Bernhard Löhr

Functional response of Orius albidipennis (Hemiptera: Anthocoridae) to Megalurothrips sjostedti (Thysanoptera: Thripidae)

Abstract A locally available anthocorid predator, Orius albidipennis Reuter, was evaluated as a candidate biocontrol agent for Megalurothrips sjostedti Trybom, a major pest of French beans, Phaseolus vulgaris (L), in Kenya. The functional response of adult O. albidipennis to larval and adult M. sjostedti was studied at 15, 25, and 28 °C and densities of 5, 10, 20, and 30 larval and adult M. sjostedti per cage, over 24 h. More larvae and adult M. sjostedti were killed at the higher densities and with an increase in temperature. The data provided good fit to both type I and II functional response models. Attack rates increased with temperature for both the second instar larva and adult M. sjostedti. Handling times for the larvae decreased with an increase in temperature while those for the adults increased with increase in temperature. The implications of these results are discussed.

A field trial of the entomogenous fungus Metarhizium anisopliae for control of onion thrips, Thrips tabaci

Abstract In Kenya, Thrips tabaci Lind., is an important constraint to onion production. The current strategy to use synthetic pesticides is inadequate and unsustainable. An isolate of Metarhizium anisopliae (Metsch.) Sorok. with high pathogenicity to legume thrips, Megalurothrips sjostedti , and western flower thrips, Frankliniella occidentalis , was tested in field trials as a potential alternative for control of onion thrips. Weekly and bi-weekly applications of the fungus M. anisopliae and bi-weekly spray of the chemical insecticide dimethoate (Rogor ® 50) were compared for 3 seasons. M. anisopliae was applied at the rate of 1×10 11 xa0conidiaxa0ha −1 and dimethoate was applied at the recommended rate of 17.5xa0gxa0a.i.xa0ha −1 . In all the trials, thrips density and damage were significantly lower in the fungal and chemical insecticide treatments compared with the untreated control. Onion bulb yield did not differ significantly among the treatments during the first season trial. However, in the second season trial, dimethoate-treated plots provided the greatest bulb yield (17xa0metricxa0tonsxa0ha −1 ) and in the third season trial, M. anisopliae applied weekly recorded the highest yield (24xa0metricxa0tonsxa0ha −1 ). With the exception of spiders, densities of nontarget organisms were higher in plots treated with M. anisopliae than in dimethoate-treated plots. The results indicate the potential of using M. anisopliae for the control of T. tabaci while protecting biodiversity in the onion agroecosystem.

Impact of an exotic parasitoid on Plutella xylostella (Lepidoptera: Plutellidae) population dynamics, damage and indigenous natural enemies in Kenya.

Diadegma semiclausum (Hellén) (Hymenoptera: lchneumonidae), an exotic diamondback moth parasitoid, was released in two pilot areas (Werugha in Coast Region and Tharuni in Central Province) in Kenya. Fifteen month before release, observations on the diamondback moth, Plutella xylostella (Linnaeus), and local natural enemy population dynamics and pest damage were initiated in both areas and continued for three years after release. The P. xylostella population was bimodal with higher records during dry seasons. At Werugha, the peak population of P. xylostella was 16.8 per plant (October 2001); at Tharuni it was 12.8 (February 2002). Populations at Werugha declined from three months after release and decreased from 5.4 per plant (before release) to 0.8 (year 3 after release). Concurrently, average damage (1.9 to 1.5) (on a 0-5 scale), proportion of attacked plants (72 to 31%) and proportion of plants in damage group >2 (plants with head damage) decreased (21.4 to 5.3%), while total parasitism increased from 14.4 (before) to 52.5% (year 3 after release, 90% due to D. semiclausum). At Tharuni, D. semiclausum was only recovered 3 months after release. Average populations of P. xylostella declined from 5.9 per plant (before release) to 2.4 (year 3 after release) and damage scores from 2.3 to 1.7. The proportion of plants in damage group >2 declined from 39.7 to 4.5% while overall parasitism increased from 4.2 to 40.6% (98.3% by D. semiclausum). Four species of indigenous parasitoids (Diadegma mollipla (Holmgren), Oomyzus sokolowskii (Kurdjumov), Apanteles sp. and Itoplectis sp., all primary parasitoids) were almost completely displaced by D. semiclausum. Possible reasons for the different parasitoid development between the two release areas and the displacement of the indigenous species are discussed.

Evidence of Adaptation of Diamondback Moth, Plutella xylostella (L.), to Pea, Pisum sativum L.

A strain of diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), adapted to peas was detected in 1999 in the export vegetable growing area south of Lake Naivasha in the Rift Valley Province of Kenya. The pea strain (DBM-P) was compared in laboratory studies to the normal crucifer strain (DBM-C). Whereas DBM-P performed comparably well on kales and peas, the cabbage strain (DBM-C) suffered heavy mortality on peas. Out of the 250 DBM-C first instars, only six reached adult stage on pea. In addition, larval development was prolonged by five days on peas as compared to kale, and larval growth was greatly reduced. Pupal weights of DBM-C survivors on pea were significantly lower (3.8 mg) than of DBM-P (4.6 mg) and those of both strains on kale (5.7 and 5.3 mg, respectively). Neonate larvae of the pea strain mined on both kale and pea but both the proportion of larvae mining and the number of mining days were lower than for DBM-C on kale. The latter failed completely to mine on pea.A laboratory culture was started with the DBM-C survivors on pea and the performance of the progeny compared on kale and pea in three additional generations of selection. Larval survival increased from 2.4% in the first generation to 28.6%, 41.3% and 49.7% in the second, third and fourth generation, respectively. Pupal weight of larvae reared on pea increased with each generation of selection, but it remained significantly lower than of larvae reared on kale. In spite of the large differences in larval mining on the two host plants, performance on peas was not related to the ability of DBM-C to mine on pea but rather to the ability to initiate feeding without the normal stimuli present in crucifers. Pupal mortality for larvae from both hosts was similar when larvae of equal weight were compared, suggesting acceptable suitability of pea for larval development once the new host is accepted. The implications of these findings on adaptability of DBM to plants beyond its normal host range are discussed.RésuméUne souche de la fausse-teigne des crucifères, Plutella xylostella (L.) (Lepidoptera: Plutellidae), adaptée au pois, a été observée dans une zone de cultures maraîchères d’exportation au sud du Lac Naïvasha dans la province de la Rift Valley au Kenya, en 1999. La souche élevée sur le pois (DBM-P) est comparée à la souche normale élevée sur le chou (DBM-C) en laboratoire. Bien que les performances de la souche DBM-P soient comparables sur chou et sur pois, la souche (DBM-C) a subi une mortalité considérable sur le pois; sur 250 larves du premier stade, six larves seulement ont pu atteindre le stade adulte. Par ailleurs, le développement larvaire est plus long de 5 jours sur le pois par rapport au chou. La croissance larvaire est aussi considérablement réduite. Le poids des chrysalides de la souche DBM-C élevée sur pois est significativement inférieur (3,8 mg) à celui des chrysalides de la souche DBM-P (4,6 mg), ainsi qu’à celui des chrysalides des deux souches élevées sur chou (5,7 et 5,3 mg, respectivement). Les larves neonates de la souche adaptée au pois ont miné les plants de chou et de pois, mais la proportion de larves mineuses et le nombre de jours pour miner sont inférieurs à ceux de la souche DBM-C sur chou. Cette dernière a été incapable de miner le pois. Un élevage a été mis en place au laboratoire à partir des survivants de la souche DBM-C élevés sur pois et les performances des descendants ont été comparées à celles obtenues sur chou et sur pois pendant trois générations. La survie des chenilles a augmenté de 2,4% au cours de la première génération et de 28,6%, 41,3% et 49,7% au cours des générations suivantes. Sur pois, le poids des chrysalides a augmenté après chaque génération; il est cependant resté inférieur à celui des chrysalides obtenues sur chou. En dépit d’énormes différences dans la capacité des larves à miner les deux plantes-hôtes, les performances sur pois ne sont pas liées à la capacité de la souche DBM-C à miner le pois mais plutôt à la capacité de commencer à s’alimenter sans les stimuli normaux présents dans les crucifères. La mortalité des chrysalides sur les deux plantes est identique lorsque les larves de même poids sont comparées. Ce résultat suggère que le pois convient bien au développement larvaire dès lors que le nouvel hôte est accepté. On discute des conséquences de ces résultats sur l’adaptation de la fausse-teigne des crucifères à des plantes non hôtes.

A PCR-based approach to distinguish important Diadegma species (Hymenoptera: Ichneumonidae) associated with diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (Linnaeus) has a cosmopolitan distribution and is one of the major pests on cruciferous plants. Biological control, especially with species of the genus Diadegma, has been successfully employed in several parts of the world, mainly in South East Asia. The taxonomy of this genus based on classical morphological characters is still unclear and misidentifications are reported. In the present study seven Diadegma species associated with P. xylostella were separated using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analyses. The second internal transcribed spacer (ITS2) of the ribosomal DNA (rDNA) was successfully amplified in all 167 individuals and digested using 11 different restriction enzymes. One restriction enzyme (CfoI) showed different restriction profiles in all species and also between two population samples of D. mollipla (Holmgren) from eastern and southern Africa. In addition, a new Diadegma species associated with P. xylostella from Ethiopia was discovered.

Host–parasitoid population density prediction using artificial neural networks: diamondback moth and its natural enemies

1 An integrated pest management (IPM) system incorporating the introduction and field release of Diadegma semiclausum (Hellén), a parasitoid of diamondback moth (DBM) Plutella xylostella (L.), comprising the worst insect pest of the cabbage family, has been developed in Kenya to replace the pesticides‐only approach. 2 Mathematical modelling using differential equations has been used in theoretical studies of host–parasitoid systems. Although, this method helps in gaining an understanding of the systems dynamics, it is generally less accurate when used for prediction. The artificial neural network (ANN) approach was therefore chosen to aid prediction. 3 The ANN methodology was applied to predict the population density of the DBM and D. semiclausum, its larval parasitoid. Two data sets, each from different release areas in the Kenya highlands, and both collected during a 3‐year period after the release of the parasitoid, were used in the present study. Two ANN models were developed using these data. 4 The ANN approach gave satisfactory results for DBM and for D. semiclausum. Sensitivity analysis suggested that pest populations may be naturally controlled by rainfall. 5 The ANN provides a powerful tool for predicting host–parasitoid population densities and made few assumptions on the field data. The approach allowed the use of data collected at any appropriate scale of the system, bypassing the assumptions and uncertainties that could have occurred when parameters are imported from other systems. The methodology can be explored with respect to the development of tools for monitoring and forecasting the population densities of a pest and its natural enemies. In addition, the model can be used to evaluate the relative effectiveness of the natural enemies and to investigate augmentative biological control strategies.

Development and reproductive potential of diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) on cultivated and wild crucifer species in Kenya

The development, survival and reproductive potential of diamondback moth Plutella xylostella (Linnaeus) were studied at 25 ± 1 °C in the laboratory in response to two cultivated Brassica oleracea cultivars (cabbage B. oleracea var. capitata and kale B. oleracea var. acephala) and four wild crucifer species Erucastrum arabicum, Raphanus raphanistrum, Rorippa nudiuscula and Rorippa micrantha. Rorippa micrantha was the most preferred species in oviposition choice tests, while cabbage and kale were least preferred. First instar larval mining period differed significantly between plant species with the longest period recorded on cabbage (3.0 days) and the shortest on R. micrantha (0.4 days). Pupal weight was significantly lower for larvae reared on R. nudiuscula, while those of the others were similar. The developmental period from first instar to adult was the shortest on R. micrantha (14.1 days) and the longest on R. raphanistrum (15.6 days). Survival to adult was not statistically affected by the host plant species. Adult longevity ranged between 18.2 days on R. raphanistrum and 24.7 days on R. nudiuscula. The females were significantly heavier than the males on all plant species. However, males lived longer than females. Moths reared on R. nudiuscula recorded the highest fecundity (326 eggs), while moths reared on cabbage had the lowest fecundity (262 eggs). Kale and R. nudiuscula recorded the longest generation time of 31.7 days, while E. arabicum had the highest net reproductive rate (126.4 eggs per day). The highest intrinsic rate of increase was calculated for R. micrantha (0.179) and the lowest for kale (0.147). This study shows the suitability of wild crucifers as hosts for P. xylostella and indicates that they may play a major role as reservoir for the pest during the absence of cultivated host plants.

Assessing the importance of self-regulating mechanisms in diamondback moth population dynamics: Application of discrete mathematical models

The applicability of discrete mathematical models for the description of diamondback moth (DBM) (Plutella xylostella L.) population dynamics was investigated. The parameter values for several well-known discrete time models (Skellam, Moran-Ricker, Hassell, Maynard Smith-Slatkin, and discrete logistic models) were estimated for an experimental time series from a highland cabbage-growing area in eastern Kenya. For all sets of parameters, boundaries of confidence domains were determined. Maximum calculated birth rates varied between 1.086 and 1.359 when empirical values were used for parameter estimation. After fitting of the models to the empirical trajectory, all birth rate values resulted considerably higher (1.742-3.526). The carrying capacity was determined between 13.0 and 39.9DBM/plant, after fitting of the models these values declined to 6.48-9.3, all values well within the range encountered empirically. The application of the Durbin-Watson criteria for comparison of theoretical and experimental population trajectories produced negative correlations with all models. A test of residual value groupings for randomness showed that their distribution is non-stochastic. In consequence, we conclude that DBM dynamics cannot be explained as a result of intra-population self-regulative mechanisms only (=by any of the models tested) and that more comprehensive models are required for the explanation of DBM population dynamics.

Interspecific competition between Diadegma semiclausum Hellen and Diadegma mollipla (Holmgren), parasitoids of the diamondback moth, Plutella xylostella (L), feeding on a new host plant.

Interspecific competition between an introduced parasitoid species aimed at controlling a herbivorous pest species and a native parasitoid parasitising the same host may influence the success of classical biological control programmes. In Kenya, interspecific competition between an introduced and a local parasitoid on two diamondback moth populations (DBM, Plutella xylostella) was investigated on two different host plants. We tested simultaneous and delayed competition of the local parasitoid Diadegma mollipla Holmgren and its exotic congenus D. semiclausum Hellen on a newly aquired DBM host plant (snowpea) in the laboratory. Under simultaneous competition, D. mollipla produced more progeny than D. semiclausum on snowpea. A head start of D. Mollipla, of four and eight hours before its congenus was introduced, resulted in a similar number of progeny of both species. In delayed competition (time intervals of 24 h, 48 h and 72 h), progeny production was similar for both parasitoids when the time interval was 24 h, irrespective of which species parasitized first. More progeny was produced by the species which attacked first, when the time interval was greater than 24 h, although it was only significant at 72 h. Competitive abilites of both parasitoids on the new host plant differed largely between laboratory and semi-field conditions. The influence of two host plants (snowpea and cabbage) on competition was studied in the greenhouse with different host and parasitoid densities. Parasitism levels of D. semiclausum were significantly higher than those of D. mollipla, regardless of host plant, host and parasitoid densities, but progeny production of D. mollipla on snowpea was still slightly higher than on cabbage. As compared to the confinement of parasitoids and larvae to small containers, D. mollipla parasitized very few larvae in the cages. Competitive ability of the two parasitoid species tested was influenced both by the density of the searching females and by parameters related to either the host plant and/or the herbivorous hosts.

Diversity, distribution and role of wild crucifers in major cabbage and kale growing areas of Kenya

An investigation of the diversity and distribution of wild crucifer species and their importance for cultivated crucifers was conducted during 2005 and 2006 in the highland and mid-altitude semi-arid areas of Kenya. Thirteen species of wild crucifers in nine genera were recorded: Raphanus raphanistrum, Erucastrum arabicum, Sisymbrium officinale, Crambe kilimandscharica, Capsella bursa-pastoris, Rorippa nudiuscula, Ro. micrantha, Ro. microphylla, Lepidium bonariense, Coronopus didymus, Brassica rapa, B. juncea and an unidentified Brassica species. Highland areas had significantly higher species diversity and species richness than mid-altitude semi-arid areas. Species richness, diversity and evenness varied with season and location. Raphanus raphanistrum was the dominant non-cultivated species in the highlands followed by E. arabicum, which was also present and dominant in the semi-arid study sites. Diamondback moth (DBM) was recorded from ten wild crucifer species and R. raphanistrum and E. arabicum were the preferred host plant species. Overall, four larval, one larval-pupal and one pupal parasitoid of DBM were recorded: Diadegma semiclausum, D. mollipla, Apanteles sp., Cotesia plutellae, Oomyzus sokolowskii and Brachymeria species, respectively. Diadegma semiclausum was the most dominant species on all crucifers. We conclude that wild crucifers act as alternative hosts for DBM and provide refugia for DBM parasitoids, which risk local extinction through pesticide application or competition from introduced exotic parasitoid species. The wild crucifers also act as recolonization sites for DBM parasitoids.