Whiteflies

• Whiteflies are tiny nutrient-sucking insects that feed off of your watermelon's leaves. You may see these insects in clusters underneath the leaves in warm weather. In addition to causing your watermelon leaves to turn yellow and fall off, these insects produce a sticky substance called honeydew. Honeydew is a favorite food source for ants. You may see a trail of ants heading toward your watermelon crops. Moreover, fungal spores like sooty mold attach themselves to watermelon leaves and cause the plants further damage.
  
  

Treatment

• Whiteflies are difficult to control through insecticides, according to the University of California. Gardeners should remove heavily infested watermelon plants. You can try to control their damage by spraying an insecticidal soap on the watermelon plants. Avoid using the insecticidal soap on watermelon plants that are water-stressed or when daytime temperatures exceed 80 degrees Fahrenheit. Monitor your watermelon plants after spraying. Whiteflies are able to build up a resistance to the soap.
  
  

Spider Mites

• Spider mites are the most common type of mites that infest gardens. Gardeners may see small webs in between the watermelon plants. These insects suck the nutrients out of watermelon leaves and cause them to yellow. Eventually, yellowing leaves will fall off and compromise the watermelon plant's ability to photosynthesize. Much like whiteflies, spider mites produce and spread during times of hot weather. Gardeners should check their watermelon plants during June and September.
  
  

Treatment

• Certain types of spider mites will actually reproduce faster when sprayed with an insecticide that contains carbaryl. In fact, spraying an insecticide can kill off the beneficial insects that eat spider mites. Use an insecticidal soap to treat spider mite problems. Make sure that your watermelon crop has been watered, because water-stressed watermelons will not tolerate the soap and can succumb to the spider mite damage. It may take several applications of insecticidal soap to see a dent in the spider mite population.