Sulfur for control of Citricola Scale
by Devin Carroll
2004 Trials
After hearing reports that sulfur can be used in citrus to control Citricola scale, and having had some earlier experience showing major reductions in red scale male catches following sulfur applications, I decided to approach my citrus growers about trying some sulfur in 2004.
The advantage in cost and labor time of applying sulfur dust, versus applying oil in a volume of 1000 gallons water, made this an attractive option for my growers.
Here is a summary of my experience in 2004. As can be seen, I did not find sulfur to be a very effective control method for Citricola or red scale. All of these growers are organic.
Grower #1 – near Del Rey, two blocks, one mixed mandarin/Washington Navel, the other Early Beck. Citricola was the main problem, especially in the Early Beck. The grower applied sulfur dust 6-2, although my recommendation was to wait for the red scale crawlers in early July. I counted 88% mortality of citricola, which was close to what I counted before the sulfur. Many citricola settlers always die due to heat, poor settling, predators and parasites, etc. In the following weeks the total mortality in quick counts varied from about 75-85%, no improvement. The grower was very nervous about the citricola, and applied oil about 6-26. In the Early Becks, the coverage was not sufficient, and the grower applied a second oil about 8-26. We had heard reports that oil is hard on Early Becks, and also warnings not to apply oil after sulfur. However, with only three weeks between the sulfur and the first oil, I could not discern any damage.
Grower #2 – near Lindsay, Washington Navel and Valencia blocks. The grower had been using sulfur with a different PCA on a different ranch, and felt it was working on citricola. We applied sulfur dust about 6-29. Before the dust I counted 74% mortality of citricola settlers. One week after the dust I counted 67%. I continued to find good numbers of E. tularensis predatory mites. In the following weeks the mortality did not increase. The grower did not want to apply oil, and applied sulfur again in August, but mortality remained about the same. I also did not see significant mortality after a PyGanic spray 8-21, aimed at GWSS.
Red scale male traps counts were about 500 before the first sulfur, and 1 or 0 per trap after. They averaged less than 5 per trap the remainder of the season. There is no doubt that sulfur dramatically reduces males in traps. I saw a small amount of red scale on fruit late in the season.
I released Aphytis beginning 8-23, and recovered one in a trap 9-13.
Grower #3 – west of Fresno, Washington Navel. The block has had a chronic red scale problem along one edge, about 10 rows deep. I attribute this partly to poor oil coverage, and possibly to interference to Aphytis due to sulfur applications to the neighboring raisin block. The block otherwise was under good Aphytis control in 2003, after several years of problems. Citricola was spotty but increasing.
We applied sulfur dust about 7-2. I saw no improvement in citricola mortality. Average red scale male counts were about 4000 before the sulfur, and about 60 after. I noticed whitecaps on fruit 7-16, and definitely new whitecaps 7-23. By August I could see that the whitecaps were surviving and growing, and anticipated a problem. Unfortunately, it was too hot to apply oil. By 7-30 trap counts were above 2000 and rising fast. By 8-13 counts in the bad area were above 40,000. We finally got oil on 8-27, but by that time some branches were dying. Yikes!
Between the sulfur and the oil I caught no Aphytis. I did not release Aphytis in the spring of 2004, anticipating the sulfur would interfere. That was probably also a mistake. I started releasing large amounts right after the oil, and by 9-10 was catching a few on the traps.
I did not notice any ill effects of the oil on the trees, which was about 8 weeks after the sulfur.
Grower #4 – south of Fresno, 1 row of citrus trees, orange and grapefruit, between a raisin vineyard and a house. Red scale has been serious. No citricola. Grower applied sulfur dust early July. I checked it 9-1 and found moderate scale, much less than I expected given my experience with #3. It seemed to me that there was less than the previous year. I also found Aphytis larvae in the red scale, and Comperiella. No Aphytis were released this year.
Discussion – one could ask why I was crazy enough to try this with three growers, especially after it seemed not to be working with the first grower. Well, the growers were eager to try due to the low cost and ease of application. I could not tell that it was not working on red scale until the third grower’s block showed problems in August.
It is possible that earlier or more frequent sulfur applications would work better. The sulfur suppressed red scale males on traps for at least three weeks. Crawlers might have been inhibited for about the same time (I couldn’t tell). Most of the citricola I was counted were settled before the sulfur application. Possibly earlier sulfur would inhibit them from settling.
I can’t be certain, but it appears to me that the sulfur inhibited the Aphytis.
I saw no ill effects of sulfur on the trees, or of oil applied three weeks after the sulfur. It is possible that the trees suffered ill effects that were not obvious.
2005 Trials
After the unsuccessful sulfur trials on citrus in 2004, grower #2, near Lindsay, wanted to try again. Here is what we learned.
In summary, the sulfur suppressed the citricola, and controlled the red scale.
Sulfur was applied six times at 40-50 lbs per acre with an orchard duster. Coverage was good over the entire trees. The dates were 4-24, 5-2, 5-14, 5-27, mid June, and mid July. The principle target was citricola scale, which began with an extreme population. The idea was to prevent the crawlers from settling, so the first application was about a week before I expected the citricola to begin crawling.
It was clear to me that the sulfur killed a high proportion of the young citricola. I’m not sure how much was prevented from settling. By the end of the year, the orchard still had too much citricola, but I am convinced the sulfur made a big difference.
After two years of applying nothing but sulfur and some Aphytis to this orchard, I could not find any red scale in the trees or on fruit. As I reported last year, traps shut down virtually to zero for several weeks after a sulfur application, so the sulfur interferes with males. Most likely, crawlers are also killed. I am convinced that repeated sulfur dusting controls red scale.
I did not notice any secondary problems. The orchard did not develop a mite problem, either two-spot or red mite.
This grower has another PCA for 2006. So I will not be continuing the trials there.
Ironic, that on the one hand I advocate reducing or eliminating sulfur in grapes to help the biocontrol of mites, leafhoppers, and other pests, yet on the other hand I am trying out new sulfur treatments in citrus and apples.