Members of the cucurbit vegetable family (melons, squashes, cucumbers) struggle to survive in our summer heat. When stressed by high temperatures and inconsistent irrigation, they become susceptible to infestations by several pest insects and several viral diseases. Flower and crop production slows or stops in the heat as well. Stress from heat and drought is what turns cucumbers so bitter in summer.
Monitor all the cucurbits in your garden regularly for aphids and whiteflies. The sucking damage from aphids causes leaf stippling (small yellow spots) and also causes the leaves to curl up around the aphids. Clouds of tiny whiteflies will fly into the air when the leaves are brushed. Aphids and whiteflies also excrete "honeydew," a sticky substance that attracts ants and that can foster the growth of sooty mold.
Stressed plants attract pest insects. Control for aphids and whiteflies on cucurbits begins with maintaining consistent irrigation. Drip emitters work well for cucurbits; overhead watering or hand watering will encourage the development of powdery mildew which coats the leaves with a white powdery mold. Increase watering times as temperatures rise. When growth and flower production slows significantly in July, reduce or stop fertilization to prevent growth spurts of new green growth that attract pest insects.
Parasitic wasps, ladybugs and green lacewings are very active beneficial predators of aphids and whiteflies. Unfortunately, ladybugs fly off to the foothills to escape the summer heat in the central San Joaquin Valley. Wash aphids and whiteflies off leaf undersides and the insides of the curled leaves or use horticultural oils or insecticidal soaps to knock down aphid and whitefly populations. Broad spectrum insecticides have long-lasting residual effects and kill beneficial insects as well as pest insects.
Squash bugs feed on cucurbit leaves. The leaf tissue around the holes turns black or brown and the leaf dies. Squash bugs are pretty easy to spot. They're blackish grey and about a half-inch long. Lift up leaves to find them and their reddish egg clusters. Hand pick the bugs off the plants and gently squish the eggs with your fingers.
The viral diseases carried by aphids and whiteflies include cucumber yellowing and several types of mosaic diseases. Pull out any obviously diseased plants and put them into the green waste bin, not the compost pile.
Squash are pollinated only by squash bees which slow down foraging for pollen in the heat when few flowers are produced. Next year, plan on putting in bee-friendly summer flowers suitable for our dry, hot climate (cone flowers or echinacea, coreopsis, goldenrod, sages, etc.) to provide a stable source of pollen and nectar for the bees during hot weather and also plan to provide a clean water source like a birdbath or a trickling fountain for them.
Plant hybridizers have recently developed several varieties of parthenocarpic or self-pollinating zucchini and cucumbers which don't need bees or other insects for pollination. I've had good luck with summer crops of the Silor Mini cucumbers (small, crunchy, sweet) and the Cavili zucchini (dark green, OK flesh and taste, not as tasty as heirloom zucchini), both grown from seed ordered from www.kitchengardenseeds.com.