Friday, September 12, 2014

Remnants of an aphid outbreak

   During last week's tour of pecan orchards in SE Missouri, I found evidence that many trees had suffered from a massive explosion of pecan aphids. By the time I visited the trees, the aphid population had crashed and largely disappeared. However, the aphids left behind some tell-tale signs of their sap-feeding activities.
   Black-margined aphids produce huge amounts of honeydew as they feed. The honeydew quickly covers the leaves with a sugary-sticky film giving the leaves a glossy appearance. Over time, the black sooty mold fungus colonizes the honeydew painting the leaves black (photo at right). The sooty mold fungus grows entirely on the honeydew and does not actually infect the leaves. In fact, you can take your finger nail and scrape the fungus off the surface of the leaves.
   The black pecan aphid is less common in northern pecan groves but I found evidence these aphids had been feeding on SE Missouri pecan trees. As the black aphid feeds on pecan foliage it causes angular yellow and necrotic blotches on the leaves (photo at left). The black aphid tends to feed on leaves on lower and interior leaves. Eventually, black-aphid-damaged leaves drop from the trees causing early tree defoliation.