In most years, multiple hatches of weevil larvae occur in Indiana, as many as four population peaks may be seen in southern Indiana during the spring. Due to the phenomenon of multiple peaks, the application of controls should be delayed somewhat to reduce the likelihood that multiple applications of an insecticide will be needed. In other words, if an insecticide is applied too early and there are weevils yet to hatch, the insecticide may no be effective enough to control the later hatching larvae.
By utilizing heat unit accumulations data (base 48?F) to determine when sampling should begin and when a management action should be taken, producers can get the greatest economic return. The management guidelines listed below should be used as a guide in determining when alfalfa weevil should be controlled in southern Indiana. Refer to heat unit information in each week’s Pest&Crop “Weather Update.” This heat unit information will help one determine when management steps should be taken.
Three facets of corn rootworm biology will dictate the potential for rootworm damage to corn in 2003: 1) beetle egg laying in 2002, 2) the success of egg overwintering, and 3) larval survival and establishment.
1) Beetle egg laying in 2002: As already outlined in Pest&Crop #1, February 21, 2003, many first-year corn fields in northern Indiana will be at risk from rootworm damage because of last year’s western corn rootworm beetle numbers observed in soybean. This is also true of most corn following corn throughout the state. Simply put, more beetles last year laid eggs, which means greater risk for root damage this year.
2) Egg overwintering and survival: We have received questions this winter concerning the effects of temperature and tillage on overwintering rootworm eggs. After all, the Midwest experienced some low temperatures this winter. Rootworm egg survivability has been researched many times and in different ways in the past. To seek answers one must consider numerous variables such as, soil temperature, duration of temperature, soil moisture, soil type & texture, soil compaction, tillage type, residue cover, snow cover, and depth of egg. Researchers have conducted experiments in order to predict overwintering survival. A recent study was summed up as follows: “... factors such as soil moisture and temperature, which vary with soil depth and texture, and vary between and within years, make development of simple and highly predictable models of overwinter survival of western corn rootworm eggs difficult.” Lab experiments with controlled conditions have studied the effect of temperatures on hatch. A two year study found that temperatures of 23?F and 32?F for 8 weeks resulted in 80% and 17% mortality respectively. Soil temperature data from Purdue’s Agronomy Research Center, W. Lafayette during January (coldest period) averaged 30?F (minimums) at 4-inch (bare soil) depth. According to the above research, we shouldn’t rely on the winter’s cold to significantly decrease rootworm numbers.
Tillage experiments (1940’s to present) to control rootworms have yielded a mixed bag of results. The strategy is to move the eggs near the soil surface where they will be exposed to lower soil moisture and temperature; desiccation causes high egg mortality. Knowing the depth of the overwintering egg is critical before tillage is considered as a control method. One study found 60% of the western corn rootworm eggs at a depth of 8-12 inches. Perhaps that is why researchers in one study concluded that “neither fall nor spring plowing can be recommended as a reliable method of controlling corn rootworms.”3) Larval survival and establishment: Here too many factors influence the survival of the newly emerged larva in the spring. Soybean sweeps in 1997 resulted in very high numbers indicating a high risk to first-year corn in 1998. However, heavy rains in late May and early June during egg hatch that season drowned many hatching larvae and damage was reduced. In short, barring environmental extremes such as drought or floods, hatched larva will find and begin feeding on corn roots within a few inches away.
Hopefully all producers have their planting equipment fine-tuned and ready for planting this spring. Just as it is important to have your planter units in good working order, your granular insecticide boxes should be cleaned, worn or broken parts replaced, and properly calibrated. The importance of this was recently brought to light while working with several producers on research trials in northwestern Indiana. For the past several years we’ve calibrated producer’s granular applicators before planting and found the settings change, even when using the same planter and product. Occasionally we find serious mechanical failures with boxes, causing significant misapplication.
Remember, each granular unit should be calibrated separately. Use calibration tubes and instructions supplied by the manufacturer or dealer for the insecticide of choice. To calibrate: 1) measuring off a distance of 500 feet along which the planter will travel at planting speed during the calibration process or operate the planter in a stationary position that allows one to simulate traveling 500 feet at planting speed, 2) set each application unit at some beginning setting (planter manual and/or insecticide label may help in this regard), 3) attach an insecticide collection device to each unit, 4) catch and weigh the insecticide that is metered out of each application unit over the prescribed distance, and 5) compare the amount of product caught for each unit with the amount that should be delivered over the prescribed distance (see table).
When calibrating, don’t forget to operate the planter at planting speed. Once you determine the correct setting for each unit, check each unit one more time to make sure that you are getting a fairly consistent reading. Also, record the setting somewhere on the insecticide box (but remember that you will still need to recalibrate next year even if using the same product). We can’t stress enough the importance of proper calibration. Happy planting!
Southwestern corn borer spends the winter as larvae in galleries at the base of corn stalks. Stubble in cornfields can be checked during early spring for damaged plants and surviving borers. This can provide an indication of what the first generation may be like for 2003. A survey of southwestern corn borer damage and larval survival was conducted in Caldwell, Daviess, Hardin and Henderson counties on March 13 and 14. These counties were selected because of the past infestation history. The purpose was to estimate the extent of SWCB damage, as evidenced by basal stalk girdling. In addition, we wanted to estimate the survival of the overwintering larvae in the crowns of these damaged plants. In each county, three to five non-Bt corn fields were evaluated. Within each field, 10 to 12 groups of 10 plants were examined for girdling. An additional minimum of 50 girdled plants were examined for the presence of live SWCB larvae.
This is the fifth year that we have conducted such a survey. In comparison to the previous winters, we had moderate levels of girdled plants, but the lowest survival of overwintering larvae because of the long cold winter. Moderate levels of girdled stalks were to be expected, because wet soils delayed planting in 2002. Delayed harvest allows SWCB time to migrate to the bottom of the stalk and girdle the plant. Late planted corn is also more attractive for late-season egg laying.
Observed levels of survival in the girdled crowns was welcome news. Of the girdled crowns sampled this spring, a large proportion had evidence of bird activity with the larva having been removed. Relatively few crowns had dead larva remaining in the overwintering chamber. The survival was the lowest observed in the last five years. While the survival of the larvae was less than last year and because there were so many larvae, the number of live SWCB larvae per stalk is almost twice of what we estimated last year, but the numbers are still very low. This survey indicates that there are relatively few SWCB moths to begin the season as compared with the past years.
Keep in mind that overwintering survival is just one of the variables that will, in part, determine the potential for SWCB problems in 2003. Historically, the date of planting of individual fields has been a key variable contributing to the potential for late season SWCB damage. Although early season numbers seem to be very low, favorable conditions, may allow SWCB numbers to rebound by the second and third generations. Typically, fields planted after May 10 have an increased potential for this type of damage. Last year we had very low numbers of SWCB, but delayed planted made much of the crop more vulnerable to second and third generation borers.
What we can conclude: