Xylostella Populations Research Articles

The Resistance Monitoring of Plutella xylostella (L.) Population Againts Residual of Emamectin Benzoate by Diagnostic Concentration Determination

The Resistance Monitoring of Plutella xylostella (L.) Population Againts Residual of Emamectin Benzoate by Diagnostic Concentration Determination

Efficacy of Azadirachta excelsa vinegar against Plutella xylostella

ABSTRACTThe diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is considered a major pest for crucifers in Malaysia. New control materials are constantly required as P. xylostella population develops resistance to most of the insecticides used against them. In this study, vinegar derived from Azadirachta excelsa (Jack) Jacobs was evaluated against the third-instar larvae of P. xylostella and an appropriate application dose required to control the pest was determined in the laboratory trial. Results indicated A. excelsa vinegar to be a useful control measure, significantly reducing P. xylostella leaf consumption and adult size of P. xylostella. Interestingly, mortality rate of larval, pupae and adult was significantly increased by the application of A. excelsa vinegar. An in-diet EC50 of 80× against larvae was achieved, whilst obvious defects in adult moth were also recorded. The results indicate that A. excelsa vinegar exhibits potential to be used against P. xylostella.

Seasonal proportion change of ryanodine receptor mutation (G4946E) in diamondback moth populations.

This study examined the seasonal proportion change of ryanodine receptor mutation (G4946E) for Plutella xylostella populations using quantitative sequencing. Results showed that the proportions of G4946E generally increased from spring through summer, but then decreased in autumn. Furthermore, the proportions in late autumn were similar to those in early spring of the subsequent year. These results suggest that diamide effectiveness for P. xylostella control in the reference year can be evaluated based on a proportion survey in the prior year.

Resistance monitoring of Plutella xylostella (L.) (Lepidoptera: Plutellidae) to risk-reduced insecticides and cross resistance to spinetoram

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae) is the major pest of the Brassicaceae family. In the Agreste region of Pernambuco (Brazil), a Brassica producing center, P. xylostella is under continuous selection pressure by insecticides leading to increased frequency of resistance genes in this species. The objective of the present study was to monitor the resistance of P. xylostella populations to spinosad, chlorfenapyr, and chlorantraniliprole as well as the susceptibility to spinetoram in the Agreste region of Pernambuco (Brazil). Concentration-response bioassays were performed with spinosad, chlorfenapyr and spinetoram to estimate the LC values. Furthermore, a concentration corresponding to label dose for registered products (chlorantraniliprole, spinosad, and chlorfenapyr) and diagnostic concentration for chlorantraniliprole and spinosad were used. The LC50 to spinosad varied from 0.017 (Recife) to 3.64 (Bezerros II) mg a. i./L (RR50 varying up to ~200-fold), while for spinetoram, they varied from 0.0013 (Alegre) to 0.198 (Bezerros II) mg a. i./L (RR50 varying up to ~150-fold). The LC50 for chlorfenapyr ranged from 0.43 (Recife) to 42.23 (Bezerros II) mg a. i./L (RR50 varying up to ~100-fold). Plutella xylostella populations developed resistance to chlorfenapyr and spinosad. Apparently, there was no alteration of the resistance levels to chlorantraniliprole in the field. Cross-resistance between spinosad and spinetoram (RR50 ~ 150-fold) as well as between the spinosyns and chlorfenapyr was observed. The field populations were susceptible to concentration corresponding to label doses of spinosad (mortality > 80 %) and chlorfenapyr (mortality > 80 %), but a considerable loss of susceptibility to both insecticides was observed. Immediate attention to correct rotation of products is necessary to delay development of resistance in this pest.

Detection of insecticidal resistance against various insect pests in vegetable crops at Raipur

Bioassay methods such as leaf dip and larvae dip were used in the laboratory conditions for testing lethal dose of insect pests conducted at laboratory of department of entomology, IGKV, Raipur (C.G.) during 2009-2010. At least five dilutions for each of the selected insecticides were tested using different methods. In each method and insecticide at least 10 larvae of 2-3 rd instars were released on each dilution in 3 replications along with untreated control. The results shown that the Helicoverpa armigera population was maximum LD 50 value to Chlorpyriphos (O.633 μg/lit) followed by Spodoptera litura population LD 50 (0.576 μg/lit) and lower value for Leucinodes arbonalis LD 50 (0.503 μg/lit) and the Plutella xylostella population showed maximum LD 50 value to Cypermethrin (0.810μg/lit) and lower LD 50 value (0.246 μg/lit) for Trichoplusia ni. Therefore, H. armigera showed higher resistance to Chlorpyriphos and P. xylostella showed higher resistance towards Cypermethrin.

Studies on intra-specific variations in the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae) under different geographical regions

Studies on intraspecific variations in insect pests are an important tool for preparation of management strategy in different geographical regions. In this paper we tried to establish that these variations are also found in the diamondback moth (P. xylostella) populations under north Indian conditions with respect to reproductive biology.The pest populations were collected from five different geographical regions viz. Hisar (800feet), Kangra (2200feet), Solan (4200 feet), Theog (7500feet) and Kinnaur (9000feet) and then reared under laboratory conditions at 25±1oC on cauliflower. The life table analysis revealed that the female from the Kangra population laid maximum eggs (332.16 eggs/female) whereas the number of eggs laid by the female from the Hisar, Solan, Theog and Kinnaur population was 189.53, 207, 252 and 270 eggs/female, respectively. The doubling time (DT) was observed to be the lowest for Kangra (3.12 days) and maximum for the Hisar (4.59 days) population, whereas weekly multiplication rate was minimum for the Solan (2.252) and maximum for the kangra (4.73) population. The true generation time was the lowest for the Kangra (18.54 days) and highest for the Hisar (24.38 days) population. The true intrinsic rate of increase (rm) was found to be maximum (0.222 female progeny/female/day) for the Kangra whereas for the Kinnaur, Theog, Solan and Hisar population it was 0.203, 0.202, 0.182 and 0.151 female progeny/female/day thereby indicating that the Kangra population is more prolific as compared to the population from other geographical regions. The results are indicative of geographical variations among different populations of P. xylostella.

Pesticide resistance in Plutella xylostella (Lepidoptera: Plutellidae) populations from Togo and Benin

AbstractThe diamondback moth,Plutella xylostella(L.) is the major insect pest of cabbage crops in Togo and Benin. For control, farmers very often resort to spraying chemical insecticides at high dosages with frequent applications. Bioassays were carried out on three populations ofP. xylostella, two from Togo (Kara and Dapaong) and one from Benin (Cotonou), to assess their level of susceptibility to currently used insecticides. A reference strain ofP. xylostellafrom Matuu in Kenya was used as a control. In the laboratory, three insecticide representatives of different chemical families (deltamethrin, chlorpyrifos ethyl and spinosad) were assayed against third instar larvae ofP. xylostella. Results revealed thatP. xylostellapopulations from Dapaong, Kara and Cotonou were more resistant to deltamethrin (13 to 59-fold at LC50level, 149 to 1772-fold at LC90level) and chlorpyrifos ethyl (5 to 15-fold at LC50level, 9 to 885-fold at LC90level) than the reference strain. Spinosad was more toxic toP. xylostellapopulations than the other insecticides with LC50and LC90values less than 1 µg/ml and 15 µg/ml, respectively. However, the population from Cotonou appeared significantly more resistant to spinosad compared to the reference strain. These results are discussed in the light of developing an integrated pest management strategy for reducing the selection pressure of spinosad.

Determinants of parasitoid assemblages of the diamondback moth, Plutella xylostella, in cabbage farmer fields in Senegal

Conservation biological control, which fosters the optimal use of indigenous natural enemies, is a promising way for reducing pesticide reliance in horticultural systems. A two-year field survey was conducted in the main cabbage-producing area in Senegal (Niayes) to assess the potential of indigenous parasitoids to control populations of Plutella xylostella (Lepidoptera, Plutellidae). Results showed an overall low level of parasitism (11.7%) which was independent of host abundance, but was highly variable among fields (0–50%). Parasitism was predominant in the late part of dry season. Insecticide use, mostly relying on broad-spectrum insecticides, had a negative effect on the overall parasitism rate. Observations conducted throughout the cabbage crop cycle showed that parasitism unexpectedly decreased with crop aging (from 41 to 60 days post transplanting), likely due to repeated insecticide applications. Four main parasitoid species including Oomyzus sokolowskii (Kurdjumov) (Eulophidae) (48.8%), Apanteles litae Nixon (Braconidae) (32.5%), Brachymeria sp. (Chalcididae) (11.3%), and Cotesia vestalis Haliday (Braconidae) (7.3%) were identified. Parasitism due to O. sokolowskii was greater during the first part of the dry season whereas parasitism due to A. litae was greater during the second part of the dry season. Parasitism due to Brachymeria sp. was not affected by time of season but was greater in the Centre and North than in the South of Niayes. Parasitism due to C. vestalis was equal in the three zones but was higher in the late part of the dry season. The diversity of parasitoids was constant across zones but was greater during the second part of dry season. A positive relationship between diversity (Shannon diversity index H′) and parasitism rate was observed, suggesting a positive effect of parasitoid diversity on natural pest control. Parasitoids have a promising role to play as biocontrol agents of P. xylostella populations in Senegal, provided significant changes to current insecticide use are made. Better knowledge of their resource requirements including crop and non-crop habitats, and provision of these in and around crops is also needed.

Biotic factors are more important than abiotic factors in regulating the abundance of Plutella xylostella L., in Southern Brazil

The serious economic loss caused Plutella xylostella L., 1758 in several regions of the world has prompted a demand for alternative management strategies. In this context, understanding the key factors governing the population dynamics of the pest is important for development of management strategies. This study aimed to identify the larval parasitoids associated with P. xylostella and investigate the biotic (crop subspecies, plant age and parasitism) and abiotic factors (minimum and maximum temperatures, rainfall, relative humidity and planting season) affecting the population dynamics of the pest in organic crops located in Southern Paraná State, Brazil. Despite the continuous and abundant availability of host plants throughout the year, P. xylostella occurred between June and November, and the largest peaks of abundance were observed between August and September, when low temperatures and rainfall were recorded. According to the stepwise regression analysis, P. xylostella was more abundant in broccoli during winter. Neither temperature, nor rainfall significantly influenced pest abundance. Four species of larval parasitoids were identified associated with the pest, of which Diadegma leontiniae (Brèthes) (Hymenoptera: Ichneumonidae), Apanteles piceotrichosus Blanchard (Hymenoptera: Braconidae) and Siphona sp. Meigen (Diptera: Tachinidae) were abundant, while Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae) was rarely found. Parasitism was the major factor influencing population dynamics of P. xylostella, contributing to 48% of the variation in pest abundance. These results show the importance of larval parasitoids complex in regulating P. xylostella population and that the temperature and rainfall recorded during field experiments did not influenced pest abundance.

Genetic differentiation of the regional Plutella xylostella populations across the Taiwan Strait based on identification of microsatellite markers.

Movement of individuals through events, such as storms or crop transportation, may affect survival and distribution of insect pests, as well as population genetic structure at a regional scale. Understanding what factors contribute to gene flow in pest populations remains very important for sustainable pest management. The diamondback moth (Plutella xylostella) is an insect pest well known for its capacity of moving over short to long distances. Here, we used newly isolated microsatellite markers to analyze the genetic structure of nine populations across the Taiwan Strait of China (Taiwan and Fujian). A total of 12,152 simple sequence repeats (SSRs) were initially identified from the P. xylostella transcriptome (~94Mb), with an average of 129 SSRs per Mb. Nine SSRs were validated to be polymorphic markers, and eight were used for this population genetic study. Our results showed that the P. xylostella populations could be divided into distinct two clusters, which is likely due to the year-round airflows in this region. A pattern of isolation by distance among the local populations within Fujian was found, and may be related to vegetable transportation. Considering the complexity of the P. xylostella population genetic structure from local and regional to global levels, we propose that developing ecologically sound strategies for managing this pest will require knowledge of the link between behavioral and population ecology and its genetic structure.

Pest control and resistance management through release of insects carrying a male-selecting transgene.

BackgroundDevelopment and evaluation of new insect pest management tools is critical for overcoming over-reliance upon, and growing resistance to, synthetic, biological and plant-expressed insecticides. For transgenic crops expressing insecticidal proteins from the bacterium Bacillus thuringiensis (‘Bt crops’) emergence of resistance is slowed by maintaining a proportion of the crop as non-Bt varieties, which produce pest insects unselected for resistance. While this strategy has been largely successful, multiple cases of Bt resistance have now been reported.One new approach to pest management is the use of genetically engineered insects to suppress populations of their own species. Models suggest that released insects carrying male-selecting (MS) transgenes would be effective agents of direct, species-specific pest management by preventing survival of female progeny, and simultaneously provide an alternative insecticide resistance management strategy by introgression of susceptibility alleles into target populations. We developed a MS strain of the diamondback moth, Plutella xylostella, a serious global pest of crucifers. MS-strain larvae are reared as normal with dietary tetracycline, but, when reared without tetracycline or on host plants, only males will survive to adulthood. We used this strain in glasshouse-cages to study the effect of MS male P. xylostella releases on target pest population size and spread of Bt resistance in these populations.ResultsIntroductions of MS-engineered P. xylostella males into wild-type populations led to rapid pest population decline, and then elimination. In separate experiments on broccoli plants, relatively low-level releases of MS males in combination with broccoli expressing Cry1Ac (Bt broccoli) suppressed population growth and delayed the spread of Bt resistance. Higher rates of MS male releases in the absence of Bt broccoli were also able to suppress P. xylostella populations, whereas either low-level MS male releases or Bt broccoli alone did not.ConclusionsThese results support theoretical modeling, indicating that MS-engineered insects can provide a powerful pest population suppressing effect, and could effectively augment current Bt resistance management strategies. We conclude that, subject to field confirmation, MS insects offer an effective and versatile control option against P. xylostella and potentially other pests, and may reduce reliance on and protect insecticide-based approaches, including Bt crops.

Insight into the Migration Routes of Plutella xylostella in China Using mtCOI and ISSR Markers.

The larvae of the diamondback moth, Plutella xylostella, cause major economic losses to cruciferous crops, including cabbage, which is an important vegetable crop in China. In this study, we used the mitochondrial COI gene and 11 ISSR markers to characterize the genetic structure and seasonal migration routes of 23 P. xylostella populations in China. Both the mitochondrial and nuclear markers revealed high haplotype diversity and gene flow among the populations, although some degree of genetic isolation was evident between the populations of Hainan Island and other sampling sites. The dominant haplotypes, LX1 and LX2, differed significantly from all other haplotypes both in terms of the number of individuals with those haplotypes and their distributions. Haplotypes that were shared among populations revealed that P. xylostella migrates from the lower reaches of the Yangtze River to northern China and then to northeastern China. Our results also revealed another potential migration route for P. xylostella, i.e., from southwestern China to both northwestern and southern China.

A Revised Calculation Method for Correcting Population Density in the Field: A Case ofPlutella xylostella(Lepidoptera: Plutellidae)

In this paper, we introduce a revised model to estimate the corrected initial number of each life stage in field life tables of insect pest populations, with a case example of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae). To validate our revised model, we used life table data of P. xylostella from 9 field surveys conducted by the corresponding author in 1996. In these surveys, P. xylostella infestations had been monitored in fields of flowering Chinese cabbage, long white radish, butter cabbage, Shanghai pak choi, and Chinese kale to investigate the impact of different pest management methods on the pest populations. In the past, the numbers of individuals per life stage on any given sampling date were calculated based on sampling data of that particular date and an average temperature determined for the entire sampling period. In several cases, this approach produced unrealistic survival rates (above 100%) for certain life stages in the resulting life tables. This problem continued even after the model was adjusted by calculating the duration of each life stage based on the actual temperature measured on each sampling date (instead of using one average temperature for the entire period). With temperature being an important factor that (i) affects the development time of insect life stages and (ii) can be determined easily during field surveys, we previously hypothesized that including corrected initial numbers of each life stage on each sampling date based on an average of the temperature between the given sampling date and its preceding sampling date will result in realistic and precise life tables. Furthermore, to estimate accurate survival rates, we here hypothesized that (i) it is important to adjust and correct the numbers of life stages on a given sampling date by including the numbers of preceding life stages from a preceding sampling date (not from the given sampling date) in the model; and (ii) the development time of the preceding life stage will determine which sampling date needs to be included in the calculation. By re-constructing and comparing 9 life tables of P. xylostella populations according to the previous and the revised new model, we confirmed these hypotheses. The revised model will allow a precise and realistic evaluation of control efforts against diamondback moth and other insect pest infestations in agriculture.

Efeito de Bacillus thuringiensis nas características biológicas do predador Orius insidiosus Say (Hemiptera: Anthocoridae) alimentado com ovos de Plutella xylostella L. (Lepidopteta: Plutellidae)

A traça-das-crucíferas, Plutella xylostella (L.) (Lepidoptera: Plutellidae), é considerada uma das pragas mais importantes de brassicáceas (Brassicaceae) em todo o mundo. A praga ocorre todos os anos no Brasil, onde o seu manejo é feito principalmente com controle químico por causa de sua conveniência e rápida ação inicial. O uso indiscriminado de inseticidas afeta organismos não-alvo e tem sido associado com o aumento da resistência de populações de P. xylostella. Estas preocupações despertaram o interesse para o uso de outras estratégias de controle, que incluem o uso de microrganismos entomopatogênicos como Bacillus thuringiensis Berliner e do predador Orius insidiosus (Say) (Hemiptera: Anthocoridae). O objetivo foi avaliar o efeito de B. thuringiensis (B. thuringiensis aizawai GC 91, Agree®) nas características biológicas de O. insidiosus. Os predadores foram alimentados com ovos de P. xylostella tratados com água destilada (controle) ou com suspensão de B. thuringiensis (0,7 g/0,5 L). Os seguintes aspectos biológicos foram avaliados: duração, sobrevivência e consumo durante o período ninfal; e consumo, número de ovos por fêmea e viabilidade de ovos durante a fase adulta. Os parâmetros de tabela de vida de fertilidade também foram avaliados para os tratamentos com ovos tratados e não tratados com B. thuringiensis. Os resultados mostraram que dentre os parâmetros de O. insidiosus a duração de ninfas de segundo ínstar, consumo ninfal e longevidade de fêmeas foram afetados pela presença de B. thuringiensis nos ovos tratados. Fêmeas alimentadas com ovos tratados mostraram menor número da progênie, que consequentemente levou a uma menor taxa de crescimento populacional.

Toxicity of Insecticides against the Diamondback moth, Plutella xylostella L. and Its Parasitoid, Cotesia plutellae, on Cauliflower Crop.

Field studies on efficacy of different insecticides against the diamondback moth, Plutella xylostella L. and their toxicity to parasitoid,Cotesia plutellae on cauliflower crop were carried out on farmers fields. The insecticides tested were abamectin, emamectin benzoate, lufenuran, spinosad, endosulfan, profenophos, and a mixture of endosulfan +lufenuran. The pretreatment observation was taken 24 hrs before and post treatment observations were recorded 48, 72 and 96 hrs, 7 and 15 days after application of insecticides. On overall basiss abamectin and emamectin benzoate were found to be the most effective insecticides against Plutella xylostella, followed by profenophos and lufenuron with P. xylostella population of 1.75, 2.12, 3.69, and 4.12 insects per plant, respectively. While, spinosad and lufenuron were found comparatively less toxic to parasitoid, C. plutellae, followed by endosulfan with parasitism of 36.74, 36.72 and 35.65%, respectively. Whereas, abamectin was highly toxic to C. plutellae, with parasitism of 19.83% only.

Incomplete control of the diamondback moth, Plutella xylostella, by the parasitoid Cotesia vestalis in a cabbage field under tropical conditions

Immature Plutella xylostella (Lepidoptera: Plutellidae) and parasitoids were sampled for 39 months in an unsprayed cabbage field near Cotonou, Benin, to determine how and when host-parasitoid interactions influence the population dynamics of the moth in a tropical environment. Eighty-three samples were taken at approximately two-week intervals. There were no seasonal patterns in the abundance of immature moths, which was not correlated with weather variables, although heavy rainfall during the principal rainy season may have temporarily affected the population. The host-parasitoid system consisted almost exclusively of P. xylostella and its larval parasitoid Cotesia vestalis (Hymenoptera: Braconidae), both species occurring at similar levels of abundance (on average 7.5 ± 0.3 and 7.2 ± 0.3 individuals per plant, respectively). The tendency for host-parasitoid dynamics to cycle was apparent in the field. P. xylostella and C. vestalis showed coupled oscillations in abundance, with a time lag of about two weeks between host and parasitoid peaks. High parasitoid abundance resulted in significant decreases in moth abundance over several weeks. However, the parasitoid population in turn decreased, could not prevent the moth from rebounding, and there was no stable control of the pest. We conclude that under tropical conditions in which P. xylostella populations grow rapidly, combined with a high probability of recolonization from surrounding areas, biological control by a well-established specialist parasitoid reaches its limits and additional control measures are necessary.

Geographical variations in life histories of Plutella xylostella in China

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most important pests of crucifers worldwide. Susceptibility to insecticides and host plants of P. xylostella vary geographically. To investigate local adaptation, we measured the variation of the biology, life-histories and life-table parameters of P. xylostella populations from five widely ranging geographical regions in China, Beijing (BJ), Shandong (SD), Shaanxi (SX), Yunnan (YN), and Guangdong (GD), using the same variety of cabbage (Brassicae oleracea L. var. capitata; var. “Qingan 80”) as the food plant at the same temperature in the laboratory. Principal components analysis showed the life-history and life-table parameters of P. xylostella differed among the five geographical populations. The first component, including female fecundity and adult longevity and male adult longevity, accounted for the most variation among the five geographical regions, which can be classified into three groups: BJ–SX, YN–GD, and SD. The intrinsic rate of increase (rm) was greatest in BJ and SX, intermediate for GD and YN, and the lowest for SD, whereas mean generation time (T) was greatest in SD, intermediate for YN and GD, and the lowest for BJ and SX. These variations reflect the importance of local genetic adaptation and should be considered when planning management of P. xylostella in China.

Geographical variation of Plutella xylostella (Lepidoptera: Plutellidae) populations revealed by mitochondrial COI gene in China

AbstractThe diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most serious pests for cruciferous vegetable growers worldwide. To determine the relations of local and regional populations, we performed a mitochondrial COI gene analysis of eight P. xylostella populations from different locations in or around the Qinling Mountains and two other populations, one from Beijing and the other from Guangdong. The mtDNA divergences among the 10 populations were high, and 32 haplotypes were detected in 149 adults. The mean haplotype divergence was 1.7% (range 0.04–4.1%). Haplotype diversity in the 10 populations varied from 0.571 (AK) to 0.885 (HZ), and the nucleotide diversity varied from 0.00286 (AK) to 0.0117 (HZ). The results also did not show significant correlation between genetic and geographical distance. Also, the effective number of migrants between populations (Nm) ranged from 1.43 to infinite, suggesting that population exchange and gene flow among the P. xylostella populations occurred. However, principal component analysis (PCA) showed that the TB and TC populations were differentiated from other populations, indicating possible across‐mountain barrier to migration and gene flow.

Natural enemies delay insect resistance to Bt crops.

We investigated whether development of resistance to a Bt crop in the presence of a natural enemy would be slower than without the natural enemy and whether biological control, in conjunction with a Bt crop, could effectively suppress the pest population. Additionally, we investigated whether insecticide-sprayed refuges of non-Bt crops would delay or accelerate resistance to the Bt crop. We used a system of Bt broccoli expressing Cry1Ac, a population of the pest Plutella xylostella with a low frequency of individuals resistant to Cry1Ac and the insecticide spinosad, and a natural enemy, Coleomegilla maculata, to conduct experiments over multiple generations. The results demonstrated that after 6 generations P. xylostella populations were very low in the treatment containing C. maculata and unsprayed non-Bt refuge plants. Furthermore, resistance to Bt plants evolved significantly slower in this treatment. In contrast, Bt plants with no refuge were completely defoliated in treatments without C. maculata after 4–5 generations. In the treatment containing sprayed non-Bt refuge plants and C. maculata, the P. xylostella population was low, although the speed of resistance selection to Cry1Ac was significantly increased. These data demonstrate that natural enemies can delay resistance to Bt plants and have significant implications for integrated pest management (IPM) with Bt crops.

Effects of plant availability on population size and dynamics of an insect community: diamondback moth and two of its parasitoids

To understand the effect of plant availability/structure on the population size and dynamics of insects, a specialist herbivore in the presence of two of its parasitoids was studied in four replicated time-series experiments with high and low plant availabilities; under the latter condition, the herbivore suffered from some periods of resource limitation (starvation) and little plant-related structural refuges. Population dynamics of the parasitoid Cotesia vestalis was governed mainly by the delayed density-dependent process under both plant setups. The parasitoid, Diadegma semiclausum, under different plant availabilities and different coexistence situations (either +competitor or -competitor) showed dynamics patterns that were governed mainly by the delayed density process (significant lags at weeks 2-4). Both the competing parasitoids did not experience beneficial or costly interferences from each other in terms of their own population size when the plant resource was limited. Variation in the Plutella xylostella population under limited plant availability is higher than that under the other plant setup. For both parasitoids, under limited plant setup, the extinction risk was lower when parasitoids were engaged in competition, while under the unlimited plant setup, the mentioned risk was higher when parasitoids competed. In this situation, parasitoids suffered from two forces, competition and higher escaped hosts.