A striped cucumber beetle (Acalymma vittatum) rests on a watermelon. SCB can cause damage to fruits as well as the roots and leaves of the plant. (Photo by John Ternest)

By John J. Ternest

Farmers must consistently make challenging decisions when protecting their crops from damaging insect pests and the pathogens they spread. These decisions include a variety of complex factors that must be considered, such as whether an insecticide application is necessary, when and what to apply and the cost of doing so, as well as potential nontarget impacts of those applications. The outcome of this decision-making process can be the difference between a successful crop and yield losses or the use of costly inputs that threaten farm sustainability.

John Ternest

In Indiana watermelon production, farmers often take a preemptive approach to pest management that relies on prophylactic use of insecticides to manage the primary pest of concern, striped cucumber beetle (Acalymma vittatum). These applications typically occur below the established economic threshold of five beetles per plant, which increases risk to beneficial predators that reduce secondary pest populations (e.g., aphids, mites) and pollinators that are essential for watermelon fruit set. Additionally, if pests are managed too aggressively, farmers may be wasting money on unnecessary insecticide applications.

Fortunately, farmer decision-making is supported by integrated pest management (IPM), which aims to manage pests at non-damaging levels via a holistic set of management practices that can lead to reduced reliance on insecticides.

In a study published this month in the Journal of Economic Entomology, colleagues and I at Purdue University assessed pest management practices and economics for Indiana watermelon production. This research is one component of a regional project led by entomologists at Purdue that seeks to better understand the impacts of pest management on pollinators in Midwestern cucurbit production.

Assessing the Pest and Developing a Scouting Protocol

To assess the threat posed by the pest we scouted for striped cucumber beetle (SCB) over a two-year period on 30 commercial watermelon fields. Across 281 individual field visits, the economic threshold (ET) for SCB was never reached and levels were seldom observed at even two SCB per plant. This indicated that farmers were effectively controlling the pest, even on farms that rarely or never applied chemicals.

Using these pest scouting data, we developed a scouting protocol that will allow farmers to quickly and effectively assess the density of SCB in their watermelon field. This was a key component for the effective implementation of IPM in this system, as it allows farmers to assess pest densities in relation to the ET and only apply insecticides when it is exceeded. Prior to this research the ET was known, but farmers had no way to effectively and confidently scout for pests.

Scouting for pests in watermelon is a quick and effective strategy for IPM implementation. It requires the plant to be visually observed by turning over vines and leaves while looking for pests and damage caused by pests. (Photo by Jacob Pecenka)

The Value of Scouting and IPM

In addition to the scouting protocol, we examined insecticide use for SCB management. Across the 30 sites, only four never used insecticides, despite the threshold never being reached on any field. This meant that nearly every farm in the study applied insecticides prophylactically rather than in response to pest densities, or they used a lower density than the ET recommended by extension entomologists. Of farms that applied insecticides, the number of unique applications ranged from one to 10 per season. Compared to pest densities, we found that after an insecticide had been applied one time, there was no further pest management benefit from subsequent applications. A single insecticide application was sufficient for season-long control. Paired with the already low SCB densities in the system, this indicates that many insecticide applications were likely unnecessary.

To further demonstrate the value of scouting and threshold-based IPM practices, we compared the cost of scouting with insecticide costs. When implemented for a season, scouting costs were quickly overtaken by savings from the reduction of insecticide applications. Eliminating just one insecticide application on the average field was estimated to pay for scouting costs for the entire season.

This research demonstrates that scouting and threshold-based IPM is an effective means of managing SCB in Midwestern watermelon production. This is particularly the case in watermelon where SCB-transmitted bacterial wilt is not a concern as it is in other cucurbits like cucumber and cantaloupe, and thus higher densities can be more readily tolerated. If implemented, we expect that watermelon farmers can reduce their insecticide inputs and potential for nontarget effects on pollinators while also reducing the cost of their pest management regimen.

John J. Ternest is a Ph.D. student in the Entomology and Nematology Department at the University of Florida in Gainesville, Florida. He completed his master’s degree in entomology at Purdue University in 2019. Twitter: @TernestJ. Email: jternest@ufl.edu.