Arthropod Pest Response to Plant Nutrient Concentration Michael J. Costello Wine and Viticulture Department California Polytechnic State University, San Luis Obispo Arthropod herbivores (insects and mites) might be kept in check by Top down effects: 3 rd trophic level (natural enemies) acts on the 2 nd trophic level (herbivores) Herbivore Bottom up effects: 1 st trophic level (plant) acts on 2 nd trophic level Plant defenses are the bottom up effects 1
Some bottom up effects Physical defenses Cuticle Trichomes Cell wall Pectin HRGP Cellulose and hemicellulose bottom up effects Chemical defenses (defense phytochemicals) Phenolics Alkaloids Caffeine Lignin Terpenes Proteins Pyrethrin Peroxidase 2
Nutrition Nutrient concentration is not thought of as a plant defense, but sometimes can influence arthropod density Of the macronutrients, most information on nitrogen, some on potassium Nitrogen in plants Important component of Amino acids, proteins Phenylalanine Chlorophyll Nitrogenous bases of DNA and RNA Adenine N is the most limiting plant macronutrient in agriculture 3
Insects and mites are much higher in N (7-10%) and K (5%) Plants are generally low in percentage of N and K (1-2%) Nitrogen can affect arthropod growth and development oviposition/fecundity ultimately population growth Arthropod response to N varies Depends in part upon the type of plant tissue exploited The mesophyll region of the leaf The whole leaf (or stem, shoot, etc) The xylem The phloem 4
For chewers, most studies have found increased damage, growth, fertility and density with increased nitrogen On corn, European corn borer & corn earworm (Lepidoptera) On roses, Fuller rose weevil, hoplia beetle 4.00 phloem Phloem feeders have a positive response, but to soluble N (free amino acids), not total N % soluble N (amino acids) 3.50 3.00 2.50 2.00 1.50 1.00 0.50 Aphid reproductive rate 0.00 5
Aphids, psyllids, whiteflies, mealybugs and scales are phloem feeders Acacia psyllid Black scale Greenhouse whitefly Green peach aphid Grape mealybug Most research has found little if any effect of N on growth and development of leafhoppers, Lygus or other mesophyll-feeding plant bugs Lygus Azalea leaf bug Rose leafhopper 6
However, spider mites are kind of a hybrid between mesophyll and phloem feeders--- They don t have fine mouthparts as leafhoppers do Mite chelicerae Pecan leaf scorch mite And their response to N is similar to that of phloem feeders--- positively correlates with free amino acids Two-spotted spider mite on tomato Most studies have been on aphids, leafhoppers or mites Generally found that aphid performance was unaffected or went down with increasing potassium Yellow sugarcane aphid not affected by K Very little response to K by Willamette mite on grape 7
Willamette mite response to potassium on grape Recent study by Cal Poly undergrad Michael Sotelo Soybean aphid does better with low K Whitebacked planthopper (rice) does better with low K But, a recent study found low K on soybean corresponded to a decline in asparagine Asparagine is an important amino acid for aphids 8
Interaction between plant nutrient concentration and defense phytochemicals In some plants, there appears to be an antagonistic interaction between nitrogen and plant defense chemicals such as phenolics A recent study from Japan found increased nitrogen decreased phenolics on soybean, with increased spider mite egg production Cotton aphid (Aphis gossypii) performed better with increasing N fertilization of cotton, but did worse with increased N on chrysanthemum Cotton defense gossypol (a phenolic), decreases with increasing N 9
There may even be interactions between non-essential elements and defense phytochemicals A recent study found spider mite egg production was lower on strawberry grown under higher salinity (NaCl) What might explain this? The high NaCl plants had lower levels of peroxidase, a defense protein Red stain shows lignified cell walls Peroxidase catalyzes the polymerization of lignin and helps strengthen the plant cell wall Lignin Summary Plants have an array of phytochemical groups to help defend themselves Nutrient concentration can play a role in plant defense to insects and mites Nitrogen can have a direct effect on arthropod herbivores, or interact with other phytochemicals, such as phenolics 10