Wireworms are ubiquitous soil insect pests of most crops grown in southern Florida, where at least 12 species have been found. They cause significant economic damage, especially to sugarcane grown on the muck soils of the Everglades Agricultural Area (EAA) [Hall et al. 2002]. To control wireworms, growers frequently apply soil insecticides at planting.
Cherry (1993) showed that rolled oats were a simple attractive bait that could be used for sampling Melanotus communis (Gyllenhal) and Conoderus sp. wireworms in the organic soils of the EAA. In that study, baits were left in fallow fields for 14 days. However, the effect of timing of bait exposure on wireworm numbers found at baits was not determined. Moreover, the effect of time of bait exposure on wireworm numbers found at baits has received little attention in general. Due to the importance of reducing insecticide use for wireworms while maintaining their populations below the economic damage threshold, more information is needed. The objective of the study described in this fact sheet was to determine the effect of time of bait exposure on the number of wireworms found at baits under the field conditions of the EAA.
All tests were conducted in 10 previously-disked fallow fields on Histosols at the UF/IFAS Everglades Research and Education Center (UF/IFAS EREC) in Belle Glade, FL. Fields were surveyed by digging to determine the presence of wireworms before tests were conducted.
Each bait consisted of 200 g of rolled oats. Four baits plus two controls (= no bait) were in each replication in a 3 x 2 pattern with baits and controls 5 m apart. Ten replications 10 m apart were used in each field. Baits and controls were placed following a randomized complete block design. Holes were dug 15 cm deep, each bait poured into the hole, a flag placed through the bait to mark for recovery, and the bait covered with soil. At the time of bait placement in the field, one unbaited control sample from each replication was dug-up to represent 0 days after bait placement. Thereafter, one bait in each replication was dug-up at 7-day intervals up to 28 days. By comparing the two control samples (0 vs 28 days), it was possible to determine if wireworm population density changed in the plots during the tests. If the wireworm population density did not change in the plots, this would indicate that wireworm populations at baits were due to wireworm attraction to baits over time and not to changes in population levels in the plots over time.
Baits were recovered by excavating the bait and adjacent soil in a 25 x 25 x 20 cm deep sample and placing the sample in a bucket. Samples were stored in a laboratory at about 23°C. Each sample was visually examined for wireworms for 30 minutes in the laboratory. Wireworms were then stored in alcohol and later identified by microscopic examination. M. communis, the most important wireworm pest in the EAA, were identified using the key of Riley and Keaster (1979). Conoderus sp. was identified by J.B. Heppner at the Florida Division of Plant Industry, Gainesville. Other wireworm species accounted for less than 10% of the total wireworms found at baits and were not identified. Ten tests were conducted in 10 different fields from October 1992 to September 1993.
Complete methodology and data analysis may be found in Cherry and Alvarez (1995). Since greater than 90% of the wireworms found at food baits were either Conoderus sp. or M. communis, statistical analysis was restricted to these two groups. A t-test analysis showed no significant differences ( = 0.05) in means numbers of Conoderus sp. or M. communis at 0 versus 28 days in unbaited samples in any of the 10 fields. These data indicate that wireworms at baits over time were due to bait attraction and not changes in population densities during the tests.
Since the main objective of the tests was to determine the overall response of wireworms to baits over time, the data from the 10 fields were pooled for regression analysis. Initially, the total number of wireworms at baits was plotted against time (0, 7, 14, 21, 28 days). Visual observation indicated a quadratic function for M. communis and a cubic function for Conoderus sp. Thereafter, these models were used for the two wireworm groups using the General Linear Model Procedure (SAS 1990).
The quadratic equation for M. communis was: Y = ao + a1Xi + a2X2i, where
Y = total number of wireworms at baits,
Xi = days baits left in the field,
i = 0, 7, 14, 21, and 28 days,
ao, a1, and a2 = intercept, linear term, and quadratic term, respectively.
The cubic equation fitted to the data for Conoderus sp. was: Y = ao + a1X1 + a2X2i + a3X3i, where
a3 = cubic term; all other terms were previously defined.
Regression results of the estimated equations are shown next:
The equation for M. communis was highly predictive of the actual observed wireworm distributions over time (Figure 1).
Both linear and quadratic terms show high statistical significance; furthermore, the coefficient of determination was a high 0.99 and the coefficient of variation was a low 5.96. None of the coefficients from the equation for Conoderus sp. were statistically significant. However, the equation is very close to the actual distribution of wireworms in the fields (Figure 2), as shown by the high coefficient of determination (0.92), but not by the coefficient of variation (39). Both equations showed that time of bait exposure was a strong predictor of wireworm numbers found at rolled oats baits under field conditions. Wireworms of both Conoderus sp. and M. communis were found in increasing numbers from 0 to 21 days and then declined in number after 21 days.
Soil insecticides are frequently applied for wireworm control when various crops are planted in the EAA. In many cases, soil insecticides are not needed since wireworm populations are too low to cause economic damage (Cherry et al. 1993). However, few growers sample for wireworms since this procedure is difficult due to digging, sorting through the soil, and low numbers of wireworms normally found. Cherry (1993) and this study have shown that rolled oats baits are attractive to wireworms in the EAA and that the time of bait exposure affects the number of wireworms found at baits. These latter two studies provide basic data for using rolled oats baits as a sampling tool to determine the necessity of applying insecticides for wireworm control when planting various crops in the EAA. Wireworms are pests of various crops, such as sugarcane, vegetables, etc., and management practices vary among Florida growers; therefore, individual growers should use data from this publication to determine how to sample for wireworms based upon their specific crops and management practices. Reduced use of soil insecticides will result in lower production costs and beneficial effects to the environment.
Recent Related Research
Sugarcane is Florida's major field crop being grown in southern Florida. Soil insecticides are typically applied at planting for wireworm control. Because of the large acreages being planted, growers plant on tight schedules and generally do not use baits for sampling because of the time involved in field exposure needed for baits. Hence, Cherry et al. (2013) developed a sequential sampling plan for wireworms at sugarcane planting as an option to using baits. Essentially, right at the time the sugarcane is about to be planted, wireworms are sampled in a transect across a field by digging and counting wireworms found in samples. There is an economic injury level based on wireworm counts above which an insecticide is suggested and below which the insecticide is probably not needed. The advantage of this sequential sampling is that a field may be sampled in 1 to 2 hours rather than waiting for baits over days. Cherry et al. (2013) should be seen for details.
Two more recent studies show the possible use of chemicals as wireworm baits or to enhance the attractiveness of existing baits. Cherry and Bhadha (2019) showed that in free choice tests M. communis wireworms were attracted to low ethanol concentrations in soil. Cooper et al. (2019) in free choice tests also showed that M. communis wireworms were attracted to carbon dioxide at a specific concentration, but not other concentrations. It is usually thought that carbon dioxide is a general attractant for many soil insects who use it to find host plants through root respiration giving off carbon dioxide.
Cherry R. 1993. "Baits for Sampling Wirewoms (Coleoptera: Elateridae) in Organic Soils (Histosols) of Southern Florida." Journal of Agricultural Entomology 10:185–188.
Cherry R. and J. Alvarez. 1995. "Effect of Time of Bait Exposure on Number of Wireworms (Coleoptera:Elateridae) Found at Baits." Florida Entomologist 78:549–553.
Cherry R., G. Powell, and M. Ulloa. 1993. "Reduced Soil Insecticide Use in Sugarcane Planted after Rice." Sugar y Azucar 88:37–38.
Cherry, R., P. Grose, and E. Barbieri. 2013. "Validation of a sequential sampling plan for wireworms (Coleoptera: Elateridae) at sugarcane planting." J. Pest Sci. 86: 29–32.
Cherry, R. and J. Bhadha. 2019. "Response of sugarcane wireworms (Coleoptera: Elateridae) and white grubs (Coleoptera: Scarabaeidae) to ethanol in soil." J. Entomol. Sci. 54: 54–60.
Cooper, J., R. Cherry, and S. Daroub. 2019. "Attraction of the corn wireworm Melanotus communis (Coleoptera: Elateridae) to Carbon Dioxide." J. Agric. and Urban Entomol. Accepted for publication in 2019.
Hall D. G., R. H. Cherry, R. S. Lentini, and R. A. Gilbert. (2002). Wireworms in Florida Sugarcane. SC013. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/SC013
Riley T. J. and A. J. Keaster. 1979. "Wireworms Associated with Corn: Identification of Larvae of Nine Species of Melanotus from the North Central States." Annals of the Entomological Society of America 72:408–414.
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