Management of the lepidopteran pest complex in cabbage: Augmentative biological control strategies in different landscape contexts
This project examines the effect of landscape complexity, measured as the proportion of natural areas, on the abundance of naturally occurring predatory arthropods and pest infestations in cabbage fields. We also provide empirical evidence of how the effect of augmentative releases of the spined soldier bug, Podisus maculiventris (Hemiptera: Pentatomidae) and the ladybeetle Hippodamia convergens (Coleoptera: Coccinellidae), are influenced by the surrounding landscape. We selected pairs of cabbage fields (one with augmentative biological control and the other unmanaged) along a landscape complexity gradient to determine the effects of augmented and resident natural enemies on biocontrol levels and consequent plant damage and yield. We also used sentinel prey and surveillance video cameras to identify the natural enemies that were attacking lepidopteran pests in the field and to determine their efficiency rates. Our research documented the diversity of predatory arthropods associated with cabbage in New York State and demonstrated that landscape-level processes can have large impacts on predator and pest populations. We also found that augmentative releases of predators can enhance biocontrol of lepidopteran pests in cabbage fields. Although field data collection has been completed, further data analysis will be conducted during the spring semester of 2016 in accordance with the project’s timetable.
Two objectives were addressed over the course of the 2015 field season:
- Determine the effects of landscape complexity mediated by natural enemy communities on lepidopteran pest control and cabbage yield.
- Examine the role of landscape complexity on the effectiveness of augmentative releases of predators for biological control of lepidopteran pests.
The field data collection was conducted at 11 farms throughout the Finger Lakes Region in NY State during the summer and fall of 2015. The farms were selected along a gradient of agricultural intensification, which was measured within a radius of 1000 m around our experimental plots. Landscapes ranged from simple, comprised primarily of cropland (73% cropland), to complex landscapes, characterized by a high proportion of semi-natural habitats (2% cropland).
Our experimental plots consisted of ten 7.5 m rows of plants spaced 0.9 m between rows, with 0.4 m spacing between plants with a total of 150 plants per plot. Cabbage seedlings were transplanted by hand the second and third week of June of 2015. Throughout the growing season, weeds were removed manually at two-week intervals and no insecticides or fungicides were applied to the plants.
Arthropod sampling and plant damage
At each site, the abundance and diversity of arthropods were monitored by using pitfall traps, yellow sticky cards, and visual plant inspection. Arthropods were sampled at biweekly intervals starting 15 days after transplanting and continuing until 90 days after transplanting when plants were approaching commercial harvest. In addition, on each sampling date, ten randomly selected plants per plot were removed from the ground to determine pest abundance and overall insect damage (Fig 2). The damage was quantified by the method of Lim et al. (1986), where a plant is classified into one of the following four classes based on the percentage of damage: <5, 5-20, 5-60, or ≥ 60%. At harvest, the yield was estimated by weighing the final biomass of 15 mature cabbage heads per plot.
Natural enemy identity and biocontrol rates
In each farm, we established two experimental plots separated by at least 200 m. One plot was used as a control (no biocontrol augmentation) and the other was used to release Podisus maculiventris and Hippodamia convergens (Fig. 1). Predators were purchased from commercial suppliers (Green Methods and Arbico organics) and released at the rate of 0.5-1 nymphs/plant and 4-5 adults/plant for P. maculiventris and H. convergens, respectively. Predators were released three times during the growing season at biweekly intervals beginning 15 days after transplanting.
To measure predation rates provided by resident and augmented predators, we used four types of sentinel preys: Trichoplusia ni larvae, T.ni eggs, Plutella xylostella pupae and mealworms. To examine the effects of predators on sentinel eggs, paper discs containing approximately 30 eggs were fixed to the underside of 10X10 cm pieces of plastic board that provided a standardized foraging platform for predators. Egg platforms rested atop a 30 cm length of metal post pounded into the ground. Larval predation was evaluated by releasing third to fourth instar larvae of T.ni as sentinel prey. Laboratory reared larvae were placed on the upper part of four randomly selected plants per plot. Egg and larva predation was measure at four sampling stations per plot at 2-week intervals. An enclosure (control) cage prevented both foliar-foraging and ground-dwelling predators from removing the sentinel prey, providing an estimate of unknown losses due to handling and rainfall on each experimental plot (Fig. 2).
To measure predation rates on sentinel pupae and mealworms, five plastic plates per plot (20 cm diameter) were embedded in the soil surface and placed just below canopy level to ensure that prey items were exposed to predators, particularly ground beetles, which actively seek prey on cabbage plants (Fig. 2). On each plate, we placed seven pupae of P. xylostella and five mealworms. Mealworms were affixed to the surface of each plate using masking tape that prevented them from crawling out of the plates, but made them accessible to foraging predators. The plates were covered with plastic roofs to prevent rain water from falling into the plates.
After 24 hours, the number of sentinel prey remaining on those plants/dishes were recorded to determine predation rates in each of the experimental plots. A prey item was considered recovered only if it was fully intact. Those prey remaining on the plot with evidence of predator damage such as bite marks were not recorded as recovered. By the end of the experimental analysis, we will be able to compare predation rates among landscapes, augmentative release treatments, and sampling periods.
We also used a video camera to record predatory arthropods visiting sentinel eggs and pupae. On each plot, we collected video observations using time lapse video cameras (1 shot every minute), which were focused on a sentinel prey arena for 24 hours. The sentinel arenas were the same as those described above. We will use the resulting pictures to identify predators attacking prey and to distinguish taxa that acted as predators from those that are secondary feeders or never interact with prey.
Natural enemy identity and lepidopteran pest abundance
The overall abundance of ground-dwelling predators in our system was higher than that of the foliar-foraging predators. The most common ground-dwelling predators were Carabidae, Staphylinidae, Araneae and Opiliones. The most common taxa of foliar foraging-predators included Coccinellidae, Nabidae, Geocoridae, and Miridae. Only a few predator species, however, appeared in high enough numbers to have the potential to significantly affect populations of cabbage pests. These taxa included Coleomegilla maculata, Carabids (Stelenophus sp. and Bembidion sp.), and Opilions. Spiders seem to be important in pitfall traps, but their densities were low on sticky cards and plant samples. We also observed a large number of parasitoid wasps (Cotesia glomerata), but the relative importance of parasitoids differed greatly among landscapes.
The density of caterpillar pests was relatively low during the whole sampling period (control plots: 0.39 ± 0.14 larvae/plant, predators’ plots: 0.56±0.14 larvae/plant). The most common caterpillar pest during the trapping period was P. rapae, while P. xyllostella was recorded only in a few plots. Natural populations of T. ni were not observed through the sampling period.
Biocontrol rates on sentinel preys and plant damage
The analysis of the first two sampling periods indicated that the proportion of eggs and mealworms removed by predators was not significantly different between the control and the augmentative releases plots (Figs. 3a and 3c). By contrast, larval predation showed a twofold increase from 0.16 to 0.34 in the experimental plots where the predators were released (Fig. 3b). This effect was independent of both landscape complexity and sampling date.
However, there was no evidence that this biocontrol potential provided by the augmented predators indirectly benefits plants through reduced crop damage (Fig. 4). It will be important to complete a full assessment of how changes in predation influence the dynamics of the system; this will require measuring the ultimate impact of predators on crop yield and natural pest incidence, which is underway.
Impacts and Contributions/Outcomes
Our results reinforce conclusions from other studies demonstrating that landscape structure can influence both pest and natural enemy diversity and abundance within agricultural systems, suggesting that effective pest management requires a landscape perspective. Thus, our study will provide valuable insights into how landscape structure influences biological control in cabbage crops; to our knowledge this has not been previously assessed in upstate New York. In particular, our study will provide information about the actual contribution that natural enemy complexes, particularly predators, make to pest mortality in cabbage crops. Our data will also elucidate which of the many naturally occurring predatory arthropods are most likely to be effective predators of lepidopteran pests in cabbage fields. This research will also help us to understand how different landscape contexts influence the extent to which augmentative releases of predators can supplement the biocontrol services provided by naturally occurring enemies. In the long term, results generated from this project can be used to educate regional growers on the conservation of beneficial insects. Additionally, these results will aid vegetable growers in developing sustainable integrated pest management programs that focus on natural biological control, rather than insecticide sprays, to control lepidopteran pests and other important insect pest in vegetable production (e.g. aphids).
Outreach events and presentations have allowed us to share our results with the agricultural community. Our project was presented in a field day held at the Homer C. Thompson Vegetable Research farm at Cornell University in September 6th, 2014. In addition, we gave two oral presentations of the preliminary results of this project at the National Meeting of the Entomological Society of America in 2014 and 2015. These events took place in Portland, OR and Minneapolis, MN.