I have my doubts that the oleander aphids and the warm season melon aphids are one and the same. It is more likely their life cycle ended around the time the melon aphids was underway.

Aphids rarely cause problems on mature plants other than dome leaf droop before the beneficials start to feast. Young ones, yes. They can be sources of virus transmission if they were feeding on an infected plant before moving onto an uninfected one. I cannot see any problem with your image or inset. They look like a healthy plant. Though it looks like maybe the aphids are on the vine stem portion of the melons---like I said you can't tell even when the image is blown up where the aphids are.

Watermelon by seed should be planted starting mid-Dec through March in PHX and likely a few weeks later in Tucson. Planting so late as July encourages unhealthy plants with lower resistance to plant pests.

Melon Aphid, Aphis gossypii

The melon (or cotton) aphid appears in high numbers in the cooler spring months, but unlike the green peach aphid, melon aphid populations can be found infesting melons when temperatures are warm. They reproduce parthenogenically and develop at a very high rate under ideal growing
conditions. They can also be serious problems on fall melons. Melon aphid has an extensive host range including, cotton, citrus, and many summer annual weeds.

Damage is similar to green peach aphids. The melon aphid is a known vector of several viruses. However, they can also cause injury to melons through their feeding. They can be a major problem on young plants where they cluster on the terminal growing points of the developing vines, distorting and curling the leaves, and producing large amounts of honeydew. Feeding damage can lead to loss in plant vigor, reduced growth rate and plant death. Melon aphids will injure all melon types grown in the southwest.

Biological
Numerous naturally-occurring predators and parasitoids will attack melon aphids and are capable of keeping them under control if not disrupted by insecticide applications.

Cultural
The same cultural control tactics used for green peach aphid should be employed for management of melon aphid populations.

Chemical
Melon aphid is very difficult to control with insecticides. No thresholds have been established for timing treatments, but applications should be made if large numbers of aphids build up early in the season and natural enemies are absent. Early treatment does not prevent virus transmission, but may reduce within-field spread of the virus.

source: http://www.ipmcenters.org/cropprofiles/docs/azmelons.pdf

http://www.ipm.ucdavis.edu/PMG/r116300711.html

http://homeguides.sfgate.com/natural-insect-sprays-watermelons-77239.html

http://edis.ifas.ufl.edu/in168

http://ucanr.edu/sites/Small_Farms_and_Specialty_Crop/files/90029.pdf

Here is a link that might be useful: Garden Planting Calendar for Tucson

This post was edited by Fascist_Nation on Sun, Sep 21, 14 at 14:00