Back to August 2012

Weevils in their native environment

A weevil circle trap posted on a pecan tree. (Photo by William Reid)

Weevil is the number one pest native pecan growers must learn to manage. Every time I think I’ve discovered the magic formula for completely stopping pecan weevil damage, nature throws me a curveball and I end up pulling weevil-damaged nuts off the cleaning table.

It seems that managing pecan weevil in a native pecan grove may be a bit more complicated than just setting out a few Circle traps and then spraying the grove after you catch a few weevils. Understanding how weevils behave in their native environment (i.e. native pecan groves) will help make control decisions both more precise and effective.

Weevil Soils

All soils found in native pecan groves share a common origin – they were deposited by
thousands of years of river flooding events. That is not to say that all the soils found in any one native grove are uniform. Heavy sand particles tend to be deposited where a flooded
river runs fastest while smaller silt and clay particles settle in areas of slow-moving or backwater.

Figure 1. Soils map of a native pecan grove in Southeast Kansas.

The soil map in Figure 1 is typical. This pecan grove in Southeast Kansas has three soil types. Verdigris silt loam (Vb) is most often found adjacent to the river and has the lowest clay content (15-25 percent). Lanton silt loam (Ln) is also found in areas near the main river channel and in major drainage ways. Lanton silt loam has a slightly higher amount of clay (20-30 percent) than the Verdigris silt loam. The Osage silty clay (Os) is found in areas where flood waters move slowly, away from the main river channel. The locals call Osage silty clay, “black gumbo ground,” because of its high clay content (40-50 percent).

Knowledge of the soils in your native grove is important because weevil populations are influenced by soil texture. Pecan weevils enter the soil in the fall as larvae to form pupal cases deep in the soil then exit the soil 2 years later as adults. During both trips, weevils expend stored energy (fat reserves) to move through the soil profile. The heavier the soil (more clay), the more difficult the journey and the more of those fat reserves are burned up. As a result, pecan weevils are much more likely to complete their life cycle in lighter texture silt loam soils as compared to silty clay soils.

To increase the effectiveness of pecan weevil traps in determining when to apply control measures, make sure to place traps on trees growing in “weevil” soils. I’ve run weevil traps on trees growing in native grove pictured in Figure 1. Traps placed on trees growing in osage silty clay (the largest portion of this grove) caught very few weevils. In fact, the trap catches were so low it was difficult to determine if we were just trapping a wandering weevil or getting a flush of adult emergence. In contrast, large numbers of weevils were captured from trees growing in the Lanton and Verdigris soil. This enabled us to get a clearer picture of the periods of peak adult emergence and plan our control measures for the entire grove accordingly.

Weevil Trees

Native pecan groves are known for their genetic diversity. Every tree in the grove is different: different nut size and shape, different limb structure, and different reaction to pests. In terms of pecan weevil, the most important attribute of a native tree is the timing of kernel fill. Like every genetic characteristic, trees in a native grove display a range of nut development rates.

If you have ever collected hickory nuts in the wild you will find that most nuts have been attacked by pecan weevil. In fact, the life cycle of pecan weevil seems to be more in tune with hickory nut development than with pecan. Pecan weevils start emerging from the soil in late July. At this time, hickory nuts are filled with kernel and provide a great spot for female weevils to lay eggs. But in a pecan grove, early emerging weevils often need to wait around until pecans start filling their kernels in mid-August to finally start laying eggs. Since female weevils don’t like to wait, they will migrate to trees that fill their kernels first. This leads to weevils populations building up on the earliest ripening native trees in the grove. Find those early ripening trees in your grove and you will be identifying some of the best trees for trapping weevil (Figure 2).

Foreign Invaders

Most native groves are not isolated islands separated from other native groves or forests of mixed hardwoods. What happens outside your grove can have a major impact on the number of weevils that attack your crop. I have observed two situations of increased weevil damage in native groves that were not detected by on-farm weevil traps.

The first situation occurred on a native grove that was bordered by a large forest stand of mixed bottomland hardwoods. Major species in the forest included pecan, bitternut hickory, shellbark hickory, pin oak and green ash. Of these species, the extremely prolific nut producer, bitternut hickory, is the absolute favorite place for pecan weevils to lay eggs. I have seen huge numbers of weevil damaged nuts on the ground under pecan’s close cousin, the bitternut hickory.

Hickories usually break bud 10 to 14 days before pecan trees and one year, a late spring frost blasted the hickories (destroying the nut crop) but left the still dormant pecan trees unscathed. Later that summer huge numbers of adult weevils emerged from under the hickory trees. Without a place to lay eggs (hickory crop destroyed by frost), these weevils migrated to the adjacent native grove in search of oviposition sites. The weevils traveled tree top to tree top bypassing carefully installed trunk traps installed by the grower. The only clue we had that a large number of weevils had migrated into the grove was a huge increase in water stage nut drop caused by weevils probing nuts. With careful observation of dropped nuts, a weevil disaster was averted by applying an insecticide to control the foreign invaders.

A similar situation can occur if you have a neighbor that practices “the lord will provide” form of pecan management. This all too common native pecan management plan involves investing zero dollars into the crop and letting the trees produce a crop every five to seven years. These same type of growers are probably better at raising weevils than pecans.

During years of low nut production, the nut crop of unmanaged native groves and wild trees in the forest is usually wiped out early in the growing season by pecan nut casebearer and hickory shuckworm.

When weevils emerge in August they will migrate to any pecan tree that has a nut crop. That means the well-tended grove becomes a magnet for all area weevils looking for a place to lay eggs. If the invasion of outside weevils comes in early August, careful observation of nut drop is good way to determine if there is a potential weevil problem. However, if weevil emergence is delayed by drought, adults weevils can enter the grove largely undetected. To avoid getting caught unaware, make sure you pay attention to nut crop produced by pecan and hickory trees on adjacent properties. If you have a crop but all surrounding trees do not, be on the lookout for migrating weevils.

Author Photo

William Reid

William Reid is a horticulturist at the Pecan Experiment Field, Kansas State University, Chetopa, Kansas.