Objectives:

1.   To determine the site-specific effect of multiple stresses (nutrients, water, insects) on the growth, development, maturation, and yield of cotton.

2.   Integrate the effect of N, H₂0, and insect injury on growth, development, maturation, and yield of cotton.

3.   Estimate photosynthesis depression by aphids, and fruit consumption by heliothine larvae as a function of site-specific pest densities and crop phenology for available cotton varieties.

4.      Develop decision support software that predicts the site-specific impact of multiple stresses on cotton crop growth, development, maturation, and yield

A.   Reporting Period: Sept. 1997 - Aug. 1999

B.     Summary of Progress (Address all applicable objectives; data must be included)

Objective 1:   Quantify the spatial and temporal variability of factors that can be addressed by precision agriculture practices.

Lamesa: The most common arthropod species found using visual leaf samples were pests including cotton aphid, whitefly, and thrips along with green lacewing larvae and spiders (both predators).  The most common arthropods from sweep net samples were pests including boll weevil adults and armyworm larvae along with predators such as spiders, ladybugs, green lacewing adults, big-eyed bugs, and nabids.  Finally, boll weevil and bollworm caused most of the injury to squares and bolls.  We examined the data using 3 different methods: (1) ANOVA to determine the effects of irrigation and nitrogen fertilizer levels on yield, % plant nitrogen, and insect distributions and damage, (2) multiple regression to determine possible correlations between insect and plant parameters and cotton yield, and (3) examination of insect spatial distributions to determine the size of management units for variable pesticide application.

Results of ANOVA indicate the lint yield was significantly greater at the high irrigation level than at the low irrigation level, but that fertilizer level had no effect on yield (Fig. 1).  (Significance in all ANOVAs was set at P < 0.05.)  The opposite was true for % nitrogen in the cotton leaves, % nitrogen was higher at the low irrigation level than at the high irrigation level, and % nitrogen increased significantly with increasing nitrogen fertilizer levels (Fig. 2).  Results for ANOVAs on aphid density and bollworm square damage were similar as that of % plant nitrogen. Both were significantly lower at the high irrigation level than at the low level.  Also, in all cases the irrigation level by nitrogen fertilizer level interaction was significant indicating that nitrogen fertilizer had no effect at the low irrigation level but did have an affect at the high irrigation level, with the medium and high fertilizer levels having higher aphid and bollworm damage than the low fertilizer level (Figs. 2, 3, and 4).  For boll weevil boll damage the irrigation level by nitrogen fertilizer interaction was also significant, because at the low irrigation level bollworm damage was significantly less at the low fertilizer level (than the medium or high levels), but at the high irrigation level bollworm damage was less at the high fertilizer level than at either the medium or low level (Fig. 5).