Unfortunately, pests don’t know when to take the day off. Even though we have bees in the field and watermelons reaching peak bloom, we still need to watch out for pest infestation and disease outbreaks. First and foremost, scouting fields to monitor pests will ensure that infestations never reach high enough populations to threaten yield. Secondly, we have a suite of insecticides and miticides that have low bee toxicity values but still can control damaging pest populations.
Since watermelons require insect pollinators it is essential that efforts are made to reduce pollinator exposure to make sure flowers receive enough visits from insects. All pesticides will pose some risk to pollinators but limit these risks whenever possible by either avoiding an application altogether or to apply products with reduced risk. In the table below, we have color-coded the products based on their toxicity to pollinators. Hopefully these options will let you pick the best product for the pest and phenology of the crop.
But it’s not just the pollinators who are working for watermelon growers in their field; natural enemy insects such as lady beetles, lacewing larvae, and parasitoid wasps are often effective at controlling pest insects before they become a problem. Aphids, for example, are often controlled by either natural enemies or even a timely rain event. If you find signs that aphids may have entered your field, mark the infested plant(s) and then return to check for mummified aphids the following week (Figure 1a). If you see aphid mummies, this means that your natural enemy populations are hard at work. Tiny parasitic wasps lay eggs inside of the aphids (Figure 1b), and the resulting wasp larvae eat the aphid from the inside out. Thus, killing your problem aphids before you even need to pull out the sprayer. Unfortunately, the figures were removed due to copyright issues.
Weekly insecticide applications or “calendar” sprays that are not being applied in response to pest infestation, may have negative off-target effects to both natural enemies and pollinators. Broad-spectrum insecticide products, like those belonging to the organophosphate and pyrethroid classes, have been associated with outbreaks of mites and aphids. This is most likely due to the insecticides disrupting natural enemy populations (like the parasitic wasps described above) that would otherwise keep these pest populations in check. We have found that outbreaks of secondary pests such as aphids or spider mites are more likely to require insecticides if that field had previously been given pyrethroid sprays, eliminating the field’s natural enemies.
Purdue entomologists continue to research both pest and beneficial insects in watermelon fields in hopes that they can provide the most effective recommendations to growers to protect yields while conserving pollinators and natural enemies.
In the table below, we have included some insecticide/miticide options for key watermelon pests (other alternatives are available in the Midwest Vegetable Production Guide, pgs. 92-102). As a reminder, always check product labels for additional safety information and timing for both field re-entry and harvest.
