According to the May climate and drought outlook, the majority of South Dakota is classified as being in a moderate drought. The less optimistic weather forecast indicates that crops are going to suffer intermittent moisture deficits during the growing season.
When plants are water stressed, levels of free amino acids and sugars in the plants increase, which can enhance the performance of plant feeding insects. While there are complex theories and conflicting studies on the negative impacts of drought or water-stress on insect activity, in general, certain pest insects are associated with severe yield losses or impacts during drought situations.
Grasshoppers are a recurring problem for farmers and ranchers in South Dakota. The majority of grasshopper species in South Dakota overwinter as eggs. Warm spring temperatures with little rainfall (conditions we are experiencing this spring) are highly favorable for the hatching and development of grasshoppers.
According to the 2014 grasshopper survey map (Figure 1), the vast majority of western South Dakota can expect low to moderate grasshopper populations in 2015. In a normal year, these densities are of less concern as there is plenty of vegetation in the field borders and ditches for the grasshoppers to feed on.
Most of the grasshopper species are generalist herbivores, which means they feed on several different grasses and broad leaf plants but have a preference for young and green plants. During a drought, lack of green vegetation along the field borders might attract grasshoppers to the field crops. Considering the current drought situation, producers and ranchers in the orange and red areas of the survey map are advised to be on the look-out for grasshopper populations in 2015.
In general heavy mite infestations are associated with drought conditions in spring and summer. There are three species of mites that attack and cause damage to field crops in South Dakota; two-spotted spider mite, wheat curl mite, and brown wheat mite.
Mites feed on plant sap and symptoms of damage range from curling of the leaf margins (brown wheat mite), stippling or mottled leaves with webbings (spider mites, Figure 2), to stunting and discolored plants (wheat curl mite). The effectiveness of chemical treatments can be minimal and not economical in drought conditions and often times the only thing that can alleviate the situation is rain.
Aphids are another group of sap feeding insects that are known to cause increased damage to water stressed plants. Aphids attacking wheat and soybean crops are of concern in South Dakota. Growers often tend to use prophylactic insecticide and fungicide treatments to get an upper hand over diseases and aphids in wheat and soybeans. However, in drought situations prophylactic sprayings are discouraged because it is an unnecessary cost, and may lead to outbreaks of other pests such as spider mites.
Unlike sap sucking insects, drought or water stress conditions have been shown to have less influence on the activity of leaf chewing insects. However, alfalfa weevils might compound the damage by drought as they feed on the nutritious green part of the leaf resulting in reduced hay quality and quantity. In drought situations, grazing and early cutting could be economical alternatives to spraying for weevils this year.
Drought does not affect corn rootworm larvae as they hatch or begin feeding on the roots. Corn plants that are drought-stressed, however, are more likely to suffer significant economic injury from corn rootworm feeding on the roots.
In drought conditions, one full node of roots consumed to within 1.5 inches from the stalk can lower uptake of nutrients and cause yield losses. If plants are severely stressed, this economic injury level can occur even with less than one full node of roots damaged by the larvae. Unfortunately, not much can be done if drought conditions and rootworm infestations are apparent in mid-summer. Prevention of corn rootworm infestations through crop rotation and Bt rotation are recommended practices to minimize impact of corn rootworm.