Feedlot Manure Application Modifies Environment For Soil Insects

Manure application rates had no effect on corn rootworm.

The manure test plots were evaluated for insect damage, as well as weed populations. Previous studies in more humid environments indicated that manuring increased corn rootworm populations and caused subsequent reductions in yield. We evaluated the relationship between manure application rates and rootworm damage to see if it held true in our semi-arid environment.

Insect damage to corn plants was evaluated at harvest on both fields in 1997. Corn rootworm damage was assessed by digging up plants and inspecting roots. Corn rootworm ratings were based on the Iowa 1-6 system, where 1 = no damage to roots and 6 = complete removal of root system. One plant per plot was evaluated for rootworm damage. Rootworm damage was very even across all treatments on both fields. Average damage at the clayey site was 2.2, and at the sandy site the damage rating averaged 2.4. Both ratings indicate very little effect on yield due to damage by rootworms. There was no significant impact on rootworm damage due to manure application rates. In 1997, both fields were treated with insecticide either at planting or at cultivation to control rootworms.

The clayey site had a grasshopper infestation in 1997. Grasshopper damage averaged 8.2% leaf defoliation, which suggests an extremely small effect on plant yield. There were no significant differences among treatments, indicating no effect of manure rate on grasshopper damage.

In 1998, all plots were sampled at harvest for corn rootworm damage. Damage ratings varied only slightly from 2.0 to 2.7, which means that only slight damage was noted, and thus, corn rootworm feeding had no effect on yield. In 1998, the clayey site received an application of rootworm insecticide, but the sandy site did not. Standing plants were rated by damage observation for second generation European cornborer. Less than 3% damaged plants were observed. The majority of the damage was found just below the tassel, thus, final yields were not affected by the damage from this insect. No significant impacts of manure application rate on insect damage were found in either year.

Rootworm and grasshopper damage by site and manure application rate in 1997. 

Site	Manure Rate Tons/Acre	Rootworm Damage*	Grasshopper Damage**
Clay	0	2.2	8.2
	10	2.2	8.4
	20	2.2	8.4
	30	2.4	7.7
Sand	0	2.4	n/a
	10	2.5	n/a
	20	2.6	n/a
	30	2.3	n/a

* 1 = no root damage; 6 = complete root removal.
** % leaf defoliation.

by Jerry Alldredge
Extension Agent, Weld County