Tuesday, September 10, 2013

Black-margined aphids coat pecan leaves with honeydew

    Our pecan leaves are getting sticky with honeydew (photo at right). A late season outbreak of black-margined aphid is currently feasting on pecan leaf sap and the sticky exudate that aphids secrete is giving pecan leaves a shiny appearance.
    Black-margined aphids feed on the underside of pecan leaflets, mainly along the midrib of primary leaf veins.  In the photo at left you can see several immature (wingless) aphids feeding. Note that the insects appear in several sizes. The smallest aphids are newly born while the larger aphids are half-way grown towards adulthood. If you look carefully you'll see white, cast-off skins of aphids that have already molted. (Aphid nymphs grow larger by casting off their old small skin, or more accurately "exoskeleton", and growing a larger skin.)
    The photo at right shows a winded adult feeding on the midrib. The black markings on the outer margin of the adult's forewing is what gives this aphid its common name. The scientific name for this pecan aphid pest is Monellia caryella.
       The appearance of large amounts of honeydew on pecan foliage attracts large numbers of green and brown lacewings. These beneficial insect are voracious aphid predators. The larvae of lacewings are common known as aphid lions.
   The green lacewing lays its egg on a silken thread (photo at left). A single female produces 200 to 800 eggs and from the number of eggs I've found on our honeydew-covered trees, we've had plently of lacewing actively already. Unlike the green lacewing,  brown lacewings lay eggs directly on the underside of leaves or in bark crevices. Both species are important biological controls of black-margined aphid.
   At this late point in the season, I do not recommend trying to control black-margined aphid chemically. Eventually, lacewings and lady beetles will cause the aphid population to crash naturally. A good thunderstorm would also help in removing the honeydew from the foliage.