Spider mites and weed hosts

Border weeds can support spider mites and are sources of mite infestations in  cotton fields.

Spider mite outbreaks in cotton are related to population levels on other host plants such as other crops or weeds where they overwinter and develop in May and June.  Spider mites move from these alternate hosts  by crawling over the soil or from plant to plant, carried by wind, man or his equipment, or by animal movements (5).

During 1998 and 1999 we sampled weeds adjacent to 2 commercial cotton fields in Poinsett County, AR,  on a weekly basis during June and July to determine which weeds were sources of spider mites on cotton.  The two fields were chosen because they have had a long history of mite infestations.

The weed species present and their abundance varied greatly during the season due to natural factors and grower efforts to control weeds.  Weeds within 25 meters of the field edges were identified to species and searched for spider mites.  When possible, at least 5 specimens of each weed species and 5 leaves on each plant were examined each week with a 10x hand lens and the number of mites counted.

The results are shown in Table 1.  A total of 39 weed species were commonly found adjacent to the two cotton fields.  Most of these, 26 species, did not support spider mite populations even when the cotton surrounding them was heavily infested with mites.  Spider mites were occasionally found  on 11 species, such as lambsquarters and cocklebur but these weeds did not appear to be important early season sources for mites.  Two species, Palmer amaranth and pitted morningglory, appeared to be the major source of  mite infestations in these fields.  Frequently, extremely small individuals of these two weed species, only inches high, were heavily infested with mites.  We never saw mite populations on red-root pigweed (a close relative of Palmer amaranth) or velvetleaf (a relative of cotton).

Early season (May and early June) control of Palmer amaranth and pitted morningglory on the borders of cotton fields with herbicides or other methods should be helpful in reducing mite movements into Midsouth cotton fields.  To be effective, weed control must be done before mites enter the cotton.

Small specimens of Palmer amaranth (Amaranthus palmeri) were identified as a frequent hosts of spider mite populations. Small specimens of pitted morningglory (Ipomoea lacunosa) were frequent spider mite hosts.

Occasionally larger Palmer amaranth plants were mite infested. Spider mites were not found colonizing velvetleaf (Abutilon theophrasti), even when surrounded by infested cotton.