The second generation can be difficult to manage economically because extended egg laying may last 3 to 4 weeks in some locations. But, scouting can determine the need and correct timing for insecticide application. Without scouting, effective management of infested fields is impossible. Second-generation scouting is completely different from first-generation scouting; it focuses on finding egg masses instead of larvae. Second-generation European corn borer usually attack the plant at the tassel stage or later. During the second generation, larvae are not concentrated in a whorl, and detection in leaf axils or behind leaf sheaths is very difficult and time consuming. Therefore, egg masses, which are laid on the underside of leaves, are scouted.
Several methods can be used to determine when scouting for egg masses should begin. Initiation of egg laying can be determined by:
(1) using the computer program discussed in the first-generation scouting information to predict egg laying dates, (2) observing the date the first moths are collected in a nearby blacklight or pheromone trap, or (3) walking waterways and ditches to flush moths from the grass when the most advanced corn field in the area has reached the pretassel stage. When none of these methods are used, frequent egg mass sampling (2 to 3 days apart) is necessary.
If the egg mass sampling method is used, keep in mind that researchers believe locating an egg mass is unlikely before 5 percent of the eggs have been deposited in the field. Based on the 20-day symmetric egg-laying period (Figure 14), initiation of egg laying would probably occur 3 days before detection of the first egg masses, if regular intensive sampling has been conducted at 2-day intervals. For example, if the first egg mass was found July 4, egg laying probably was initiated on July 1. Timely sampling will reduce discrepancies between adult flight and egg deposition in nearby fields. It is very important to accurately establish egg-laying initiation. A proper assessment of the egg-laying period is crucial to economic analysis and a management decision.
Scouting to determine the density of the egg population (Figure 14B) should occur 8 to 10 days after the date of the first egg laying. Sampling 8 to 10 days into the egg-laying period reduces sampling errors and allows time to start chemical control, if it is needed. Scouting earlier may reveal low egg mass densities, which is misleading. To determine the egg mass density, 20 consecu-tive plants in a row should be examined at five representative locations within the field. Avoid sampling within 100 feet of the edge of the field.
Late-planted corn typically has the largest populations of second-generation European corn borer, and this is largely influenced by crop growth stage. Fields that are shedding pollen and have green silks present are more attractive and frequently have more problems than fields approaching brown-silk stage corn. After the brown-silk stage has started, there is no need to scout for new egg masses, if the economic threshold has not been reached by this time, because this stage of corn is no longer attractive to egg-laying females.
In corn that has tasseled, scout the field for egg masses by examining the underside of corn leaves. Mark off 20 plants in a row. Count the number of egg masses on the middle seven leaves of each plant (the ear leaf and the three leaves above and the three leaves below).
Repeat this procedure at four more representative field locations. This will give a total of 100 plants that have been scouted for egg masses. Divide the number of egg masses counted by 91. The resulting number will estimate the egg masses per plant as if all of the leaves had been examined. For instance, if 27 egg masses had been found on the middle seven leaves of 100 plants (0.27 egg mass per plant), dividing this number by 91 gives an estimate of 0.30 egg mass per plant. Use this value for the number of egg masses per plant in Table 3 to estimate the population density of larvae per corn plant. Research has indicated that this is an accurate, time-saving procedure that eliminates the need to look on the underside of each leaf.
Table 3. Predicted potential population densities of European corn borer larvae for various egg mass densities per whole corn plant (or corrected egg mass densities if 7-leaf count useda) collected on different dates during the oviposition (egg-laying) period.
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Predicted number of larvae per plant at designated days after initiation of egg laying
and proportion of egg laying complete (in parentheses)
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Number of egg 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
masses/plant .125 .180 .245 .320 .405 .500 .595 .680 .755 .820 .875 .920 .955 .980 .995 1.00
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0.02 0.72 0.50 0.37 0.28 0.22 0.18 0.15 0.13 0.12 0.11 0.10 0.10 0.09 0.09 0.09 0.09
0.04 1.44 1.00 0.74 0.56 0.44 0.36 0.30 0.26 0.24 0.22 0.21 0.20 0.19 0.18 0.18 0.18
0.06 2.24 1.56 1.14 0.88 0.69 0.56 0.47 0.41 0.37 0.34 0.32 0.30 0.29 0.29 0.28 0.28
0.08 2.96 2.06 1.51 1.16 0.91 0.74 0.62 0.54 0.49 0.45 0.42 0.40 0.39 0.38 0.37 0.37
0.10 3.68 2.56 1.88 1.44 1.14 0.92 0.77 0.68 0.61 0.56 0.53 0.50 0.48 0.47 0.46 0.46
0.12 4.40 3.06 2.24 1.72 1.36 1.10 0.92 0.81 0.73 0.67 0.63 0.60 0.58 0.56 0.55 0.55
0.14 5.12 3.56 2.61 2.00 1.58 1.28 1.08 0.94 0.85 0.78 0.73 0.70 0.67 0.65 0.64 0.64
0.16 5.92 4.11 3.02 2.31 1.83 1.48 1.24 1.09 0.98 0.90 0.85 0.80 0.77 0.76 0.74 0.74
0.18 6.64 4.61 3.39 2.59 2.05 1.66 1.40 1.22 1.10 1.01 0.95 0.90 0.87 0.85 0.83 0.83
0.20 7.36 5.11 3.76 2.88 2.27 1.84 1.55 1.35 1.22 1.12 1.05 1.00 0.96 0.94 0.92 0.92
0.22 8.08 5.61 4.12 3.16 2.49 2.02 1.70 1.49 1.34 1.22 1.10 1.10 1.06 1.03 1.02 1.01
0.24 8.80 6.11 4.49 3.44 2.72 2.20 1.85 1.62 1.46 1.34 1.26 1.20 1.15 1.12 1.11 1.10
0.26 9.60 6.67 4.90 3.75 2.96 2.40 2.02 1.76 1.59 1.46 1.37 1.30 1.26 1.22 1.21 1.20
0.28 10.32 7.12 5.27 4.03 3.19 2.58 2.17 1.90 1.71 1.57 1.47 1.40 1.35 1.32 1.30 1.29
0.30 11.04 7.67 5.63 4.31 3.41 2.76 2.32 2.03 1.83 1.68 1.58 1.50 1.45 1.41 1.39 1.38
0.50 18.40 12.78 9.39 7.19 5.68 4.60 3.87 3.38 3.05 2.80 2.63 2.50 2.41 2.35 2.31 2.30
0.75 27.60 19.17 14.08 10.78 8.52 6.90 5.80 5.07 4.57 4.21 3.94 3.75 3.61 3.52 3.47 3.45
1.00 36.80 25.56 18.78 14.38 11.36 9.20 7.73 6.76 6.09 5.61 5.23 5.00 4.82 4.69 4.62 4.60
1.25 46.00 31.94 23.47 17.97 14.20 11.50 9.66 8.46 7.62 7.01 6.57 6.25 6.02 5.87 5.78 5.75
1.50 55.20 38.33 28.16 21.56 17.04 13.80 11.60 10.15 9.14 8.41 7.89 7.50 7.46 7.23 6.93 6.90
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