Fall 2013: Do insects proliferate in response to better plant nutrition?

Regional Report San Diego and Riverside Counties by James A. Bethke

Lots of folks believe that the more fertilizer you add to a plant, the more attractive the plant is to pests and the more plant pests will proliferate. Unfortunately, this theory is not proven in the scientific literature because there are studies that provide evidence in both directions. As such, the goal of one of our research projects, conducted in a greenhouse at UC Riverside, was to investigate the effects of two different levels of water and three different levels of nitrogen fertilizer on whiteflies, thrips, aphids and leafminers on six different cultivars of poinsettias, chrysanthemums and gerbera daisies.

The plant physiological effects we observed were somewhat predictable and are summarized here (although specific data are not shown). The responses of each cultivar of the three plant species were variable, but plants fertilized with the low nitrogen rate or irrigated at the deficit level were generally shorter with lower leaf and stem dry mass and less leaf area; in some cases the leaves had reduced soluble nitrogen. Plants fertilized with the low level of nitrogen were commercially unacceptable. Chrysanthemums fertilized with the high rate of nitrogen had reduced soluble protein, likely due to the increase in EC values observed in the potting media.

Effects of the treatments on the insects were also somewhat expected, but they were mixed. Plant hosts stressed by low levels of nutrients and water significantly reduced survivorship and development rate of whiteflies on poinsettias (data not shown) and supported fewer adult or nymph thrips on chrysanthemums (table 1). In addition, cultivars with the lowest soluble protein content supported the lowest number of thrips, and cultivars with the highest protein content supported the greatest number of thrips (data not shown). This agrees with the hypothesis that high concentrations of soluble protein are associated with increased preference of these plants to phytophagous insects.

In aphids on chrysanthemums, nitrogen and irrigation rates did not affect insect longevity but the host cultivar did (table 1). Another important cultivar effect on aphids was the intrinsic rate of increase (performance) and population doubling times, which were the highest in ‘Iridon’.  In addition, the highest level of nitrogen applied (240ppm) adversely affected aphid performance. This follows, since we also found that high rates of fertilizer were related to adverse effects on the physiology of the plants, including reduced soluble protein levels, as stated previously.

Physiologically, the best plant comes from a well-fed plant, but overfeeding the plant can adversely affect it, although this doesn’t necessarily make it more susceptible to pests and can actually cause adverse effects on the insect. We can also conclude that much of the pest population effects are associated with soluble protein levels, which are associated with the cultivar and the fertilizer level. Our final conclusion from these experiments was that the most significant reductions in pest population could be achieved by simply making good decisions on cultivar selection rather than by plant growth and cultural techniques.

Table 1. Responses of thrips and aphids to different levels of fertilizer and irrigation and to cultivar of chrysanthemum.

References

Bethke JA, Redak RA, Schuch UK. 1998. Melon aphid performance on chrysanthemum as mediated by cultivar and differential levels of fertilization and irrigation. Entomologia Exper. Appl. 88: 41-47.

Schuch UK, Redak RA, Bethke JA. 1995. Whole-plant response of six poinsettia cultivars to three fertilizer and two irrigation regimes. J. Amer. Soc. Hort. Sci. 121(1): 69-76.

Schuch UK, Bethke JA, Redak RA. 1996. The role of water in floricultural crop/insect interactions. Annual Meeting of the American Society for Horticultural Science. San Jose, California, USA. Hortscience. 31(6): 917-918.

Schuch UK, Redak RA, Bethke JA. 1998. Cultivar, fertilizer, and irrigation affect vegetative growth and susceptibility of chrysanthemum to western flower thrips. Journal of the American Society for Horticultural Science. 123(4):727-733. 

James A. Bethke
Farm Advisor, Nurseries and Floriculture
UC Cooperative Extension San Diego, North County Office
151 E. Carmel St., San Marcos, CA 92078
(760) 752-4715 phone
(760) 752-4725 fax

jabethke@ucdavis.edu

http://cesandiego.ucdavis.edu/