Post by Dominic Reisig, with sampling information from Alejandro Del Pozo-Valdiva, NCSU Dept. of Entomology PhD student.
I’ve received many calls about the presence of nymphs in soybeans that are not being picked up in the sweep net. This is not surprising, since populations of nymphs in nearby fields can be anywhere from mostly large (see first photo) to mostly small. Small nymphs are green, round, and are less likely to land in the net except in dense (read treatable!) populations. Entomologists are aware that the sweep net “undersamples” nymphs compared to adults. The sweep net threshold, then, is calibrated for this undersampling. You will begin to pick more nymphs up as they become larger. Moreover, there are some things you can do to improve your sampling technique for this insect (more on that below).
As an aside, if you have a field where you are going to apply a herbicide or fungicide and kudzu bugs are present at levels you think might hit one nymph per sweep in the future (the photo represents a several nymph per sweep situation), you might want to tank mix in the insecticide to eliminate the nymphs. Keep in mind that small nymphs are not causing as much damage as large nymphs and the soybean plant has an amazing ability to compensate. Kudzu bug reduces number of seed and seed size, but not pod number. Therefore it might pay to wait and see what happens. Also remember that a spray at R2 or R3 will kill all beneficial insects, potentially flaring worm populations. Our first major corn earworm moth flight and egg lay into soybeans generally happens in the end of July/beginning of August. It almost never pays to tank mix and insecticide automatically hoping to kill pests that are there. You will kill what is there, but you will also set yourself up for future problems.
Full-season soybeans are all large enough to sweep for kudzu bug. Double-cropped beans planted in stubble are difficult to sweep, but are not attractive to kudzu bug as seedlings. Sweeping is still an excellent technique to estimate the abundance of small nymphs, which blend in with soybean stems and are difficult to see. By the time nymphs are large enough to see up and down the stem, yield may already be compromised. You should be able to prevent this by spraying when you hit the one nymph per sweep threshold. Keep in mind that most fields will have kudzu bugs at some level. As discussed in the first paragraph, even if you can see insects when you peel back the canopy, you might not have a threshold-level population or you might start picking them up as they get older. Kudzu bugs take a long time to develop relative to other insects, so this will not happen overnight. Some tips for using the sweep net are to:
1) Take 15 – 20 sweeps per sample away from field edges. Ideally, keep the same number of sweeps per sample and per field to compare them.
2) Try to get the net as low as possible between sweeps to hit the middle portion of the plant. More than a half of the insects are located this section of the plant. I like to imagine that I’m stripping the bugs off the main stem, which hopefully buries my net pretty deep in the canopy.
3) Sweep at a comfortable pace that you can maintain throughout the sampling bout.
4) Kudzu bug is more active from 11AM to 2PM, resting near the bottom of the plant in quiescent times in the morning and late afternoon and evening. To facilitate capture and reduce the chance of underestimate adult population, scout during the midday period above.
5) Look closely at the bottom of the sweep net because nymphs cluster in this section of the net.
For both visual samples and sweeping, you can check field edges to see if the bugs are present. However, this, insect congregates much more heavily on these edges. The insect is also attracted to structures with height, often aggregating on taller plants, fence posts, or volunteer corn plants in a field. Treatment decisions should be based on field interiors of average sized plants. Start sampling at least 50 feet into the field, being sure to visit several parts of the field.