Too Much of a Good Thing

*Cloyd 9/29/05 1:49 PM Page 32

pests & diseases

Can too much fertilizer affect pest populations? New research shows just how much mealybugs like to feed on over-fertilized plants.

By Brian Hogendorp and Raymond Cloyd

o feed, or not to feed — that is the medium for black sooty mold fungi, which reducing the number of insecticide/miticide question greenhouse producers can reduce a plant’s ability to manufacture applications needed. contemplate for every crop. Am I food via photosynthesis. Citrus mealybug The second school of thought suggests that using enough fertilizer? Can I use females are capable of laying up to 600 eggs plants receiving excess nutrients will allocate more without jeopardizing the per generation, with overlapping generations additional energy toward growth and the crop?T What are the ramifications of too much likely in greenhouses. In short, this pest production of new leaves, which tend to be fertilizer? In order to address these questions, reproduces quickly and may become a seri- succulent. This succulent new tissue will have the following study was initiated to assess the ous threat within a very short period of time higher levels of enriched vascular fluids interaction of various nitrogen fertilizer treat- if left unchecked. (phloem and xylem), which in turn will bene- ments and the insect citrus mealybug, There are two schools of thought concern- fit insects and mites feeding on the plant. Planococcus citri. ing fertilization and pest outbreaks. The first Enriched plant fluids increase plant suscepti- A common greenhouse pest, citrus mealy- suggests that plants need nutrients via fertiliz- bility to phloem-feeding insects such as bug is classified as a soft scale with piercing- ers to grow and maintain their vigor. If green- aphids, whiteflies and mealybugs. As a result, sucking mouthparts. Infestations can result in house producers fertilize plants with enough plants receiving higher-than-recommended reduced plant vigor, which is characterized nutrients, plants have the ability to defend amounts of nutrients may have more prob- by yellowing and wilting of the host plant, themselves from pests naturally, mainly by lems with insects, which means more insecti- premature leaf and fruit drop, and the excre- manufacturing their own chemical defenses. cide applications may be required in order to tion of honeydew. Honeydew is a clear, sticky These chemical defenses may result in fewer prevent crop losses, thus increasing the cost liquid that serves as an excellent growing insects and mites feeding on plants, thus of production.

Figure 1. Citrus mealybug egg load on green and red-variegated coleus.

Number of citrus mealybug eggs on green coleus

Number of citrus mealybug eggs on red-variegated coleus 350

300

250

200

150

100 Number of eggs

0 0 25 50 100 200 400 Close-up of adult citrus mealybug, Planococcus citri. (All photos courtesy of Ray Cloyd) Nitrogen fertilizer (ppm)

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The Study opmental time from first instar to reproduc- Although it appears that fertilizing coleus The purpose of our study was to deter- tively mature adult were recorded. plants with high nitrogen concentrations may mine if providing plants with “too much” lead to more marketable plants, in actuality, fertilizer increases the performance of insect Plant response this may be more costly. For example, the pests. The study system we used consisted of The plant parameters measured in the coleus plants receiving 400 ppm nitrogen green and red-variegated coleus plants treat- study were height, leaf number, branch num- grew excessively tall (more than 3.3 feet high) ed with six different fertilizer levels — 0, 25, ber, plant moisture content and leaf nitrogen and were prone to lodging or bending over, 50, 100, 200 and 400 ppm — supplied in a concentration. Results indicated that plants with stem breakage a major problem. Most constant liquid feed program. The industry receiving higher concentrations of nitrogen plants needed extra support in order to standard for fertilization practices of coleus is fertilizer (200 and 400 ppm) were taller and accommodate the excessive stem elongation. 150-200 ppm. The fertilizer treatments we had more leaves and branches than those Coleus plants receiving 100 and 200 ppm used ranged from a very deficient and un- plants receiving lower concentrations of nitro- nitrogen were also very tall, with some green realistic fertilizer regime of 0 (it doesn’t get gen (100 ppm or less). Plant water content plants needing extra support. Those plants any more lean than 0 ppm nitrogen) to a lux- increased with the higher fertilizer treatments, receiving less than 100 ppm nitrogen were urious range of 400 ppm. as did leaf nitrogen concentration. small, weak and chlorotic. ➧ Coleus were grown for 32 days, which allowed the plants receiving the 0-ppm-nitrogen application to deplete their nitrogen reserves, while the other plants acclimated to the new supplemental nitrogen concentrations (25, 50, 100, 200 and 400 ppm). Thus, any dif- ferences in growth rates and other plant parameters were due to the varying nitrogen fertilizer concentrations. After growing the coleus at the selected fertilizer regimes for 32 days, plants were artiﬁ- cially inoculated with ﬁrst instar citrus mealybugs using a leaf disc transport procedure. The life history parameters that were used to determine the performance of the citrus mealybugs under the different fertilizer treatments were egg load, mature size and development time (in days). After allowing the citrus mealybugs to complete their life cycle, female mealybugs that were laying eggs were harvested, and the number of eggs was counted. In addition, mealybug size and the devel- Left: Citrus mealybugs on coleus. Right: Close-up of adult female citrus mealybug, Planococcus citri.

Figure 2. Citrus mealybug size (mm) on green and red-variegated coleus. Figure 3. Citrus mealybug development time (days) on green and red-variegated coleus.

Citrus mealybug size (mm) on green coleus Citrus mealybug development time (days) on green coleus Citrus mealybug size (mm) on red-variegated coleus Citrus mealybug development time (days) on red-variegated coleus 3 60

2.5 50

2 40

1.5 30 Size (mm) 1 20

0.5 10 Development time (days)

0 0 0 25 50 100 200 400 0 25 50 100 200 400

Nitrogen fertilizer (ppm) Nitrogen fertilizer (ppm)

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pests & diseases

Mealybug Response ing on coleus plants receiving 200 and 400 inﬂuenced the life history parameters of citrus The response of citrus mealybugs to the ppm nitrogen had the largest egg loads and mealybugs in a way that is unfavorable to various fertilizer treatments was interesting. shortest development times (from first instar greenhouse producers and favorable if you Apparently, greenhouse producers who share to egg-laying adult) and were larger in size. happen to be a mealybug. These results trans- the concept that excess fertilization leads to (See Figures 1-3, pages 32-33.) Also, increased late into larger pest outbreaks in greenhouses. “stronger” bugs are correct. Mealybugs feed- water content and leaf nitrogen concentration It was interesting to note that mealybugs ➧

Left: Close-up of adult female citrus mealybug, Planococcus citri, with crawlers emerging from eggs. Right: Citrus mealybug, Planococcus citri, crawlers.

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34 GPN October 2005 *Cloyd 9/29/05 1:51 PM Page 36

pests & diseases

Watch for Aphids Too

Mealybugs aren’t the only pests affected by fertilizer. A study by Fred Davies and others in the Department of Horticulture Science at Texas A&M University, College Station, Texas, shows that fertilizers high in nitrogen may enhance insect pest populations of cotton aphid. The effect of nitrogen fertilization on the abundance of cotton aphid, Aphis gossypii, was stud- ied on chrysanthemum. Davies tested three fertilization treatments consisting of 0.5x, 1x and EPA Grants TriStar™ Insecticide 2x the recommended nitrogen levels. Thirty-ﬁve days after four rooted cuttings per pot were established, pinched, treated with B-Nine (Chemtura Corp.) at 3,500 ppm and placed under New Expanded Insect Label short-day conditions, ﬁve apterous aphids were transferred to each pot. Aphids were counted Dayton, N.J. — Cleary Chemical Corporation weekly over a 7-week period. The aphids were harvested after 78 days, and all treatments announced today that the Environmental evaluated for plant growth and leaf tissue elemental analysis. Protection Agency (EPA) has granted a new The research showed highest leaf elemental nitrogen and magnesium and lowest phospho- expanded insect label for TriStar™ 70 WSP rus and potassium occurred at the highest fertility level. There was no difference in leaf ele- insecticide. mental calcium among fertility treatments. Plants at the low fertility level had the lowest leaf elemental nitrogen and the highest phosphorus. Plants at the highest fertility level had highest TriStar, introduced to iron, manganese, zinc, copper and boron and lowest molybdenum and sodium. Both boron the ornamental market and sodium were lowest in aphid-colonized plants. From two to seven weeks after inoculation in 2003, has quickly of aphids, aphid populations were lower at the low fertility (0.5x) than at the 1x or 2x levels. established itself as the Aphid levels were comparable between the medium and high fertility rates. Therefore, leading neonicotinyl although lower fertility reduced populations of cotton aphid on chrysanthemum, plant quality foliar insect spray for suffered, and the nutritionally stressed, low-fertility-level the greenhouse, field, plants were not commercially acceptable. nursery and landscape Davies’ concluded that the long-term goal of this markets. The new research was to develop management systems for maxi- expanded label increases mizing crop quality and quantity while minimizing inputs, its already broad energy costs, and environmental and worker safety risks. spectrum control to He believes benchmarks need to be determined for opti- key, economically important insect categories like mal production inputs and that the incorporation of new aphids, mealy bugs, caterpillars , scales, whiteflies, production systems to reduce fertilizer and pesticide thrips, leaf eating beetles and leaf miners. usage without reducing plant quality is one of the most “We are extremely pleased with the acceptance Research shows lower fertility may important challenges facing the industry. — Carrie Burns that TriStar continues to receive from leading reduce aphid populations. (Photo courtesy of Ray Cloyd) ornamental growers and landscapers. The fact that TriStar is a foliar spray with rapid knockdown that controls a wide variety of insects is a winning feeding on green coleus performed “better” rus mealybugs, thus reducing insecticide than mealybugs feeding on red-variegated use and minimizing worker exposure to combination of benefits for the customer”, says coleus. However, within coleus color, spray residues. When greenhouse produc- Don Rossi, Director of Sales and Marketing, trends in the citrus mealybug life history ers attempt to “push” their crops by fertil- Cleary Ornamental Products. “At present”, says parameters measured were similar. izing at rates higher than those recom- Rossi, “no other foliar insecticide can deliver the Higher nitrogen concentrations in the mended by manufacturers they increase exceptional control of TriStar. The product form of supplemental fertility used in the likelihood of plants falling over and performance factors of contact, systemic, greenhouse production systems result in elevated pest outbreaks. Fertility manage- enhanced performance of citrus mealybugs ment is a critical component of an integrat- ovicidal and translaminar activity make based on higher egg loads, larger mature ed pest management strategy that can TriStar the foliar insecticide that truly works females and shorter developmental times reduce having to deal with plant-feeding in minutes and lasts for weeks.” on coleus plants. The results of our study insects and mites. GPN are similar to those dealing with insect Cleary Chemical Corporation has the exclusive pests, particularly phloem-feeders such as marketing rights from Nippon Soda Co., Ltd., for sweet potato whitefly (Bemisia tabaci), Brian Hogendorp is a graduate research assis- TriStar™ 70WSP insecticide in the United States greenhouse whitefly (Trialeurodes vaporari- tant and Raymond Cloyd is an associate profes- for the greenhouse, nursery and ornamental orum), melon aphid (Aphis gossypii) and sor/extension specialist in ornamental entomol- markets. green peach aphid (Myzus persicae), when ogy at the University of Illinois, Urbana, Ill. Cloyd exposed to high nitrogen concentrations. can be reached by phone at (217) 244-7218 or E- For 65 years, Cleary Chemical has supplied Previous studies have attributed mail at [email protected]. the turf market and ornamental market with increased fecundity on plants treated with superior, quality plant protection products, high nitrogen concentrations to an utilizing the most up-to-date technology. enhanced nutritional value of the host plants. The results from our study demon- LearnMore Cleary markets innovative products such as strate that proper fertilizer practices will For more information related Cleary 3336™, Endorse™, Spectro™ and to this article, go to help greenhouse producers avoid dealing www.gpnmag.com/lm.cfm/gp100502 new Alude™ fungicides, TriStar insecticide, with extensive insect outbreaks such as cit- and Nutri-Grow Magnum foliar nutrient.

Read and follow label directions. The Cleary logo, 3336, Spectro and Alude are trademarks of Cleary Chemical Corporation. Endorse is a trademark of Arvesta Corp. Nutri-Grow Magnum is a trademark of Biagro-Western, Inc. TriStar is a trademark of Nippon Soda Co., Ltd.

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