A bad case of pecan leaf phylloxera

    I have been scouting pecan trees for signs of pecan nut casebearer activity and have made a couple of important observations. I haven't seen any casebearer as of today, June 7th and every native pecan I've looked at is loaded with nuts. It looks like 2019 will be a year of over-cropping and it may beneficial to skip the casebearer spray and to allow casebearer to help thin the nut crop.

    During my rounds of looking at wild pecan trees for casebearer activity, I came across a tree with the worst case of pecan leaf phylloxera that I have seen in a very long time (photo at right). The pecan leaf phylloxera is an aphid-like insect that causes the tree to form galls around these plant sap-feeding insects. Note that the galls are confined to the leaf blades.
   

    Once the galls are formed, there are no control measures for this pest. Later this summer, all seriously gall infested leaves fill drop from the tree. 
    Insecticide treatments are effective for control of this pest but must be applied before galls form over the insect. For pecan leaf phylloxera, a single insecticide application made at leaf burst will control this pest. Mark infested trees this summer and plan on spraying those trees next spring.
    You can read more about the pecan leaf phylloxera in a previous post.

Galls on pecan leaves: Pecan leaf phylloxera

    This is the time of year when folks seem to notice bubble-like galls on the blades of pecan leaves (photo at right). These galls are caused an insect called the pecan leaf phylloxera. At this point during the growing season, the small aphid-like phylloxerans are feeding on plant sap from inside the gall. The seasonal life cycle of the phylloxera starts at leaf burst when spring crawlers emerge from the bodies of over-wintering females and crawl to rapidly growing new leaf tissues. As these wing-less females feed on the leaves, they secrete a growth regulating substance that causes the tree to grow a gall around the insect. Once inside the protective gall, female phylloxera give birth to hundreds of young.  

   Flip a phylloxera infested leaf over and you might find that some of the galls have developed an opening that almost looks like an exploded volcano (photo at left). A new generation of winged adults will emerge from this hole. The winged adults will mate and impregnated females will migrate to a hiding spot in pecan bark. The phylloxerans will remain dormant until next spring when a new generation of crawlers emerge from the body of the overwintering female. 

    I cut open a couple of galls with the hopes of photographing phylloxerans inside (photos above). However, what I found in each case was a syrphid fly larvae working to devour all the phylloxerans inside the gall. In photo "A", the syrphid fly larva had cleaned out the gall completely. In photo "B", there were just a few phylloxerans left.  Syrphid flies are probably the reason why pecan leaf phylloxera is only a minor pest of pecans. 

Galls on seedling pecan trees

  

   I was trimming up some young trees when I came across a seedling tree that had leaves sprinkled with light green galls (photo at right). These galls are formed by the plant in response to the feeding of an insect called the pecan leaf phylloxera. I find leaf phylloxera most commonly on juvenile pecan trees that have not yet been grafted.

    I photographed a couple of leaf phylloxera galls on the upper side of the leaf blade then flipped the leaflet over so you can see the same galls from the under side (photo at left). On the upper leaf surface the galls a raised, irregular-shaped and smooth. On the lower side of the leaf, the galls are raised with a nipple-like projection in the center of the gall.  

     I cut open one of the galls, then photographed both halves (photo at right). The gall was filled with aphid-like insects. I found both winged adults, and wingless nymphs inside the gall. These small insects feed on pecan plant tissues that make up wall of the gall.

  Eventually the gall splits open on the underside of the leaf allowing winged adults to leave the gall (photo at left). After winged adults mate,  female phylloxerans will find a secluded spot in the rough bark of the tree to over winter.
   I usually don't bother trying to control these insects on juvenile pecan trees. Once I graft the tree, the problem seems to disappear. 

Galls on pecan stems

   This spring has provided excellent conditions for the development of galls created by pecan stem phylloxera (photo above). Pecan growers in Kansas, Missouri, and Kentucky are all finding trees covered with stem galls. The scientific name for this insect is Phylloxera devasatrix and like the name implies, this phylloxera species can have a devastating effect--distorting pecan tree growth and eliminating nut production.

    If you open the galls at this time of year you will find it filled with hundred of winged aphid-like insects (photo at left). When the gall cracks open, the winged phylloxerans will fly out, mate, then mated females will find a hiding place in the rough bark of the tree.  While hidden in the bark the female's body will become filled with eggs that will remain dormant until budbreak the following year.
    At budbreak the eggs hatch and "stem mothers" crawl out to the emerging pecan buds. Once she finds green tissue,  a stem mother begins to feed on plant sap, secreting a compound that caused the tree to grown a gall over the insect. After the gall is formed stem mothers lay eggs inside their galls. These eggs hatched into the winged form of the insect.

    To control phylloxera an insecticide needs to be applied at the outer scale split stage of nut development (photo at right). The insecticide is aimed at controlling crawlers before they become imbedded inside a gall.
    If you see a tree infested with pecan stem phylloxera, mark that tree now. Next spring, spray only the marked trees.
    The pecan leaf phylloxera is a related species that creates galls on pecan leaf blades. The control timing for pecan leaf phylloxera is at leaf burst; so make sure you identify which species is attacking your trees before planning insecticide treatments for next spring.

Pecan leaf phylloxera

   During this time of year you may notice small, yellow galls growing between the veins of pecan leaves (photo at left). These galls are created by the pecan leaf phylloxera.
     Phylloxera are small, aphid-like insects that overwinter in the rough bark of pecan trees. In early spring, just when the first new leaf bursts from expanding buds, phylloxerans crawl out onto the emerging leaves and start to feed. As leaves expand,  phylloxerans secrete a substance that causes pecan leaves  to grow a galls around each insect.

    Once inside the gall,  phylloxerans produce two generations of offspring that all feed on plant sap from inside the gall. In June, the gall opens up on the underside of the leaf (photo at right shows galls cracking open ) to allow winged adults to fly out of the gall and mate. Mated females then hide under rough bark where their bodies become filled with the eggs that will start next year's population of phylloxerans.
    If phylloxera galls become so numerous they seriously distort leaves, you will see early defoliation of infested leaflets starting in July. To control phylloxera, an insecticide should be applied early in the spring at leaf burst before galls start to form.