Anticipated Pecan Weevil Emergence in 2006. Normal or Drought-delayed?
(by Marvin K. Harris)
Research shows that the pecan weevil adults typically emerge from their soil cells (4”-10” beneath the surface) from mid-August to mid-September to mate and oviposit in pecan nuts (emergence can be monitored by using a number of different emergence cages, checking dropped nuts in August for feeding punctures, etc.). Successful management strives to prevent adult females from successfully laying eggs in the developing kernels (nuts still in the water stage are not yet susceptible to oviposition, but as they transition through the gel stage, kernel development beginning at the tip allows eggs to be placed in them). There is a 3-5 day period from when the adult emerges from the soil to when they can successfully begin to lay eggs in susceptible nuts. Emerging weevils should be killed before oviposition begins and carbaryl is the standard pesticide used for this purpose. Pecan varieties like ‘Pawnee' begin kernel formation in early August and should be protected based on when the first emerging weevils are found; other varieties, like ‘Stuart', may not begin kernel formation until early September, and pecan weevils emerging earlier can be allowed to accumulate until ‘Stuart' reaches the gel stage, when treatment must be made to prevent successful oviposition in them. The residual of carbaryl is about 10-14 days and, if pecan weevils continue to emerge from their soil cells following an initial treatment, a second or even a third treatment may be needed to prevent economic damage from occurring.
A major problem occurs when summer drought conditions result in soils becoming very hard during August. The problem is worst in clay soils and least in sandier soils because the sandiest of the latter do not harden sufficiently to delay pecan weevil emergence. Research shows that soil hardness greater than 60 kg/cm 2 will result in a drought delay in pecan weevil emergence; the soil is just too hard to allow the adults to move from their soil cells to the surface. This soil condition is easily determined by checking the soil beneath the tree canopies in your orchard or grove. The expensive way is to use a commercial soil penetrometer that will allow you to determine the exact soil hardness (the salesperson will call this “mechanical impedance” of the soil). Since we are only interested in whether the soil hardness is at or greater than the 60 kg/cm 2 threshold for pecan weevil emergence, this can be determined with a simpler tool—a ½” dowel about 8” long embedded in a sturdy handle (see Fig. 1). The flat surface of the ½” (=1 cm 2 ) dowel is placed against the soil surface and 132 lbs (=60 kg) of pressure is applied to the handle. If the dowel rod penetrates into the soil to the handle, drought delay is unlikely to occur at that spot and normal pecan weevil emergence is to be expected. Obviously, numerous spots in the orchard/grove should be checked to allow for variations in soil type, etc. Orchards under flood irrigation should already be sufficiently watered to have soils with sufficient friability to allow normal weevil emergence. Orchards/groves with no irrigation and some clay in their soils are likely to have weevils delayed when drought conditions occur in August and this condition will remain until rainfall or flooding is sufficient to soften these soils (generally 1-2” is needed to penetrate the top 8” or so—check the soil after rain to verify the effect). The really problematic situation occurs in soils under drip, micro-sprinkler, soaker hose, or other form of irrigation that does not uniformly wet the area under the drip-line where the pecan weevil cells are found. The wetted soils may be quite soft, but adjacent areas under the same tree may remain quite hard. Pecan weevils will emerge normally in the softened areas and may be delayed in the hard surfaced soils. This is the worst situation and treatments with carbaryl may be needed at the normal time and again at the time rainfall occurs to release the delayed weevils. Checking for soil hardness throughout the orchard and using emergence cages under the various conditions will allow the detection of pecan weevils in time to take action under such circumstances, but 3 or more treatments may be needed to prevent damage from occurring in infested orchards where selective irrigation occurs.
Figure 1. A soil hardness tool for detecting drought delay in pecan weevil infested pecans.
What kind of a year is 2006? The attached map (Fig. 2) shows drought prevails over the pecan weevil infested areas of Texas and that infested, un-irrigated orchards with some clay in their soils are likely to need rain to allow emergence to occur at the normal time. Producers at risk should check their soil hardness to confirm this and then monitor pecan weevil emergence, and nut development to ensure needed treatments are made at the correct time to prevent damage.
Figure 2. Drought conditions in Texas in July, 2006.