Seasonal Ecology and Population Dynamics of Sweetpotato Whitefly in a Complex Cropping System
Steve Naranjo, Peter Ellsworth (UA), Luis Cañas(OSU)
Bemisia tabaci is a key pest of cotton, cantaloupe and other crops in the US desert southwest. The insect reproduces year-round with adults colonizing crop and non-crop hosts sequentially throughout the year. Populations are at their lowest levels and most vulnerable during the winter. From a systems perspective, we are conducting comprehensive studies to examine many of the biological and ecological processes that govern the year-round dynamics of this insect. Plots of six representative host plants (broccoli, cantaloupe, cotton, alfalfa, Lantana and various weeds) have been established at the Yuma, Maricopa and Marana Agricultural Centers in Arizona. These sites represent the range of geographic and climatic areas of the state for cotton production.
One full year of studies have been completed. Cantaloupe and Lantana, provide for the buildup of B. tabaci populations during the fall. Populations declined steadily with decreasing temperatures during the winter. Low temperatures affected the host plants, with Lantana plants losing all foliage due to freezing temperatures. After the decline during the winter, populations survived in broccoli and Malva parviflora (cheeseweed), and begin to build up in broccoli, cantaloupe and Lantana. Spring populations are extremely high in Lantana and cantaloupe, which allowed for significant densities in cotton during the summer. Summer weeds such as ground cherry also host large densities of B. tabaci. Alfalfa appears to be a poor host.
Life table analyses demonstrated differences in survivorship patterns on the different hosts. In general, the highest survivorship was observed in cantaloupe at all sites. The highest B. tabaci survival at Marana occurred in cantaloupe (about 47%) followed by Lantana (15%), weeds (15%) cotton (4%), alfalfa (3%) and broccoli (2%). Yuma had the highest survival in cantaloupe (45%) followed by broccoli (31%), Lantana (19%), weeds (17%) and cotton (7%). Maricopa had the highest survival in cantaloupe (45%) followed by broccoli (45%), Lantana (33%), alfalfa (28%), weeds (15%) and cotton (4%). From the different mortality factors observed, both predation and dislodgment accounted for a significant portion of the mortality at all locations. Dislodgment and desiccation were important in some host plants and probably were influenced by low temperatures. For instance, broccoli cohorts suffered high mortality due to desiccation and dislodgment. Rates of predation were relatively high in all host plants but more so in cotton across the different locations. Parasitism by Encarsia and Eretmocerus varied from host to host and from location to location. Parasitism was highest in Lantana, especially at Maricopa (32%). Despite high levels of B. tabaci mortality at the three locations, it was insufficient to prevent population outbreaks in most plant hosts. The examination of the biodemographic characteristics of B. tabaci during the year, especially during the winter months will provide important information relative to the cold hardiness and survival of this species. Such information will aid the prediction, and possibly the prevention, of outbreak populations in the late spring and summer. (PDF)
Cold Tolerance of Sweetpotato Whitefly
Steve Naranjo, Peter Ellsworth (UA), Luis Cañas (OSU)
Studies are underway to examine population level responses of B. tabaci to near zero and subzero temperatures over varying durations of exposure. Studies are being conducted with four plants that commonly host B. tabaci during winter months, broccoli, alfalfa, Lantana, and cheeseweed. Three developmental stages are being examined; eggs, 4th instar nymphs and adults. A factorial design is being used to examine exposure periods of 0.25, 0.5, 1, 2, 4, 6, and 8 hours at temperatures of 4, 2, 0, -2, -4, -6, -8°C for each stage on each host plant. Following exposure insects are placed at 25°C with 60-70% RH and observed for mortality. Eggs and nymphs are considered dead if they fail to eclose or emerge, respectively, within 5 days. Adult survivorship is observed after 24 hours. Sublethal effects are being examined for surviving adults by measuring oviposition and survival over a two week period following cold exposure.
Survey and Evaluation of Arthropods Inhabiting Lequerella, a New Crop for the Arid Southwestern US
Steve Naranjo, David Dierig,Peter Ellsworth (UA)
Lesquerella spp. (Family Brassicaceae) are native to North America and their seeds contain industrially-valuable hydroxy fatty acids used for the production of bio-lubricants, motor oils, resins, waxes, nylons, plastics, corrosion inhibitors, cosmetics and coatings. Lesquerella fendleri occurs in the southwestern US and northern Mexico and research to develop this species as an alternative crop for the arid southwestern US has been underway in since the mid 1980s. Nothing is known about the arthropods inhabiting this new crop. Survey studies initiated in 2004 revealed that L. fendleri hosts large populations of Lygus spp. (primarily L. hesperus) along with a high diversity and abundance of other arthropods, many of which are beneficial species shared with cotton, alfalfa and other crops in Arizona. Experimental studies in 2005 showed that increasing densities of Lygus bugs were significantly related to increasing levels of fruit abortion, and flower bud and seed pod damage. This overall damage resulted in significant decreases in yield and marginally significant reductions in seed oil content. Further analyses and additional field studies will be needed to determine if the suppression of Lygus bugs and the development of management strategies are economically justified for this crop. Further work also will be needed to explore the effects of other potential pests such as webworms and thrips which also occur at high densities, and to examine the potential role of L. fendleri as a source of natural enemies for other crops such as cotton.
Impact of Lygus spp. on damage, yield and quality of lesquerella, a potential new oil-seed crop
Steve Naranjo, Peter Ellsworth, David Dierig
Lesquerella, Physaria fendleri (A. Gray) S. Watson is a mustard native to the western U.S. and is currently being developed as a commercial source of valuable hydroxy fatty acids that can be used in a number of industrial applications including bio-lubricants, biofuel additives, motor oils, resins, waxes, nylons, plastics, corrosion inhibitors, cosmetics and coatings. The plant is cultivated as a winter-spring annual and in the desert southwest it harbors large populations of arthropods, several of which could be significant pests once production expands. Lygus spp. (Hemiptera: Miridae) are common in lesquerella and are known pests of a number of agronomic and horticultural crops where they feed primarily on reproductive tissues. A four year replicated plot study was undertaken to evaluate the probable impact of Lygus spp. on production of this potential new crop. Plant damage and subsequent seed yield and quality were examined relative to variable densities of Lygus spp. (0.3 – 4.9 insects/sweep net) resulting from variable frequency and timing of insecticide applications. Increasing damage to various fruiting structures (flowers, buds, seed pods) was significantly associated with increasing pest abundance, particularly the abundance of nymphs, in all years. This damage, however, did not consistently translate into reductions in seed yield, individual seed weight or seed oil content, and pest abundance generally explained relatively little of the variation in crop yield and quality. Negative effects on yield were not sensitive to the timing of pest damage (early vs. late season), but were more pronounced during years when potential yields were lower due to weed competition and other agronomic factors. Results suggest that if the crop is established and managed in a more optimal fashion, Lygus spp. may not significantly limit yield. Nonetheless, additional work will be needed once more uniform cultivars become available and yield effects can be more precisely measured. (PDF)
Feeding Behavior of Lygus hesperus on Four New Industrial Crops for the Arid Southwestern USA.
Steve Naranjo, Melissa Stefanek
Camelina (Camelina sativa), guayule (Parthenium argentatum), lesquerella (Physaria fendleri), and vernonia ( Centrapalus pauciflorus [formerly Vernonia galamensis]) are either under limited commercial production or being developed for production in the southwestern USA. Insect pests are a potential economic threat to all these new crops, with Lygus hesperus, the western tarnished plant bug, among the most prominent due to its regional abundance and propensity to feed on reproductive plant tissue. The objectives of this study were to establish baseline data on the feeding behavior and potential impact of L. hesperus on camelina, guayule, lesquerella and vernonia. Behavioral observations of adult females and males, and nymphs of this insect were made in the laboratory. Insects spent ≈35% of their time either probing (=tasting) or feeding on various reproductive and vegetative tissues of guayule, lesquerella or vernonia, but only 20% on camelina. When insects did probe and feed they preferred reproductive tissues, primarily flowers and siliques/achenes, and there were differences in these behaviors relative to crop but not generally to insect stage or sex. Insects probed and fed more on flower tissue of guayule and vernonia compared with camelina and lesquerella, and more on siliques of lesquerella compared with achenes of vernonia. When probing and feeding on vegetative tissue, there was generally a preference for stems compared with leaves in all crops except guayule. Results show that L. hesperus will readily feed on the economically important tissues of all crops, and although research has shown that this feeding did not consistently affect lesquerella yield, further work will be needed to determine if such feeding poses a risk to commercial production of camelina, guayule or vernonia. (PDF)