess the effects of irrigation method on PMs, plants were inoculated with TSMs 6 weeks after seeding, PMs were released 10 days later, plants were irrigated about once per day, and the number of predatory mites on plants was counted 3 weeks after release. To assess the effects of irrigation method on the biological control of TSMs by PMs, plants were inoculated with TSMs and PMs were released as before, but then plants were irrigated either three times every 2 days or three times every 4 days using either drip or overhead irrigation. The number of TSMs on plants and the number of leaves showing TSM feeding injury were measured 3 weeks after predator release. Overhead watering significantly reduced TSM and PM populations as much as 68- and 1538-fold, respectively, compared to drip irrigation with microtubes. Perhaps more important, overhead watering with or without predators significantly reduced the number of leaves sustaining TSM feeding injury as much as 4-fold compared to drip irrigation. These results confirm the common observation that TSM infestations and injury may be reduced by irrigation systems that wet plant foliage. However, predators still reduced TSMs even though overhead irrigation had a suppressive effect on predatory mites. Predators are particularly useful for reducing TSM injury when plants are watered infrequently. Overhead watering could be used in tandem with biological control as a component of an integrated crop management program for TSMs in ornamental greenhouses by rapidly lowering TSM population levels in hot spots before PMs are released.

Host plant-induced changes in detoxification enzymes and susceptibility to pesticides in the twospotted spider mite (Acari: Tetranychidae).

Adult female twospotted spider mites, Tetranychus urticae Koch, reared on lima bean plants were moved to cucumber, maize, or new lima bean plants (the latter being a control) and evaluated after 24 h or 7 d for changes in susceptibility to three pesticides and in levels of related detoxification enzymes. The largest and most consistent changes were observed in mites feeding on cucumber. Susceptibility of mites on cucumber to the synthetic pyrethroids bifenthrin and lambda-cyhalothrin was greater than that of mites reared on lima bean and maize after only 24 h on the plants, and remained higher after 7 d. Mites on cucumber also were more susceptible to the organophosphate dimethoate than were mites on lima bean, but only after 7 d on the host. Susceptibility was inversely related to activities of both general esterase and glutathione S-transferase (GST) in mites on cucumber; general esterase and GST activities were 60 and 25% lower, respectively, than activities of twospotted spider mite on lima bean after 7 d of feeding. Mites on maize were slightly but significantly more susceptible than those on lima bean to bifenthrin, but not to lambda-cyhalothrin, after 7 d and to dimethoate after 24 h but not after 7 d. General esterase and GST activities in twospotted spider mite fed on maize for 24 h were 20 and 16% higher, respectively, than activities in twospotted spider mite on lima bean, but general esterase activity was 30% lower than lima bean-fed mites and GST was not different after 7 d. Thus, plant-induced changes in general esterase activity, perhaps in combination with GST activity, in twospotted spider mite appear to be inversely related to, and possibly responsible for, changes in susceptibility of twospotted spider mite to several pesticides, particularly the synthetic pyrethroids. General esterases appear to play less of a role in the detoxification of the organophosphate insecticide dimethoate.

A Rickettsia in the Ovary of the Twospotted Spider Mite, Tetranychus Urticae KOCH (Acarina: Tetranychedae)

Rickettsias are small, gram-negative Alpha Proteobacteria that are, with one exception, obligate intracellular parasites. They are found in a wide range of arthropods. Some of these bacteria are arthropod-vectored disease agents of vertebrates. For example, rickettsias are the causative agents of human typhus and Rocky Mountain spotted fever. Others, such as Wolbachia, are important manipulators of their arthropod host's reproductive capabilities and are not currently known to be disease agents of vertebrates.Research on Wolbachia and related rickettsias has increased dramatically in recent years. Most of the attention has been focused on insects, although there are occasional reports of rickettsiae being found in isopods and mites. Most recently, the use of molecular techniques has established that Wolbachia is widespread in spider and predatory mites. However, Wolbachia or other rickettsias have not been demonstrated ultrastructurally in the spider mites. This report documents the presence of a rickettsia in the ovary of the adult female twospotted spider mite, Tetranchus urticae Koch, an economically important pest of ornamental and crop plants.

"Inert" formulation ingredients with activity: toxicity of trisiloxane surfactant solutions to twospotted spider mites (Acari: Tetranychidae).

Organosilicone molecules are important surfactant ingredients used in formulating pesticides. These methylated silicones are considered inert ingredients, but their superior surfactant properties allow them to wet, and either suffocate or disrupt important physiological processes in mites and insects. Aqueous solutions of the tri-siloxane surfactants Silwet L-77, Silwet 408, and Silwet 806 were bioassayed against adult female two-spotted spider mites, Tetranychus urticae Koch, with leaf dip methods to compare their toxicity with organosilicone molecules containing bulkier hydrophobic components. All three tri-siloxanes in aqueous solutions were equivalently toxic (LC50 = 5.5-8.9 ppm), whereas Silwet L-7607 solutions were less toxic (LC50 = 4,800 ppm) and Silwet L-7200 was nontoxic to mites. In another experiment, the toxicity of Silwet L-77 was affected by the wettability of leaf surfaces. The LC50 shifted from 22 to 84 ppm when mites were tested on bean and strawberry leaf disks, respectively. Droplet spreading on paraffin and surface tension were both related to the toxicity of surfactant solutions. Surface tensions of solutions below 23 mN/m caused > 90% mite mortality in leaf dip bioassays. A field test of Conserve SC and its formulation blank, with and without Dyne-Amic adjuvant (a vegetable oil-organosilicone surfactant mixture) revealed that Dyne-Amic had the greatest miticidal contribution, reducing mite populations by 70%, followed by formulation inactive ingredients. Spinosad, the listed active ingredient in Conserve, only contributed miticidal activity when synergized by Dyne-Amic. Researchers should include appropriate surfactant or formulation blank controls when testing insecticides or miticides, especially when using high spray volumes.

Nitrogen availability and defense of tomato against two-spotted spider mite.

The aim of this work was to study how nitrogen availability affects within-plant allocation to growth and secondary metabolites and how this allocation affects host selection by herbivores. Tomato plants (Lycopersicon esculentum) were grown at six levels of nitrogen availability. When nitrogen availability increased, plant relative growth rate increased, but tissue carbon/nitrogen ratio in the second oldest true leaf and allocation to large glandular trichomes (type VI) as well as to the defense compounds rutin and chlorogenic acid decreased. Leaf protein concentration increased. Two-spotted spider mite (Tetranychus urticae) females responded significantly to these changes: in dual choice tests they preferred leaf disks from plants grown at high nitrogen availability, with a low C/N ratio. This preference persisted in an olfactometer in which the mites were offered only the odors released by leaves with damaged trichomes. We conclude that in a tomato leaf increased nitrogen availability leads to decreased allocation to defenses, and that repellent volatiles released by trichomes play a key role in affecting leaf selection by two-spotted spider mite females.

Binomial Sequential Sampling Plans for Adult Female Twospotted Spider Mites (Acari: Tetranychidae) on Hops

Binomial sequential sampling plans, based on Wald’s sequential probability ratio test, were developed for adult female twospotted spider mites, Tetranychus urticae Koch, on main stem hop leaves at the 2 m elevation for early season sampling and at the 2 + 4 m heights combined for mid- to late season sampling. The 2 + 4 m sampling plan was tested by sampling data sets, which were generated by the Monte Carlo method. Increasing the tally threshold improved the operating characteristic curves and sampling plan robustness and caused the average sample number and percent of maximum sample termination curves to be more peaked around the critical density. Increasing the tally threshold generally reduced the magnitude of sequential sampling errors. Sequential decision errors made at means above the critical density were more serious than errors made below it. When the maximum of 100 samples was taken, the data set mean was generally above the critical density. Therefore, when 100 samples have been taken and no decision has been reached, the sampler should be aware that the field mean is near, and likely greater than, the critical density. The sampling plans were based on a nominal threshold of 5 adult female spider mites per leaf. The plans could be modified using data in this article to develop sampling plans based on an economic threshold when one is determined.