Monday, August 31, 2015

Late season aphids

   The leaves of our pecan trees are starting to take on a sticky and shiny appearance (photo at right). That's because foliage is now covered with honeydew, the by-product of aphids feeding on the underside of the pecan leaves. This year, honeydew coverage of the leaves has been minimized by an unusually large amount of summer rainfall washing the leaves clean. Despite the rain, aphids are still at work, creating even more honeydew.
    I've seem some development of black sooty mold on the leaves (photo at left). This black fungus grows on honeydew and does injure the leaves directly. Indirectly, sooty mold harms the foliage by blocking sunlight and reducing photosynthesis.

   Turn over a leaflet and you'll see the insects that are creating all that honeydew. During the peak of an aphid outbreak, you will find all life stages of the black-margined aphid feeding on pecan tree sap. In the photo above, a winged adult can be seen feeding on the leaflet's main mid-rib. Note the black markings along the outer edges of the insect's otherwise clear wings. Its obvious how this insect received the common name--black-margined aphid.
    Also in the photo you can see various sizes of wingless aphids nymphs. If the nymphs can avoid being eaten by lady beetle or lacewing larvae, they will all grow into winged adults. As a nymph grows in size it must shed its old, smaller exoskeleton before growing into a new larger exoskeleton. Cast-off exoskeletons appear in the photo as white to grey bits of fluff.
   Aphid populations can grow rapidly but eventually crash as pecan leaves become less suitable for aphid feeding. Spraying pecan trees for aphid control seems only to create insecticide resistant strains of the aphid. In our orchard, we have chosen to let the aphids run their course and not spray to control the population.