Primary symptoms
Stalk borers, which can cause wilting and tillering in young plants and lodging in older plants, are often associated with symptoms that look like moisture stress. Some of the small sucking insects that attack leaves can cause damage similar to nutrient deficiencies. Other insects feed on silks, leading to poor pollination.
Confirm the problem by checking below.
Summary
| Causes of insect problems | Additional evidence required |
|---|---|
| Continuous maize cropping allows insect populations to build up, especially where residues are not incorporated. | Ask the farmer about rotations. Observe residue management. |
| Maize cycle coincides with the natural annual buildup of insect populations. | Ask the farmer about other crops planted earlier in the year which might serve as hosts for insects. |
| Dry weather can cause outbreaks of spider mites. | Review weather records. |
Stalk borers, which can cause wilting and tillering in young plants and lodging in older plants, are often associated with symptoms that look like moisture stress. Some of the small sucking arthropods that attack leaves can cause damage similar to nutrient deficiencies. Other insects feed on silks, leading to poor pollinations. The main goal of this section is to help agronomists recognize these problems.
See the pest key for insects and pests to see more on specific leaf-chewing insects. The damage due to these insects can be easily identified, and economic thresholds for damage will have to be worked out for individual regions. See the page on defoliation to get some idea of the economic thresholds for leaf loss.
Are above ground insects a problem?
Evidence: observations.
- Stalk borers.Do young plants look wilted? Pull upward on the whorl. Stalk borers often cut through the whorl, and you can easily pull it out of the surrounding stem. Plants that are affected frequently produce tillers if the growing point is damaged. Borers also produce rows of "shot holes" on the leaves of young plants. Later in the cycle, look for small entry holes in the stalk. If plants lodge, cut open the stalks and look for tunnels caused by the larvae. Stalk borers often cause low plant densities and lodging (see low plant densities and lodging to estimate economic importance), but early infestation can also leave plants severely stunted with low yields.
- Sucking arthropods that attack leaves (spider mites and thrips). Look for silvery or yellow patches on the leaves. In arid zones, carefully examine the lower leaf surface for the tiny insects or for their delicate webs. In humid areas, examine the top surface of the leaf. (These insects are seldom of economic significance; it is simply useful to recognize that nutrient stress, disease, or chemical damage are not the causes for the symptoms.)
- Silk-cutting insects. Examine the crop at flowering. Are the silks eaten away to the husk? Visit the field in the late evening or early morning and look for worms or beetles on the silks. At harvest, look for a high incidence of poorly pollinated ears. If seed set is reduced by more than 20% on more than 20% of the plants, the yield loss is significant.
Possible solutions
Note: All solutions require the correct identification of the insect. Use an insect identification guide or request help from an expert if you are uncertain.
- Apply an insecticide.
- Rotate maize with another crop.
- Plant a resistant variety if one is available.
- Plant earlier in the year to avoid high insect populations.
- Incorporate residues immediately after harvest.