Citrus Pest Management

My approach toward the management of the mite pests of citrus followed from my interest in quantifying the impact of herbivorous insects on plant fitness. I continued my approach to determine the actual impact of pest damage on crop value.  The most important mite pest of citrus in California is the Citrus red mite, Panonychus citri (Fig. 1), and it was most troublesome on “Navel” orange in the San Joaquin Valley. It usually does not feed on fruit directly but feeds on leaves, causes them to stipple, and presumably reduces rates of photosynthesis (Fig. 2).  Prior to my research, there was no published information on either the impact of mite damage on rates of photosynthesis or on crop production.  Perhaps because of their conspicuousness, the conventional treatment threshold was two adult female mites per leaf, and citrus groves were often sprayed for mites twice per year. 

Figure 1.  Adult female Citrus Red Mite.

Figure 2.  Leaf "stippling" caused by feeding by the Citrus Red Mite.

When I first measured the rates of photosynthesis on mite-damaged and undamaged leaves, I found that the rates were largely independent of feeding by mite populations differing in size and duration (Figs. 3 and 4) [1].  We confirmed this through other studies and showed, among other things, that rates of photosynthesis are more strongly influenced by irrigation rate than by mite feeding [2].

Figure 3.  Mean adult female Citrus red mites per leaf from a commercial citrus grove on acaricide-treated and untreated trees in 1987.  Mite populations naturally decline with the onset of high summer temperatures. Areas under density curves are “mite-days,” describing variation in both the size and persistence of mite populations.  From Hare et al. 1992.

Figure 4.  Relationship between mite-days per tree from acaricide treated and untreated trees and rates of photosynthesis.  From Hare and Youngman 1987.

Detailed analyses of total fruit yield and average fruit diameter showed that feeding by mites rarely reduced total yield but often increased mean fruit diameter (Fig. 5) [3, 4].  Because larger fruit are, on average more valuable than smaller fruit, the economic return of fruit from mite-damaged trees often exceeded that from undamaged trees (Fig. 6).  This can result in an economic loss from successfully suppressing mite populations with pesticides [3].  In some cases, the counterintuitive result of successfully suppressing mite populations was the production of a larger proportion of fruit that were culled due to small size.

Figure 5.  Relationship between mite-days and total yield at harvest, and mite-days and average fruit diameter from a commercial citrus grove in 1987.  Best fitting regression lines are:  Yield = 239 - 0.002 * mite days, P < 0.01 and Diameter = 81.3 + 0.005 * mite-days, P < 0.001.  From Hare et al. 1990.

Figure 6.  Frequency distribution of the total number of 40-lb cartons of fruit by size class (number of fruit per carton) from two commercial citrus groves in 1989.  Solid bars designate fruit from acaricide-treated trees and open bars are fruit from untreated trees. From Hare et al. 1992.

We hypothesized that feeding by mites in the spring may cause a slight increase in the rate of natural abortion of fruit, known as the “June drop” such that photosynthate is allocated to a smaller number of surviving fruit that reach a larger size on mite-damaged trees compared to undamaged trees.  Our results showed that the costs of pesticide application often were not recovered [4].  This research resulted in an increase in the treatment threshold from two adult females per leaf to eight adult females per leaf, a density that is rarely reached; this treatment threshold is still in effect some 30 years later.  At the time, the cost of a pesticide application was estimated at about $250 per acre, and there are upwards of 100,000 acres of oranges in the San Joaquin Valley.  The economic impact of this study justified the cost of the research, including salaries, many times over.  In addition to these direct effects, there were also indirect benefits by also preserving predatory mites that are valuable in the natural control of other citrus insect pests.

We obtained similar results on a study of the effect of feeding by the citrus red mite on coastal California lemons.  In a two-year study, we found no significant relationship between the variation in density and duration of citrus red mite populations peaking at more than 19 adult females per leaf and variation in total yield, size, or grade of lemon fruit [5].  Finally, we also studied the impact of the citrus bud mite, Aceria sheldoni, on yield and crop value in commercial groves of coastal lemons.  Although pesticide treatments effectively suppressed citrus bud mite populations, this provided no consistent benefit to crop volume, grade, or value. For all groves, the value of fruit from treated trees was not significantly greater than that from untreated trees, even before the cost of the pesticide treatment was taken into account. In four of six cases, crop value was numerically lower in the treated treatment [6].  These results have also been incorporated into current treatment recommendations for the citrus bud mite.

All of these studies point out the need to study carefully the impact of presumed insect pests on all aspects of crop production, especially including the commercial value of the crop.  I was happy to provide our results to the citrus industry both for their value in reducing the cost of crop production and for their value in minimizing unintended consequences of pesticide applications to the environment.  It also was clear that the pest status of these two mite species was highly exaggerated, and that there was no need for further research on mite pests after our research had been disseminated.  I was ready to move on to other systems and other questions as well.

1 Hare, J.D. and Youngman, R.R. (1987) Gas Exchange of Orange (Citrus sinensis) Leaves in Response to Feeding Injury by the Citrus Red Mite (Acari: Tetranychidae). J. Econ. Entomol. 80, 1249-1253.DOI: 10.1093/jee/80.6.1249

2 Hare, J.D., et al. (1989) Combined Effects of Differential Irrigation and Feeding Injury by the Citrus Red Mite (Acari: Tetranychidae) on Gas Exchange of Orange Leaves. J. Econ. Entomol. 82, 204-208. DOI: 10.1093/jee/82.1.204

3 Hare, J.D., et al. (1990) Effects of managing citrus red mite (Acari, Tetranychidae) and cultural practices on total yield, fruit size, and crop value of navel orange. J. Econ. Entomol. 83, 976-984.  DOI: 10.1093/jee/83.3.976

4 Hare, J.D., et al. (1992) Effect of citrus red mite (Acari, Tetranychidae) and cultural practices on total yield, fruit size, and crop value of navel orange - year-3 and year-4. J. Econ. Entomol. 85, 486-495.  DOI: 10.1093/jee/85.2.486

5 Hare, J.D. and Phillips, P.A. (1992) Economic effect of the citrus red mite (Acari, Tetranychidae) on southern california coastal lemons. J. Econ. Entomol. 85, 1926-1932.  DOI: 10.1093/jee/85.5.1926

6 Hare, J.D., et al. (1999) Citrus bud mite (Acari: Eriophyidae): An economic pest of California lemons? J. Econ. Entomol. 92, 663-675.  DOI: 10.1093/jee/92.3.663