Although troublesome at times, grasshoppers really are fascinating and important creatures in our prairie ecosystem. There are over 100 species of grasshoppers in Kansas and only a handful of these cause problems in crop fields. The rest make up a large part of the food chain and are eaten by other insects, birds, mammals, reptiles, amphibians, and some crazy entomologists.
The USDA samples the High Plains every year to help predict grasshopper outbreaks, and their 2019 grasshopper hazard map for Kansas (Figure 1) is pretty close to what we are seeing right now. During dry spells like western Kansas has been experiencing the last few weeks, grasshoppers have their compound eyes set on greener areas like emerging alfalfa and wheat seedlings as the mixed forb and grassy areas around crops die back. But it’s not just crops that are being mowed by grasshoppers right now, CRP, pastures and rangeland are also on their menu (Figure 2). See UNL’s excellent guide to rangeland grasshopper control here for more information: http://extensionpublications.unl.edu/assets/pdf/g1630.pdf
Figure 1. Grasshopper Hazard predictions for 2019 based on 2018 surveys. Map from https://www.aphis.usda.gov/plant_health/plant_pest_info/grasshopper/downloads/hazard.pdf.
Figure 2. Examples of rangeland that has been stripped of vegetation by grasshoppers. Photos by Justin Waggoner, K-State Research and Extension.
It’s usually a combination of several species that will move into crops and most are full grown by this time of year which makes them tricky to control. Grasshoppers will hatch starting in spring and may continue hatching until early July. After ~5 molts and 40-60 days later, they develop into adults. Since grasshoppers can eat half their body weight in vegetation every day, it’s important to keep an eye out for these gluttons around field edges and ideally treat before the ‘hoppers reach adulthood and prior to them moving into the crops.
Recent questions concerning grasshoppers
Q. Will grasshoppers move out of a crop field and into the surrounding pastures? (A recent disagreement occurred between two neighboring farms over this very question.)
A. The grasshoppers could hatch out of no-till fields if eggs were laid the previous season, but most often eggs will be laid in the areas surrounding crop fields and will move into fields after the vegetation around the field has either been sprayed with herbicide, if it dries down in hot, dry weather, or if the populations get too crowded in those areas.
Q. Do grasshoppers need to be controlled in grain sorghum?
A. Grasshoppers may need to be controlled in sorghum, especially young sorghum with 5-8 grasshoppers/square yard. There is no hard threshold for sorghum in grain fill, but older sorghum can take more pressure. As a general rule for all our crops, 15+ in field and 41+/square yard along the field margin would justify treatment (see “Sorghum Insect Management Guide” here for chemical control of grasshoppers: https://www.bookstore.ksre.ksu.edu/pubs/mf742.pdf)
Q. Should I spray grasshoppers before or after planting alfalfa?
A. If there are more than 20-40 grasshoppers/square yard before planting, consider treatment. Keep in mind growth regulators like dimilin and prevathon take a few days to a week to kill nymphs. Once planted, watch carefully for migration into the field and consider treatment if 3-5 grasshoppers/square yard are found in the seedling alfalfa field.
Sarah Zukoff, Entomology Extension Specialist, Southwest Research-Extension Center
snzukoff@ksu.edu
Tags: insects