Striped cucumber beetle, Acalymma vittatum, is key a pest of pumpkins. Field tests in 2004 evaluated the efficacy of two potential biological control methods to suppress adult striped cucumber beetle. Enhancement of a natural parasitoid, Celatoria setosa (Diptera: Tachinidae), by floral resource borders was evaluated as a conservation biological control method. An augmentation biological method of inundative application of nematodes was assessed for the efficacy of reducing adult striped cucumber beetle using Heterorhabditis bacteriophora, an insect parasitic nematode. Data indicated that providing floral resources increased the percentage of beetles parasitized by C. setosa in the field, and that applications of insect parasitic nematodes resulted in reduced numbers of striped cucumber beetle.
Striped cucumber beetle is pest of pumpkins and other cucurbit crops early in the season, and the beetles can damage flowers and fruits near the end of the season. Natural enemies of striped cucumber beetle include Dipteran and Hymenopteran parasitoids, microorganisms, predatory invertebrates, and birds. Celatoria setosa, a tachinid fly parasitoid was observed in Ohio and the life history was reported in an agricultural bulletin in the 1920s (Houser and Balduf, 1925). Houser and Baldfuf (1925) found the percent parasitism of field-collected beetles was at a maximum of 39% in October, and on average 17% when collecting from different dates and fields. An allantonematid nematode, Howardula benigna (Cobb), has been reported as a natural enemy of striped cucumber beetle (Gould 1944, Houser and Balduf 1925). Generalist natural enemies have little impact on specific pest populations in the field because of their ability to utilize a wide range of insects as food. Highly specific parasitoid natural enemies and microbial agents have a potential for success, and other researchers have reported the use of biological control species to target striped cucumber beetle.
Natural enemies attack different life stages of the beetle and parasitize at different rates, but the presence of these beneficial invertebrates is not currently controlling striped cucumber beetle populations at a level acceptable to Ohio growers. The economic threshold for striped cucumber beetle in cucurbits is low due to its ability to transmit disease, and voracious herbivory and damage to marketable fruit. Growers use some cultural pest control methods, but few curative pest control options are available other than chemical insecticides for reduction of striped cucumber beetle populations in pumpkins. Recommendations need to be developed for biological control methods that utilize natural enemies to suppress populations of the adult beetle. Potential biological control methods using two different biological control organisms were investigated in the following research project.
The objectives of this project were to evaluate methods of biological control for striped cucumber beetle in pumpkins. The initial collaborative objective was to assess the potential of adoption of biological control practices by pumpkin farmers, which was determined by a questionnaire survey. The research objective was to conduct experiments on the efficacy of applying and enhancing biological control organisms in the field. Output objectives were the following: report research findings at agricultural field days and for growers organizations, publish research results in Master of Science thesis and scientific journals, and develop a fact sheet on the natural enemies of striped cucumber beetle for use by entomology extension. The overall educational objective of this project was to increase awareness of the presence of natural enemies in pumpkin crops and to report research results that could lead to sustainable pest management methods for striped cucumber beetle.
Grower survey of interest in biological control:
In 2004, a survey was taken on pumpkin crop management during the Pumpkin Field Day held at the Western Agricultural Research Station in South Charleston, Ohio. Twenty-five growers were personally handed a one-page questionnaire and 20 of those growers returned the questionnaire with responses to some or all of the questions.
Resource enhancement of the effects of C. setosa:
The field experiments in 2004 were designed as a randomized complete block with two treatments and two blocks. One experimental block had three replications of 470 square meter pumpkin fields at an agricultural research site, and the other experimental block had three replications of 470 square meter pumpkin fields, one at each of three commercial vegetable farms. The agricultural research site was located at the Western Agricultural Research Station in Clark County, Ohio, and the other three replicates were located on separate commercial farms in Clinton, Licking, and Franklin Counties. The control treatment and experimental treatment were applied at each site. The experimental treatment was a habitat created by planting a mixed flowering border of purple tansy, Phacelia tanacetifolia, and nasturtiums, Trapoleum majus, planted on two edges of a pumpkin field. The control treatment was the natural habitat of weeds and grasses at the edge of the pumpkin field. The two flower species were chosen for their known high nectar production, tolerance of poor soils, and horticultural availability.
The variety ‘Magic Lantern’ pumpkin was used at all sites, and plot dimensions of 27 meters by 20 meters were used for each treatment. Pumpkin seeds were planted 0.3 to 1 meter apart within the row, and rows were spaced as four pairs with 1.5 meters spacing and a large 5 meter space as a driving lane in the middle of each plot. The buffer between treatment plots in Clark County was 67 meters of soybean, and the sites located on commercial farms had buffers of 30 meters of pumpkin. Flowering border treatment plots on commercial farms were modified to plant flowering borders in an L-shape along the field due to the need for sprayer access for weed and disease management. At the research station, pre-emergent herbicide was used immediately after pumpkin seed planting, but no insecticides were used on the pumpkin fields. Two of the three commercial farms sprayed their pumpkin fields with a broad spectrum insecticide at planting and in the late season, depending on severity of beetle infestations. In Licking County, the commercial farm was under organic crop management practices and used no chemicals.
To compare parasitism in pumpkin plots with flowering borders to plots without flowering borders, beetles were collected by hand-held aspirator from pumpkin flowers in September. Beetle collection was delayed until the end of the season, early September, to allow parasitism in the beetle population of each plot to accumulate undisturbed. In treatment plots where stands of flowering borders did not properly establish, beetles were not collected and the block was omitted from analysis. One hundred beetles was the target sample size for the end of the season beetle evaluations. Beetles were collected down each row of pumpkins and across the entire plot and held for observation.
Calculations and statistical tests were attempted to detect difference between the percentage parasitism of beetles found in pumpkin fields that had flowering borders versus beetles found in unaltered pumpkin fields.
Inundative application of insect parasitic nematodes:
The randomized complete block design of the experiment included six replicates and four spray treatments:
1. water control
2. high rate nematode solution (15 nematodes per 10 microliters water)
3. low rate nematode solution (5 nematodes per 10 microliters water)
4. standard rate of carbaryl, Sevin XLR (1 quart per acre)
Treatment sprays were scheduled for the late season only, to target the summer generation of striped cucumber beetle. The experiment was designed to have approximately 24 plants per treated plot, with 6 meters within row spacing and 10 meters between rows. Each plot was 75 meters by 45 meters and adjacent plots were separated by 30 meters of bare-ground or untreated pumpkins. In June 2004, field plots at Waterman Research Laboratory in Columbus, Franklin County, OH and the Western Agricultural Research Station in South Charleston, Clark County, OH were direct seeded with ‘Magic Lantern’ pumpkin with imidacloprid (Admire 2F®) applied in furrow. Imidacloprid was applied as a low rate solution, 16 fluid ounces per acre, to protect cotyledons from defoliation by the overwintering generation of striped cucumber beetle and other pests and to allow pumpkin establishment. Curbit® was used to control weeds and bimonthly sprays of a fungicide mix (Quadris, Bravo Ultra, Nova, Ridomil Gold, Terronil, or Amistar) were applied in rotation from July 21 until August 27 to control fungal diseases. These chemical were used to ensure good pumpkin stand development, and therefore allow us to collect beetle data. Experimental treatments were applied and data were collected during the flowering and fruiting stages.
Nematode solutions were based on rates used to control turfgrass grubs by foliar backpack application. The recommended rate for turf is 25 infective juvenile (IJ) nematodes per square centimeter, and the high rate is 50 IJ nematodes per square centimeter, equivalent to 2500 IJ /m² and 5000 IJ /m² respectively (Grewal 2004). In this experiment, approximately four liters of nematode solution was applied to each site for each spray. By calculation, low and high treatments were 2500 IJ /m² and 7500 IJ /m². Before applying nematode solution, concentrations of nematodes were verified under the microscope, and the high rate nematode solution for this field experiment was 15 IJ / 10 µl of water and low rate was 5 IJ / 10 µl of water. Sprays began after weekly field scouting showed that flowers were present and beetles were infesting the pumpkin fields at both sites at a minimum density of one beetle per flower. Four inundative sprays were applied at each site at 1 to 2 week intervals, which covered the flowering stage of the crop. Time intervals between sprays varied due to weather and availability of Hb GPS-11 infective juveniles. Nematode sprays were applied by backpack sprayer (Deluxe model 425, SOLO Inc., Newport News, VA) operated at 15 psi with a size 10 cone nozzle and a size 50 screen. Nematode sprays were directed into open flowers in the morning from 9 to 10 AM. Carbaryl sprays (56 milliliters of Sevin XLR in 5.8 liters of water per 8640 square feet) were also applied by backpack, but sprays were full coverage sprays that covered the entire canopy in the treatment plot. Carbaryl sprays were applied in the afternoon, when flowers were closed and bees were not active. Sprays began in early August and ended in mid-September.
Scouting for adult beetles in flowers was performed in each plot 48 hours after sprays were applied. The number of beetles in 30 flowers was recorded for individual species: A. vittatum; western corn rootworm, Diabrotica virgifera virgifera; spotted cucumber beetle, Diabrotica undecimpunctata howardi; and northern corn rootworm, Diabrotica barberi. The number of flowers present in each plot varied from a high of 30 in early August to 10 flowers by mid-September. At harvest, for each treatment plot, pumpkins were weighed in kilograms with a sample size of 20 fruit at Clark County, where productivity was higher and 10 fruit at Franklin County, where fruit production was lower. Pumpkins were evaluated for beetle feeding damage on the rind and stem using a 0, 1, 2, and 3 rating. Striped cucumber beetle density, expressed as the mean number of beetles per flower, was calculated by the scouting data for each treatment plot and for each spray. The sum of all beetle species (including western corn rootworm and spotted cucumber beetle) density was calculated as the mean number of beetles, any species, per flower for each treatment plot and each spray. For each research site, striped cucumber beetle density and the sum of all species density for each scouting date were statistically analyzed by two-way ANOVA using MINITAB 14.
Sixteen of the growers had produced pumpkins in 2003 or 2004. Six of the farms had 10 acres of pumpkins or less. Four growers planted between 15 and 45 acres of pumpkins, and two of the farms were large commercial farms that planted 50 acres of pumpkins or more. One grower out of two with over 50 acres of pumpkins responded to having “little” interest in biological control, and seven other growers responded with “very high” or “high” interest in biological control. The other growers that responded had little or no interest in biological pest control.
Resource enhancement of the effects of C. setosa:
Results from the research farm (Clark County) and the collaborative farms could not be statistically analyzed separately as blocks. This was due to poor development of the flowering borders from one of the plots at Clark County. Using data from the remaining five replications that had successful stands of pumpkin and treatment borders. Comparing the standard error of the mean for each treatment was the only statistical analysis.
Inundative application of insect parasitic nematodes:
At the Franklin County site, striped cucumber beetle density declined in all treatments, including the water control, from early August until the last scouting in mid-September. Striped cucumber beetle density ranged from x= 0 to 0.88 beetles per flower in treatment plots. After the last spray application, mean beetle density for the high rate nematode treatment and the low rate nematode treatment was significantly lower than the water control treatment (F = 1.41; df = 5, 23; P<0.05; LSD= 0.139). Calculations using data from the final scouting showed the reduction of A. vittatum density in plots treated with a high rate of nematodes was 53% compared to the water check. Western corn rootworm beetle was the most common species overall, its density ranged from 0.2 to 2.3 per flower. The total beetle density, sum of all species per flower, was compared among treatments, but treatment differences were not significant. The carbaryl (Sevin XLR) treatment was effective at reducing striped cucumber beetle and the total beetle density (total beetle density= sum of all beetle species present) throughout the period of the spray applications. At harvest, no significant differences were found among treatments for the percentage of fruit with any damage. Weighted scale ratings for rind damage and stem damage, statistics performed separately, showed no significant differences among treatments at the Franklin County site. At the Clark County site, mean density of striped cucumber beetle ranged from 0.2 to 0.7 beetle per flower, declining from a peak in August to a low after the last spray in September. After the last spray application, mean striped cucumber beetle density for the high rate nematode treatment was significantly lower than the water control treatment (F = 3.07; df = 5, 23; P<0.05; LSD= 0.185). Calculations using data from the last scouting showed the reduction of striped cucumber beetle density in plots treated with a high rate of nematodes was 50% compared to the water check. Not many dead beetles were found, but a single confirmation of beetle infection by nematodes was found at the Franklin County site.
Educational & Outreach Activities
Educational materials about natural enemies for pumpkin pests will be available to growers through an extension fact sheet that is currently being drafted. Research on natural enemies was presented at two agricultural extension field days to over 50 pumpkin growers and other vegetable growers in Ohio. Research was also presented at the 2004 and 2005 Ohio Vegetable Growers Congress, Toledo, Ohio. A M.S. thesis that includes data, results, and discussion on this research project was finished March 2005 and is held at The Ohio State University Libraries.
Online reports of this project have resulted in inquiries that show interest from researchers in other parts of the country. Results from the experiments in this project demonstrated the potential for biological suppression to be used in cucurbits, specifically for the suppression of adult striped cucumber beetle.
No economic analysis conducted for this project.
The research we conducted showed potential for increased use of biocontrol of cucumber beetles and the data were promising, but no recommendations can be made at this point. Further research will be needed to guide growers towards the most efficient use of these natural enemies and if the efforts of these methods will be applicable and efficient enough to recommend grower use.
Areas needing additional study
– Although striped cucumber beetle is the key beetle species, there are 3 other beetle species that are also common, and the susceptibility of these species to biocontrol agents needs to be evaluated.
– Evaluation of needed ratio of flowering border to crop; is one row on each side of field adequate, or is strip down center of field needed? Or 1 flower row for every 20 crop rows?
– Methods of faster establishment of flowering border.
– Possible use of additional cold tolerant flower species that could be planted earlier than the cucurbit crop.
– Evaluate the minimum number of nematode sprays needed.
– Evaluate whether higher nematode rates result in better control.
– Evaluate use of nematodes against overwintering generation of beetles, and/or against beetle larvae as well as adults.