Monday, October 27, 2014

Tight green shucks point to pecan weevil attack

     By this date in late October, all northern pecan cultivars and local native pecans have fully ripened and split shuck. But as leaves start to drop, you might notice a few pecans up in the trees that still have bright green shucks that are still tightly bound to the shell. On closer inspection, you might find a hole in the shuck. This hole could be the exit tunnel for a hickory shuckworm moth but if you take your knife and cut away the shuck you might find the hole continues straight through the nut shell (photos at right).  If the hole goes straight into the nut, what you have discovered is the exit hole for a pecan weevil larvae.
    The weevil larvae that developed inside these pecans completely devoured the kernel inside then chewed a hole through the shell and shuck to exit the nut. Once outside the nut weevil larvae drop to the ground, burrow into the soil, and form an underground cell. The larvae will pupate in the soil before emerging as and adult 22 months later (weevils have a two year life cycle).    

    I pulled a nut cluster off a tree because I noticed all three nuts in the cluster had not split shuck (photo at left). At first look, I saw only two of the nuts had weevil exit holes. But what about the third nut?

    Turning the cluster over, I found the third nut was also destroyed by pecan weevil (photo at left). Since a single adult female weevil can lay eggs in 7 to 15 nuts, it seems likely that the larvae in these three nuts originated from the egg laying activities of a single mother. It is also interesting to note that exit holes occur at seemly random locations on each nut. This may mean that once a larva finishes eating all the kernel inside the shell, the closest exit point is always the best.  

Friday, October 24, 2014

Pecan cultivars ripening in late October

Maramec split 20 Oct 2014
   This week, the latest ripening cultivars in our trials split shuck. I don't even bother testing later ripening cultivars because there is a 10% chance of a hard freeze  (28 F) by October 22 at my location in southeastern Kansas. In fact, I have seen all four of the cultivars pictured here freeze in the shuck following a cooler-than-normal summer and early fall freeze.
    Although all of the photos in this post were taken today, several of the cultivars ripened earlier in the week. Maramec was shuck-split by Monday, Oct. 20. Caddo and Oconee were next, splitting open by Wednesday, Oct. 22. 
    And finally, there is Stuart. It took me a little while to find a Stuart nut cluster with split shucks to photograph. I estimated that less than 10% of the nuts on the tree had split shucks by today, Oct. 24th. Stuart might be one of the most widely recognized pecan cultivars in the world but is is definitely not adapted to growing in our northern pecan climate.

Caddo split 22 Oct 2014

Oconee split 22 Oct 2014
Stuart, 24 Oct 2014

Tuesday, October 21, 2014

What's up in pecan tree canopies

    While I'm hand harvesting nut samples using a hydraulic lift, I often spot interesting and unusual things up in the tree's canopy that I wouldn't notice with my feet firmly on the ground.

    We have all noticed nut shells on the ground a harvest time that have been chewed by mice. You can see the gnaw marks on the shell and a hole chewed large enough to allow easy access to the kernel inside. But I never realized that mice would actually climb up the tree and start feeding on nuts long before they drop to the ground. The photo above shows two pecans neatly cleaned out by a hungry mouse. Since this was the only nut cluster I found like this, its likely that red-tailed hawks find tree-climbing mice an easy catch.

    In the canopies of unsprayed trees, I find lots of nuts infested with hickory shuckworm larvae. The small white caterpillar with a red head tunnels through the shuck of the pecan and can cause the shuck to stick to the shell (photo at left). 
      Hickory shuckworm larvae eventually pupate inside one of their tunnels before turning into an adult moths. But have you ever though about how the moth gets out of the shuck. The photo at right shows a shuckworm pupal case partially shoved out a hole created at the end of a tunnel. Still trapped in its pupal case, this shuckworm moth managed to wiggle out of the shuck until it could break free outside. I've seen several of these pupal cases and every time the case emerges from the shuck tail-end first. I guess makes sense. The moth could use the force of spreading wings to crack open the case then simply back out.  

    In one tree, I found nuts with unusually shaped shucks (photo above). The nuts appeared flattened with a distinctive crease down the middle. Although these nuts had not yet split shuck, the shuck had separated from the shell and I could easily peel out the nut. What I found inside was a small nut with a kernel that had grown too big for its shell. 
    This condition occurs when nut size and final shell dimensions are determined during a dry period in late July. A lack of soil moisture during the nut sizing period causes the creation of smaller-than-normal nuts.  Later, ample rain fell during the kernel filling process in late August which, in turn, promoted rapid kernel expansion to such an extent it popped open the shell.  What is interesting to see is how the shuck reacted to the over-expanded kernel. The crease in the shuck is located directly above the crack in the shell. Fortunately, not all the nuts on this tree appeared like this. Most nuts had normal shucks and already shuck split.

    Every fall, I enjoy taking photos of ripe pecans on the verge of falling out of their shucks. This year, I found a nut that was not only ready to fall but one that tickled my fancy (photo at right). Almost like a Rorschach diagram this pecan reminded me of something totally unrelated to pecans. Can you see the helmet, eye slits, and open mouth with pecan falling out?  Or maybe I have an overactive imagination!

Friday, October 17, 2014

Shuck split acoss the midwest

   Last week, I visited 6 pecan orchards across 3 states to check on how pecan shuck split varies with location. Below is a map of the orchards I visited all in the space of 3 days.

      By looking at the map you can see that I traveled both east and north from my home base in Chetopa, KS. What the map doesn't show you is how the seasonal climate differs between these pecan orchards. In the Table below I have listed the annual average cooling degree days for each location as well the the average number of frost free days (above 32 F), and the average number of freeze free days above 28 degrees F.
    Cooling degree days are calculated by the National Weather Service (NWS) as a measure of summer heat used to estimate the energy demand needed for refrigeration and cooling. But cooling degree days also can be used to estimate the amount of heat a pecan tree receives during the summer. Pecan is a heat loving crop and even the earliest ripening pecan cultivars require significant summer heat to fill out kernel and ripen (see pecans and climate).
    The freeze free days above 32 degrees is the most common measure of length of growing season and a number most gardeners are familiar with. But, unlike common annual garden plants, the green tissues on a pecan tree do not freeze at 32 degrees. Pecan leaves and shucks freeze at about 26 degrees which makes the freeze free days above 28 degrees (data reported by the NWS) a closer approximation of average length of the pecan growing season.        

    As you might expect, locations farther north had a shorter growing season than more southerly locations. The two most southerly locations (Chetopa and New Madrid) have the most summer heat. However, there seems to be east-west heat gradient among the 4 other locations that all lie approximately on the same northern latitude. Paola, KS is the hottest during the summer while Carlyle, IL  is the coolest.
    At each location, I checked on the development of two of our most popular pecan cultivars -- Pawnee and Kanza. Lets look at the photos, starting with Kanza.

     On Average, Kanza ripens a few days after Pawnee so I was little surprised to find shuck-split Kanza nuts all across the Midwest.  As you look at the photos you'll note that Kanza nuts look pretty much the same anywhere they are grown. The one exception was at New Madrid where the nuts were covered with black sooty mold that developed following a huge aphid outbreak.
    The one orchard I did not find Kanza fully shuck-split was located in a bottom-land orchard near Paola, KS. From the table above, you can see that Paola has the shortest growing season of all the sites I visited but this location gets plenty of summer heat. What's interesting is that just a few miles away and on top of a limestone ridge, Kanza nuts were fully split open.
    Now here's a valuable lesson. Macro-climatic conditions (as measured by the NWS) may be useful for predicting cultivar adaptation on a general level but micro-environmental conditions will determine the exact timing of pecan bud-break and nut shuck-split at any specific location.
    In the Spring and Fall, cold air flows like water down hill and accumulates in the valleys and low areas. This natural downhill movement of cold air is the reason we have areas in the landscape called "frost pockets".
    The cumulative impact of cold air movement into the bottom-land site near Paola means that things warm up later in the spring and they cool off sooner in the Fall than the nearby upland site. As a consequence, a location where cold air settles has a slightly shorter the length of growing season and a reduced number of cooling degree days. This movement of cold air also explains why the upland Kanza nuts were ripe last week while the bottomland Kanza nut were still a few days away from splitting shuck.

     Now let's look at the Pawnee photos (at left and below). Unlike Kanza, I found a wide range of ripening stages among Pawnee nuts across the Midwest.  At the warmest and most southerly locations, Pawnee was split wide open and had been split for quite some time. When I drove northwards into Illinois I found Pawnee was just starting to split shuck. This was exactly the kind of observation I was expecting to make on the trip.
   Locations with a shorter and cooler climate should ripen pecans later in the year.

     But then I moved into central Missouri and onto eastern Kansas. At these locations, I found that micro-environmental conditions impacted Pawnee nut ripening. The Columbia orchard is in an upland position while the New Franklin site is located in the Missouri river flood plain. The Pawnee nuts I saw in Columbia were similar in ripening stage to the Pawnee I saw in Illinois. But just down the road from Columbia at the Horticulture and Agroforestry  Research Center (HARC) near New Franklin, Pawnee was still tight in the shuck.
    In this case, Pawnee nut development at New Franklin was delayed in comparison to Columbia by two factors--position in the landscape and soil type. The HARC orchard lays at the foot of the river hills in the Missouri river flood plain--a perfect spot for cold air to settle. Pawnee trees at this location are also growing in a heavy clay soil. This type of "gumbo" soil warms so slowly in the Spring that bud-break can be delayed pushing back the entire nut development process.  
    When I arrived back in Kansas, I found that stage of Pawnee ripening mimicked the observations I made for Kanza at the same two orchards. Pawnee growing at the upland site in Kansas was split much like Columbia, MO and Carlyle, IL. At the bottom-land site in Paola the shucks were still tight just like the nuts I saw at HARC.
    In comparing Pawnee and Kanza across all locations this year, the ripening dates for these two cultivars were nearly similar at the four most northerly locations. In Chetopa and New Madrid, Pawnee ripened significantly earlier than Kanza. But every season is a little bit different. If I were to repeat this trip next Fall, I would probably find slightly different patterns of pecan ripening.