Robert L. Stoltz
The bean aphid, Aphis fabae Scopoli, overwinters as an egg on snowball bush, euonymous, or dock. In the spring several generations occur on the winter host before migration to summer hosts such as beans, corn, sugar beets, lambsquarters, and pigweed.
Bean aphids generally occur in noticeable numbers in sugar beets in September. Most of these infestations are spotty and do not cause appreciable yield losses. If bean aphid infestations occur earlier, however, significant yield loss can occur.
Data in Table 1 show that early infestations can reduce yields by almost 50%. These data assume, however, that 100% of the plants are infested. Also, percent sugar is not affected by aphid infestations.
Table 1. Sugar beet yields as affected by length of bean aphid infestation Infestation Sugarbeets/ha Sucrose Sucrose/ha date lbs/A (%) lbs/A Early July 25416 a 15.8 a 4033 a Late July 35305 b 15.6 b 5530 b August 48107 c 15.6 a 7469 c Check 53384 d 16.1 a 8593 d a Numbers within columns followed by the same letter are not significantly different (P=0.05) by Tukey's KSD.
Bean aphids prefer to colonize the young inner leaves instead of the intermediate aged or older leaves. Thus, consider a plant infested when from 4-8 of the new leaves have any number of aphids on them. The plant is considered not to be infested if less than four inner leaves have bean aphids on them. Also , of major importance, is to inspect plants in several sections of the field to determine the actual percent of plants infested. Bean aphid infestations are generally spotty and control decisions could be incorrect without a thorough examination of the field.
To determine whether or not to treat for bean aphids use the graph in Figure 1. Once the percent infestation is determined, check where that compares to the days before harvest. If the point lies above the curve, the field should be treated. If the point lies below the cureve, do not treat. For example, a 10% infestation 100 days before harvest should be treated and a 10% infestation 25 days before harvest should not be treated.
Figure 1. Treatment level curve based on percent aphid infested plants and number of days until harvest
Several different spicies of caterpillars can attack sugar beets at various times during the growing season. Some of the more common ones are the variegated cutworm, loopers, zebra caterpillars, western yellow-striped armyworm, beet webworm, and beet armyworm. All of these caterpillars defoliate the plant to some extent.
The age and size of the sugar beet plant should be consided when determining if a treatment should be applied to control these caterpillars. Young plants can often recover from caterpillar feeding without much yield loss. Old plants can lose some leaf tissue without yield loss. Plant defoliation diring the middle of the season appears to be the most critical period.
General guidelines for making control decisions are presented in Table 2. The age and growth potential of the plant and amount of leaf surface consumed by caterpillars were taken into account when developing the table.
Table 2 Suggested Caterpillar Injury Thresholds to Determine Need for Insecticide Treatment Growth Stage Number of Post-emergence (weeks) Larvae/plant 3 - 6 3 - 5 6 - 12 5 - 10 15 and over 10 - 15