Broccoli growth parameters and level of head infestations in simple and mixed plantings: Impact of increased flora diversification

The impacts of interplanting broccoli, Brassica oleracea var. italica L., with chili pepper, Capsicum annuum L., or yellow sweetclover (YSC), Melilotus officinalis L., on broccoli growth parameters and levels of lepidopteran head infestations were studied during two field seasons (winter & summer). During both seasons, several broccoli growth characteristics (e.g. plant vertical area, whole plant biomass, etc.) were significantly reduced in broccoli‐YSC habitats. However, broccoli heads in broccoli‐YSC habitats did not differ significantly in size from other treatment habitats. Additionally, these heads contained the lowest mean lepidopteran contaminants per broccoli crown during the winter season. During this period, the average number of lepidopteran contaminants per broccoli head was more than twice that in monoculture and pepper intercropped broccoli than in broccoli‐YSC habitats. During the summer experiment, the average number of lepidopteran contaminants per broccoli head was greater in pepper intercropped broccoli than in both monoculture and YSC interplanted broccoli. Potential causes of broccoli growth differences and their relationship to lepidopteran ovipositional behavior are discussed.     

Population densities of herbivorous lepidopterans in diverse cruciferous cropping habitats: Effects of mixed cropping and using a living mulch

Two field experiments were conducted in Hawaii to determine the responses of lepidopteran pests and their associated natural enemies to broccoli (Brassica oleracea), grown in monoculture or interplanted with chili pepper (Capsicum annuum), or yellow sweetclover (YSC) (Melilotus officinalis). The population responses of Artogeia rapae and Trichoplusia ni to habitat types were not consistent during the field trials. Additionally, their reactions to habitat diversification differed according to non-host plant species. For example, Trichoplusia ni densities were significantly lower in the broccoli–YSC habitat contrasted with the broccoli–pepper habitat on most sampling dates. Similarly, the proportion of plants damaged by Hellula undalis was significantly lower in the broccoli–YSC habitat contrasted with broccoli–pepper habitat. Findings from the study, suggest that biological control agents (e.g., spiders, parasitoids, and␣entomopathogens) are the most significant factors influencing the abundance of A.␣rapae in habitat type and that these agents inflict greater mortality on A. rapae in diculture than monoculture habitats.     

Effects of crop diversification levels and fertilization regimes on abundance of Brevicoryne brassicae (L.) and its parasitization by Diaeretiella rapae (M’Intosh) in broccoli

1 The effects of intercropping via competition on crop yields, pest [cabbage aphid Brevicoryne brassicae (L.)] abundance, and natural enemy efficacy were studied in the Brassica oleracea L. var. italica system. 2 From May to December 2004, insect populations and yield parameters were monitored in summer and autumn in broccoli monoculture and polyculture systems with or without competition from Brassica spp. (mustard), or Fagopyrum esculentum Moench (buckwheat), with addition of organic (compost) or synthetic fertilizer. 3 Competition from buckwheat and mustard intercrops did not influence pest density on broccoli; rather, aphid pressure decreased and natural enemies of cabbage aphid were enhanced in intercropping treatments, but this varied with the intercropped plant and season (summer vs. autumn). 4 In compost‐fertilized broccoli systems, seasonal parasitization rates of B. brassicae by Diaeretiella rapae (M’Intosh) increased along with the expected lower aphid pressure compared with synthetically fertilized plants.     

Odour masking of tomato volatiles by coriander volatiles in host plant selection of Bemisia tabaci biotype B

The silverleaf whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), is one of the most important pest insects in tomato crop systems worldwide. It has been previously demonstrated that intercropping tomato [Solanum lycopersicum L. Mill. (Solanaceae)] with coriander [Coriandrum sativum L. (Apiaceae)] reduces the incidence and severity of damage caused by B. tabaci. However, it is not yet known how coriander affects the insect′s behaviour. We evaluated the attractiveness of tomato constitutive volatiles to B. tabaci and what effect coriander constitutive volatiles have on the insect′s behaviour. To this end, we conducted three bioassays in a multiple‐choice four‐arm olfactometer (‘×’ type), measuring B. tabaci behaviour when offered tomato and coriander constitutive volatiles presented alone as well as together. We also evaluated the colonisation and establishment of B. tabaci in experimental plots with only single tomato plants and tomatoes intercropped with coriander in a greenhouse. Bemisia tabaci males and females recognised tomato constitutive volatiles as a positive stimulus (kairomonal effect), indicating that semiochemicals from this plant can play an important role in the insect’s host plant selection. Coriander constitutive volatiles reduced the attractiveness of tomato volatiles but no repellency to these volatiles was observed. Greater numbers of adults and nymphs of B. tabaci per plant were observed in tomato monoculture plots than in tomato intercropped with coriander. We suggest that coriander constitutive volatiles have an odour masking effect on tomato volatiles, thus interfering in the host plant selection of B. tabaci.     

Chitinolytic enzymes from Streptomyces albidoflavus expressed in tomato plants: effects on Trichoplusia ni

Tomato (Lycopersicon esculentum ) cultivars were transformed with genes that encode bacterial chitinolytic enzymes (i.e., endochitinase and chitobiosidase) from Streptomyces albidoflavus. Transgenic tomato plants producing these enzymes were found to have enhanced resistance to cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), consistently reducing the growth rates of larvae. Mortality was significantly increased in two of three feeding trials. Ingestion of endochitinase and chitobiosidase not only affected development of larval T. ni from neonate to ultimate instar, but they also caused mortality and decreased insect weight when exposure began during the third instar. The results of this study provide some insight into the mode of action of the chitinolytic enzymes, by supporting the hypothesis that ingested chitinolytic enzymes damage the chitin component of the peritrophic envelope, leading to increased permeability. The size of marker molecules (FITC-dextrans) that permeated the peritrophic envelopes of T. ni feeding on transgenic plants were 50% larger than those permeating the peritrophic envelopes of T. ni feeding on the control plants. Further research is needed to more clearly identify the sites and modes of action of these chitinolytic enzymes, and the potential for synergy between these enzymes and pathogens, allelochemicals, and other environmental factors.     

Neighbouring monocultures enhance the effect of intercropping on the turnip root fly (Delia floralis)

Knowledge of insect behaviour is essential for accurately interpreting studies of diversification and to develop diversified agroecosystems that have a reliable pest‐suppressive effect. In this study, we investigated the egg‐laying behaviour of the turnip root fly, Delia floralis (Fall.) (Diptera: Anthomyiidae), in an intercrop‐monoculture system. We examined both the main effect of intercropping and the effect on oviposition in the border zone between a cabbage monoculture [Brassica oleracea L. var. capitata (Brassicaceae)] and a cabbage‐red clover intercropping system [Trifolium pratense L. (Fabaceae)]. To investigate the border‐effect, oviposition was measured along a transect from the border between the treatments to the centre of experimental plots. Intercropping reduced the total egg‐laying of D. floralis with 42% in 2003 and 55% in 2004. In 2004, it was also found that the spatial distribution of eggs within the experimental plots was affected by distance from the adjoining treatment. The difference in egg‐laying between monoculture and intercropping was most pronounced close to the border, where egg‐laying was 68% lower on intercropped plants. This difference in egg numbers decreased gradually up to a distance of 3.5 m from the border, where intercropped plants had 43% fewer eggs than the corresponding monocultured plants. The reason behind this oviposition pattern is most likely that flies in intercropped plots have a higher probability of entering the monoculture if they are close to the border than if they are in the centre of a plot. When entering the monoculture, flies can pursue their egg‐laying behaviour without being disrupted by the clover. As the final decision to land is visually stimulated, flies could also be attracted to fly from the intercropped plots into the monoculture, where host plants are more visually apparent. Visual cues could also hinder flies in a monoculture from entering an intercropped plot. Other possible patterns of insect attack due to differences in insect behaviour are discussed, as well as the practical application of the results of this study.     

Indirect plant-mediated effects on insect immunity and disease resistance in a tritrophic system

Host plant quality can significantly influence the growth and condition of phytophagous insects, and consequently their susceptibility to pathogens. This study examined the relationship between host plant quality, insect condition, immune responsiveness and resistance to pathogens in the cabbage looper, Trichoplusia ni. Two baseline and induced immune parameters were estimated, haemocyte numbers and haemolymph phenoloxidase (PO) activity, for larvae on two host plants, broccoli and cucumber. Haemolymph protein concentration was assessed as an indication of insect condition, and the susceptibility of larvae to T. ni single nucleopolyhedrovirus (SNPV) was used as a measure of disease resistance. T. ni growth, survival and condition was much higher on broccoli than cucumber. Haemocyte numbers were significantly higher in broccoli-reared larvae, whereas PO activity was not. An immune challenge induced significantly elevated numbers of haemocytes for larvae reared on both host plants, but did not affect PO activity or protein concentrations. Susceptibility to T. ni SNPV was markedly higher in larvae reared on cucumber than on broccoli. These results clearly indicate that host plant quality can affect both immune response and disease resistance of T. ni larvae and that bottom-up effects could be important in interactions between insects and entomopathogens.     

Does a multi‐plant diet benefit a polyphagous herbivore? A case study with Bemisia tabaci

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a highly polyphagous herbivore. This research was conducted to compare the development of B. tabaci reared in a multi‐plant treatment (polyculture) with those in single‐plant treatments (monocultures). Adult B. tabaci females fed on a mixture of tomato, cabbage, cotton, cucumber, and kidney bean survived longer and laid more eggs than those fed exclusively on one of these plant species. Egg numbers per plant laid in the polyculture treatment were positively correlated with those laid on the same plant species in the monoculture treatments, and egg numbers per plant laid on tomato, cotton, and cucumber in the polyculture were significantly higher than those laid on the same plants in the monocultures. Concentrations of total protein and trehalose in B. tabaci were not significantly different after 7 days of feeding in the respective treatments, but activities of superoxide dismutases (SOD) and alkaline phosphatase (AKP) of B. tabaci in polyculture were lower than those in monoculture. Conversely, activities of trehalase, sucrase, and amylase in B. tabaci kept in polyculture were higher than those of insects from the monoculture. In each of the monoculture treatments, there was a negative correlation between AKP in B. tabaci and oviposition, and also between AKP and amylase. SOD and sucrase activities in B. tabaci were positively correlated with polyphenol oxidase (PPO) and peroxidase (POD) activities in plants. In the plants damaged by whiteflies in the polyculture treatment, activities of SOD in cucumber, PPO in cotton and kidney bean, and POD in tomato and cucumber were lower than those in the monoculture treatments, whereas SOD in cabbage and catalase (CAT) in tomato in the polyculture treatment were higher than those in the monoculture treatments.     

The effect of host plant species on performance and movement behaviour of the cabbage looper Trichoplusia ni and their potential influences on infection by Autographa californica multiple nucleopolyhedrovirus

• 1 Cabbage loopers Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) are serious pests in greenhouses growing tomatoes, cucumbers and bell peppers. A potential microbial control, now in development, is the broad host‐range virus Autographa californica multiple nucleopolyhedrovirus (AcMNPV).
• 2 The relationships between the three host plants and the feeding behaviour, larval movement and performance of cabbage looper larvae that might relate to their interaction with AcMNPV applications were investigated.
• 3 Larvae reared on cucumber plants consumed approximately ten‐fold more leaf area than larvae reared on pepper plants and almost five‐fold more than larvae reared on tomato plants. This could influence the amount of AcMNPV consumed if it were used as a microbial spray because increased consumption can be associated with increased probability of infection. Survival from neonate to pupa also varied, with the greatest being on cucumber, followed by tomato and pepper plants. Larvae fed cucumber were approximately four‐fold heavier than larvae fed tomato and over 15‐fold heavier than larvae fed pepper plants.
• 4 The distribution of larvae on plants in commercial greenhouses where a single crop was being grown also varied with food plant with 73% being found on the bottom and middle portions of tomato plants and 87% occurring in the top portions of pepper plants. Larvae tended to be distributed on the middle portion of cucumber plants, the lower portion of tomato plants and the top portion of pepper plants. Larval movement did not vary between AcMNPV‐infected and uninfected controls.
• 5 It is predicted that the higher leaf area consumption and location of larvae in the middle portion of cucumber plants may make them more susceptible to viral sprays. Furthermore, given their greater survival than larvae fed tomato and pepper, there may be a greater need for virus applications.

     

Intraspecies mixture exerted contrasting effects on nontarget arthropods of Bacillus thuringiensis cotton in northern China

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Effects of host age on parasitism and progeny allocation in Trichogrammatidae

The impact of host age on the number of hosts killed, survival of progeny, progeny allocation, and sex allocation was examined for several Trichogrammatidae (Hymenoptera) species in laboratory choice tests. Individual female parasitoids were provided with young, medium-aged and old eggs of one of three lepidopterous host species: Trichoplusia ni (Hübner) (Noctuidae), Pieris rapae (L.) (Pieridae), or Plutella xylostella (L.) (Plutellidae). Trichogrammatid species behaved as gregarious parasitoids with the first two host species, and as solitary parasitoids with eggs of the smaller latter one. They mostly preferred young eggs of T. ni, but did not discriminate among P. rapae eggs of different ages, and often preferred young or medium-aged P. xylostella eggs over old eggs. Survival of progeny did not vary constantly with host age, although it was often very low in P. rapae eggs of any age. Clutch size frequently decreased with host age in both T. ni and P. rapae. Offspring sex ratio did not change with age of T. ni and P. rapae eggs, and rarely did so in P. xylostella eggs. In regard to host age, the results with T. ni are the ones which are the most in agreement with optimal foraging theoretical predictions, as clutch size was the highest in preferred younger eggs.     

Maternal behaviours may explain riffle‐scale variations in some stream insect populations

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Evaluating the enemies hypothesis in a clover‐cabbage intercrop: effects of generalist and specialist natural enemies on the turnip root fly (Delia floralis)

• 1 The relative importance of the resource concentration hypothesis and the enemies hypothesis was investigated for the turnip root fly Delia floralis in a cabbage–red clover intercropping system compared with a cabbage monoculture.
• 2 Delia floralis egg densities were measured as well as the activity‐densities of generalist predators in a field experiment during two growing seasons. In the second year, a study of egg predation with artificially placed eggs was conducted, in addition to a predator exclusion experiment, to estimate total predation during the season. Parasitization rates were estimated from samples of pupae.
• 3 Delia floralis oviposition was greater in the monoculture during both years. The predator activity‐densities differed between treatments and study years. The known natural enemies of Delia spp., Bembidion spp. and Aleochara bipustulata showed a strong response to a cultivation system with higher activity‐densities in the monoculture. The response, however, appeared to be caused primarily by habitat preferences and not by D. floralis egg densities.
• 4 The reduction in the number of D. floralis pupae in the intercropping may be explained by a disruption in oviposition behaviour caused by the presence of clover because neither predation, nor parasitization rates differed between cultivation systems.

     

Within‐field spatial distribution patterns of corn planthopper,Peregrinus maidis,and severity of hopperburn and Maize mosaic virus symptoms as influenced by sunn hemp intercropping

Habitat management (e.g., intercropping) may alter within‐field spatial distribution patterns of herbivores, from a typical pattern as observed in a monoculture, and may influence patterns of crop injury. Field trials were conducted to study the effect of intercropping maize, Zea mays L. (Poaceae), with sunn hemp, Crotalaria juncea L. (Fabaceae) strips on within‐field spatial distribution patterns of corn planthopper, Peregrinus maidis (Ashmead) (Hemiptera: Delphacidae), and combined severity of hopperburn and Maize mosaic virus (MMV) (Rhabdoviridae: Nucleorhabdovirus) symptoms. In each field trial, spatially explicit data on P. maidis counts and ratings of severity of symptoms were obtained by sampling maize plants at weekly intervals. These data were used to examine the spatial patterns of P. maidis and severity of symptoms in maize‐intercropped and monoculture plots with Spatial Analysis for Distance IndicEs (SADIE) methodology. Spatial aggregation patterns of P. maidis in each treatment plot were not consistent among the field trials and tended to be mediated by their population densities. Interpolation of local cluster indices showed that P. maidis were more often aggregated at the field edges, irrespective of treatment. At times of MMV incidence in field trials (fall 2010 and spring 2011), the patch clusters of P. maidis and symptomatic plants were located at the field edges, but were spatially unassociated in both treatment plots. The results provided an approximation of the unpredictability of P. maidis spatial patterns at different population densities and their association with severity of symptoms in two maize‐cropping systems. However, the gap clusters of symptomatic plants were primarily located at the field interiors and were larger in intercropped than in monoculture plots. Such spatial pattern of symptomatic plants resulted in the reduced incidence of MMV in the intercropped plot compared with the monoculture plot, suggesting intercropping sunn hemp can be a useful tool in the management of MMV in maize fields.     

The effect of host developmental stage at parasitism on sex‐related size differentiation in a larval endoparasitoid

• 1 For their larval development, parasitoids depend on the quality and quantity of resources provided by a single host. Therefore, a close relationship is predicted between the size of the host at parasitism and the size of the emerging adult wasp. This relationship is less clear for koinobiont than for idiobiont parasitoids.
• 2 As size differentiation in host species exhibiting sexual size dimorphism (SSD) is likely to occur already during larval development, in koinobiont larval endoparasitoids the size of the emerging adult may also be constrained based on the sex of the host caterpillar.
• 3 Sex‐specific growth trajectories were compared in unparasitised Plutella xylostella caterpillars and in second and fourth instar hosts that were parasitised by the solitary larval koinobiont endoparasitoid Diadegma semiclausum. Both species exhibit SSD, where females are significantly larger than males.
• 4 Healthy female P. xylostella caterpillars developed significantly faster than their male conspecifics. Host regulation induced by D. semiclausum parasitism depended on the instar attacked. Parasitism in second‐instar caterpillars reduced growth compared to healthy unparasitised caterpillars, whereas parasitism in fourth‐instar caterpillars arrested development. The reduction in growth was most pronounced in hosts producing male D. semiclausum.
• 5 Parasitism itself had the largest impact on host growth. SSD in the parasitoid is mainly the result of differences in growth rate of the parasitoid–host complex producing male and female wasps and differences in exploitation of the host resources. Female wasps converted host biomass more efficiently into adult biomass than males.

     

Impact of flowering buckwheat on Lepidopteran cabbage pests and their parasitoids at two spatial scales

We assessed the potential of annual buckwheat, Fagopyrum esculentum Moench, to lead to improved parasitism of lepidopteran cabbage pests over four years. Pest, parasitism, and hyperparasitism rates were monitored in replicated cabbage plots (12 × 20 m) with or without 3 m wide buckwheat borders from 2000 to 2003. Floral borders did not significantly increase egg, larval, or pupal densities of cabbage looper, Trichoplusia ni (Hübner), imported cabbageworm, Pieris rapae (L.), or diamondback moth, Plutella xylostella (L.). Buckwheat increased parasitism rates by Voria ruralis (Fallen) on T. ni larvae and Cotesia rubecula (Marshall) on P. rapaelarvae over four years. Parasitism by Diadegma insulare (Cresson) on P. xylostella larvae was higher in buckwheat than control plots in the first year, and parasitism by Euplectrus plathypenae (Howard) on T. ni larvae was lower in buckwheat than control plots in the second year. The hyperparasitoid Conura side (Walker) attacked D. insulare all four years, but buckwheat did not affect hyperparasitism rates. The effect of spatial scale on pest densities and parasitism in 2001 was evaluated by comparing plots separated at least 67 m (nearby) versus 800 m apart (isolated). T. ni pupae and P. rapae eggs and pupae were more abundant in plots in closer proximity, whereas P. xylostella densities did not vary by the spatial separation of plots. Tachinids and Pteromalus puparum (L.) attacked more P. rapae in nearby plots. E. plathypenae responded to the treatment × scale interaction, parasitizing more in control than buckwheat when plots were isolated but not when plots were nearby.     

Effect of cropping systems on cereal stemborers in the cool‐wet and semi‐arid ecozones of the Amhara region of Ethiopia

1 Field experiments were conducted on maize and sorghum at three locations in the Amhara state of Ethiopia to determine the effects of mixed cropping on stemborer infestation, borer natural enemies and grain yields. In the cool‐wet ecozone of western Amhara, sole maize was compared with maize intercropped with faba bean, mustard, potatoes and cowpea. In the semi‐arid ecozone of eastern Amhara, the trial was conducted on both maize and sorghum with the companion crops haricot bean, sesame, cowpea and sweet potatoes. 2 The results showed that the predominant borer species in western and eastern Amhara were, respectively, Busseola fusca and Chilo partellus. In Addis Zemen, western Amhara, maize intercropped with mustard and potatoes had significantly lower pest numbers and percent tunnelling than other intercrops and the maize monocrop during the vegetative stage. In eastern Amhara, the cropping system did not significantly affect pest densities but damage to stem, ear or heads tended to be greatest when cereals were intercropped with sweet potatoes. 3 Parasitism of C. partellus by the braconid Cotesia flavipes was greater on maize than sorghum, and on maize it was greater with sweet potatoes than in other intercrops or sole maize. Cocoon mass number per plant did not vary significantly between treatments. 4 There were significant differences between treatments in yields of both sorghum and maize (per plant and per unit area) with the lowest yields observed when they were intercropped with a tuber crop. 5 The results suggest that simultaneous planting of the crop species selected has little advantage over monocropped maize.     

Effects of crop type on Bacillus thuringiensis toxicity and residual activity against Trichoplusia ni in greenhouses

We assessed the efficacy and persistence of a Bacillus thuringiensiskurstaki (Btk) formulation (Dipel) against Trichoplusia ni (Hubner) (Lep., Noctuidae), the cabbage looper, on three greenhouse vegetable crops (tomato, bell pepper and cucumber). First, T. ni larvae were fed leaf discs treated with Btk to assess how Btk toxicity varies with host plant. Secondly, T. ni larvae were fed leaf discs harvested from plants that had been sprayed with Btk 1, 5 and 9 days previously to assess the residual activity of Btk toxicity in greenhouse environments. Mortality of T. ni larvae fed tomato leaf discs was significantly higher than T. ni fed cucumber or pepper leaf discs. The toxicity of Btk had declined by less than 50% after 9 days, which suggests that Btk persistence is lengthy in greenhouse environments. No crop effects on the residual activity of Btk were found. These results demonstrate that the greenhouse environment and the crop should be considered when using Btk for insect management on greenhouse crops.     

Insect-crop interactions in a diversified cropping system: parasitism by Aleochara bilineata and Trybliographa rapae of the cabbage root fly, Delia radicum, on cabbage in the presence of white clover

Parasitism of the cabbage root fly, Delia radicum (L.) by the staphylinid Aleochara bilineata Gyllenhal and the cynipid Trybliographa rapae Westwood was examined in a cabbage monoculture and a mixed stand of cabbage undersown with white clover. Number of overwintering cabbage root fly pupae per plant was consistently reduced in the mixed stand, and the incidence of plants attacked by cabbage root fly was either reduced or not different in the mixed stand compared to cabbage monoculture. For both parasitoids, the probability of D. radicum attacked plants having at least one parasitized pupa increased with density of cabbage root fly pupae around the plant. For A. bilineata, this positive relation between presence of parasitism and host density was consistently stronger in cabbage monoculture than in cabbage undersown with clover. Location of a host plant by T. rapae was not consistently affected by the presence of clover. D. radicum attacked plants situated in the cabbage and clover mixture were found by T. rapae as easily as in cabbage monoculture. Overall, the total risk of parasitism for a cabbage root fly pupa by A. bilineata was reduced in the mixed stand compared to the cabbage monoculture, whereas the risk of parasitism by T. rapae was not consistently affected by clover. For both parasitoids, intensity of parasitism showed a variable relationship with host density on individual plants attacked by the cabbage root fly. Overall, in spite of consistently lower total density of pupae in the mixed cabbage—clover than in cabbage monoculture, the density of unparasitized pupae was reduced by the presence of non-host plants only in two of the four experiments. The results emphasize the need to include not only herbivore and crop, but also other plant species as well as natural enemies when evaluating management methods.