Degree-day model will help growers time citrus leafminer sprays

04/15/2013 12:17:00 PM
Moneen M. Jones, Philip A. Stansly and Joseph M. Russo

Editor's note: A special thanks to Dr. Monica Ozores-Hampton at the Southwest Florida Research and Education Center for coordinating The Immokalee Report, of which the article below is part.

The citrus leafminer (CLM), Phyllocnistis citrella Stainton (Lepidoptera: Gracellariidae) has been a serious citrus pest in Florida since its accidental introduction in 1993. Within a single year, it spread throughout the state from Miami-Dade, causing serpentine mines in young leaves.

Figure 1. citrus leafminerLyle Buss, University of FloridaFigure 1. Citrus leafminerLeafminers are larvae of tiny moths (Fig. 1) that fly at night and find their mates and hosts by smell. The female lays her egg generally on the underside near the midrib of a tender emerging leaf.

The tiny caterpillar bores directly from the egg into the cuticle, working back and forth over the leaf surface and growing larger until it finishes at the leaf margin.

mines created by citrus leafminerLyle Buss, University of FloridaFigure 2. Mine damage of leaves by citrus leafminer larvae. The damage causes malformed leaves (Fig. 2), reduced photosynthesis, and increased susceptibility to citrus canker, a severe disease resulting in leaf loss and fruit drop.

The current research seeks to evaluate the usefulness of traps baited with synthetic pheromone to monitor CLM populations in six citrus grove locations in Collier, Lee and Hendry counties. The pheromone is a mixture, which emulates substances emitted by the female that can attract males from long distances.

Trials were conducted to determine optimal trap density, correlation between CLM damage and adult trap counts, and the number of CLM generations per year in southwest Florida. The objective is to describe the phenology of CLM in Southwest Florida under four management scenarios, including an unmanaged block of citrus.

The information derived from this research, along with temperature data, will be used to develop a preliminary degree-day model for prediction of CLM flight.

Research citrus groves include:

1. a 63-hectare block (about 155 acres) of 46-yr-old Hamlin sweet orange trees;

2. a trial in a 4.4-hectare block (10 acres) of Valencia orange in Lee County;

3. a replicated field study conducted in a 5.4-hectare commercial block (13 acres) of young Valencia orange trees; 9 blocks of grapefruit (Flame, Ray) in a commercial grove in Immokalee, and an unmanaged 6.1-hectare grove (15 acres) of Murcott honey tangerine on Cleopatra rootstock in Immokalee.

P. citrella flight throughout the growing season was monitored weekly using pheromone traps baited with P. citrella CLM lures February through November for 2011-2012 and monitored bi-weekly December through January.

citrus leafminer phenology modelFigure 3. Phenology or flight of CLM in a grove with 3 applications of nutritionals/year, frequent flushing and 7 generations. (Click image to view larger version)Results so far show strong deviation in the number of generations (2-7) of CLM per year (Fig 3). The number of peak flights coincides with the number and intensity of flushing periods as well as grove management practices.

Published records of P. citrella growth based on temperature will be used to develop a prediction model for moth flights. Using the lower threshold of 10.4 degrees Celsius (50.7 degrees Fahrenheit), the egg, larval and pupal periods in days can be translated to degree-days.

Degree-day accumulations at the research sites will be based on temperature data obtained from the Immokalee weather station, which is part of the Florida Automated Weather Network or FAWN.

Prediction of first and second generations will be calculated by adding the number of degree-days between the start of egg laying (i.e. moth flight) and peak moth flights. This information will be available on our website in a degree-day table and an online degree-day calculator that will benefit citrus growers by informing them of the best possible time to spray for CLM control.

Dr. Philip A. Stansley is a professor of entomology and nematology at the Southwest Florida Research and Education Center. He can be reached at pstansly@ufl.edu. Dr. Moneen M. Jones is a post-doctoral research associate in integrated pest management. Dr. Joseph Russo is president of ZedX Inc., Bellefonte, Pa.



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