Too Much of a Good Thing
To feed, or not to feed — that is the question greenhouse producers contemplate for every crop. Am I using enough fertilizer? Can I use more without jeopardizing the crop? What are the ramifications of too much fertilizer? In order to address these questions, the following study was initiated to assess the interaction of various nitrogen fertilizer treatments and the insect citrus mealybug, Planococcus citri.
A common greenhouse pest, citrus mealybug is classified as a soft scale with piercing-sucking mouthparts. Infestations can result in reduced plant vigor, which is characterized by yellowing and wilting of the host plant, premature leaf and fruit drop, and the excretion of honeydew. Honeydew is a clear, sticky liquid that serves as an excellent growing medium for black sooty mold fungi, which can reduce a plant's ability to manufacture food via photosynthesis. Citrus mealybug females are capable of laying up to 600 eggs per generation, with overlapping generations likely in greenhouses. In short, this pest reproduces quickly and may become a serious threat within a very short period of time if left unchecked.
There are two schools of thought concerning fertilization and pest outbreaks. The first suggests that plants need nutrients via fertilizers to grow and maintain their vigor. If greenhouse producers fertilize plants with enough nutrients, plants have the ability to defend themselves from pests naturally, mainly by manufacturing their own chemical defenses. These chemical defenses may result in fewer insects and mites feeding on plants, thus reducing the number of insecticide/miticide applications needed.
The second school of thought suggests that plants receiving excess nutrients will allocate additional energy toward growth and the production of new leaves, which tend to be succulent. This succulent new tissue will have higher levels of enriched vascular fluids (phloem and xylem), which in turn will benefit insects and mites feeding on the plant. Enriched plant fluids increase plant susceptibility to phloem-feeding insects such as aphids, whiteflies and mealybugs. As a result, plants receiving higher-than-recommended amounts of nutrients may have more problems with insects, which means more insecticide applications may be required in order to prevent crop losses, thus increasing the cost of production.
The purpose of our study was to determine if providing plants with "too much" fertilizer increases the performance of insect pests. The study system we used consisted of green and red-variegated coleus plants treated with six different fertilizer levels — 0, 25, 50, 100, 200 and 400 ppm — supplied in a constant liquid feed program. The industry standard for fertilization practices of coleus is 150-200 ppm. The fertilizer treatments we used ranged from a very deficient and un-realistic fertilizer regime of 0 (it doesn't get any more lean than 0 ppm nitrogen) to a luxurious range of 400 ppm.
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 differences 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 artificially 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 developmental time from first instar to reproductively mature adult were recorded.
The plant parameters measured in the study were height, leaf number, branch number, plant moisture content and leaf nitrogen concentration. Results indicated that plants receiving higher concentrations of nitrogen fertilizer (200 and 400 ppm) were taller and had more leaves and branches than those plants receiving lower concentrations of nitrogen (100 ppm or less). Plant water content increased with the higher fertilizer treatments, as did leaf nitrogen concentration.
Although it appears that fertilizing coleus plants with high nitrogen concentrations may lead to more marketable plants, in actuality, this may be more costly. For example, the coleus plants receiving 400 ppm nitrogen grew excessively tall (more than 3.3 feet high) and were prone to lodging or bending over, with stem breakage a major problem. Most plants needed extra support in order to accommodate the excessive stem elongation. Coleus plants receiving 100 and 200 ppm nitrogen were also very tall, with some green plants needing extra support. Those plants receiving less than 100 ppm nitrogen were small, weak and chlorotic.
The response of citrus mealybugs to the various fertilizer treatments was interesting. Apparently, greenhouse producers who share the concept that excess fertilization leads to "stronger" bugs are correct. Mealybugs feeding on coleus plants receiving 200 and 400 ppm nitrogen had the largest egg loads and shortest development times (from first instar to egg-laying adult) and were larger in size. (See Figures 1-3, pages 32-33.) Also, increased water content and leaf nitrogen concentration influenced the life history parameters of citrus mealybugs in a way that is unfavorable to greenhouse producers and favorable if you happen to be a mealybug. These results translate into larger pest outbreaks in greenhouses. It was interesting to note that mealybugs ç feeding on green coleus performed "better" than mealybugs feeding on red-variegated coleus. However, within coleus color, trends in the citrus mealybug life history parameters measured were similar.Higher nitrogen concentrations in the form of supplemental fertility used in greenhouse production systems result in enhanced performance of citrus mealybugs based on higher egg loads, larger mature females and shorter developmental times on coleus plants. The results of our study are similar to those dealing with insect pests, particularly phloem-feeders such as sweet potato whitefly (Bemisia tabaci), greenhouse whitefly (Trialeurodes vaporariorum), melon aphid (Aphis gossypii) and green peach aphid (Myzus persicae), when exposed to high nitrogen concentrations.
Previous studies have attributed increased fecundity on plants treated with high nitrogen concentrations to an enhanced nutritional value of the host plants. The results from our study demonstrate that proper fertilizer practices will help greenhouse producers avoid dealing with extensive insect outbreaks such as citrus mealybugs, thus reducing insecticide use and minimizing worker exposure to spray residues. When greenhouse producers attempt to "push" their crops by fertilizing at rates higher than those recommended by manufacturers they increase the likelihood of plants falling over and elevated pest outbreaks. Fertility management is a critical component of an integrated pest management strategy that can reduce having to deal with plant-feeding insects and mites.