Crop Report

I’ve received several reports of corn rootworms breaking through pyramided Bt traits over the last couple of weeks. Most of these are in the usual areas north of I-80, but I was able to collect a population from Sangamon County last week and had reports from Putnam and Bureau. Continuous corn is virtually always the culprit in these cases. We are always looking for both western and northern corn rootworm populations to bioassay; if you have large numbers of adults and wouldn’t mind us coming out to collect them, please don’t hesitate to contact me.

I remain concerned about spider mites in areas which are under drought stress; I received a report this morning about mite problems in White County. Hopefully the rains this past week have slowed them down, but stay vigilant in areas under drought stress. Corn rootworm adult emergence should be well under way throughout the state. As most of the rootworm adults will still be in the field they emerged from, this is a good time to inspect fields to get a feel for the extent of your rootworm problem and how your traits/insecticides are performing. If you have high adult emergence, consider digging some roots to assess the extent of larval damage.

Speaking of damage, we learned how corn rootworm emergence tents hold up to an 85-mph derecho; not well, as it turns out. However, it was probably worth the damage to get some healing rains.

On a trip to Monmouth this past week I saw the beginning of some likely spider mite infestations; if the drought continues, these become increasingly likely in soybean. This is a great reason to hold off on broad spectrum insecticide applications, which don’t touch the mites but eliminate their natural enemies. (Also, a good reason not to mow your grassways if you don’t have to).

I’ve started to receive a few reports of damage from early season caterpillar pests, including true armyworm, black cutworm, and variegated cutworm. Concentrate your scouting efforts on fields that border wheat, other small grains, or had a grass cover crop for armyworm, and fields that had sub-optimal winter annual weed control or a legume cover crop for the cutworm species. If you find larvae, note the sizes – larger (> 1 inch) larvae do the most damage, but are also the closest to pupating and “cycling out” of the damaging stage. Populations of larvae with mixed sizes or that include predominantly smaller larvae will be with us for longer.

We should be in the middle of corn rootworm egg hatch in much of the state by the time this report is published. Continue to look out for armyworm injury in & near wheat and other dense grasses (cover crops, weeds, waterways, etc). Black cutworm larvae should be at cutting size throughout the state now, so include them in your scouting efforts, particularly in fields that had sub-optimal winter annual weed control. I have received relatively few reports of early season injury so far.

Reports of insect activity so far have focused on large/early flights of black cutworm and true armyworm, likely helped along by storms out of the southwest a couple of weeks ago. Both species are more likely to be a problem if dense vegetation is present in the field; black cutworms are especially attracted to winter annual weeds, while armyworm prefers dense grassy vegetation. Be on the lookout for seedling pests (including seed corn maggot, slugs, wireworms, etc.), which can become a problem when cool conditions delay emergence.

We are likely in the cutting window in much of the state for black cutworm larvae. I have not received reports of damage yet; in addition to the timing of moth flights, note that the fields at risk of cutworm damage are generally those that have a problem with winter annual weeds – especially when those weeds are dying at around the time the crop emerges. If you had adequate, early control of these winter annuals, even a large moth flight is unlikely to result in much cutworm damage.

Continue to monitor corn and soybean fields for cutworms and armyworms. I have not had many reports of damage to this point – good early season emergence conditions go a long way towards helping the crop “outrun” this sort of damage. We ought to start seeing rootworm egg hatch over the next couple of weeks – I browsed degree day accumulations for several weather stations throughout Illinois, and most are relatively close to the 11 year average for this time of year (some a bit ahead, and some a bit behind).