EFFECT OF TETRANYCHUS URTICAE POPULATIONS ON PEACH PRODUCTION IN SOUTH CAROLINA 1.2

Joe Kovach and Clyde S. Gorsuch Department of Entomology

Clemson University Clemson, SC 29631

Abstract: Forty-eight 'Redhaven' peach trees were infested with three different density levels of Tetranychus urticae to determine the effect of mite feeding on peach tree yield, leaf loss and bloom density. Mean fruit weight and fruit size were not I'educed on trees that accumulated 852 mite-days prior to harvest with peak densities of 105 mites/leaf. Bloom density the following season was larger and leaf drop was earlier on high mite density trees. Medium mite density trees that accumulated 467 mite-days (peak 48 mites/leaf) produced a higher percentage of large fruit (~ 6.4 em) and a lower percentage of small fruit (.:$: 5.0 cm) when compared to mite-free trees.

Key Words: Telranychus urticae, twospotted spider mite, Prunus persica, peach yield, Tetl'anychidae, peach insects.

J. Agric. Entomol. 2(1): 46-51 (January 1985)

Effects of mite populations on fruit crops vary with mite species, crop species, cultivar, tree vigor, time of attack and crop load. Because mites are foliage feeders, economic losses to fruit crops are indirectly induced. These losses may occur through removal of cell contents resulting in a reduction of photosynthesis, an increase in leaf transpiration or premature loss of leaves causing an overall reduction in tree vigor, fruit size, quality and yield.

European red mite (ERM), Panonychus uimi (Koch), populations up to 2.5 mites/cm:! (75 mites/leaf) on 'Lord Lambourne' apple trees had no inDuence on rate of fruit drop or fruit yield. Although considerable bronzing occurred, there was no effect on Dower bud initiation the following year (Briggs and Avery 1968). Reduction in the number of large fruit on apple trees infested with ERM was found in only 2 of 5 years with peak populations in these years occurring before mid July (Light and Ludlam 1972). On 'Newton' and 'Golden Delicious' apple trees up to 30 ERM/leaf, with peaks greater than 70 mites/leaf, could be tolerated without any adverse effects (Zwick et al. 1976).

Mixed populations of ERM and twos potted spider mite (TSSM), Telranychus urlicae Koch, (about 28 mites/leaD did not reduce fruit size on 'Cortland' and 'Delicious' apple trees, but yield of 'Cortland' trees was reduced by 65% and fruit set the following season was affected by 50 mites/leaf (Chapmen et a1. 1952; Lienk et al. 1956). The most significant damage to 'Red Delicious' apples by TSSM was a reduction in fruit growth rate. An estimated 3000 mite-days (peak L20 mites/ leaf) were required to produce an approximate 12% yield loss (Hoyt et al. 1979).

1 Technicill Contrihution No. :!317. s. C. ,'~ric. Exp. Stn. Puhlished hy l'~rnlis.lli,," of the Director. '2 R~cei,'ed for r",hlicot;olt )·r August 1984: ilcccl'lcd 19 Octohu 198.1.

KOVACH and GORSUCH: Effect of Mites on Pecan Trees 47

During a 3 years study in Oregon, fruit size, fruit finish, preharvest drop and fruit set on mature 'Anjou' pears were unfavorably influenced by 5 TSSM/leaf. However, after the first season 16 mites/leaf had no influence on yield (Westigard et aJ. 1966).

On ;Wright' peach trees grown in Australis, no significant relationship could be established between mite density and fruit yield, but peach trees could tolerate up to 40 - 50 TSSM/leaf without reducing yield (Bailey 1979).

As in other fruit growing regions of the world, outbreaks of TSSM are induced in South Carolina orchards by the use of organophosphate insecticides used to control other orchard pests. These outbreaks are usually controlled by 1 or 2 miticide applications. Some S. C. growers do not apply any miticides and claim that. yield is not affected. This study was conducted to determine the effect of TSSM populations on peach tree yield, leaf loss and bloom density.

MATERIALS AND METHODS

Forty-eight ;Redhaven' peach trees located at the Sandhill Experiment Station in Richland County, se, were used for the yield study during 1981. 'Redhaven.' an early mid-season peach, is a semi-free st.one cultivar that is harvested in late June or early July in South Carolina. Trees were in their peak production years and were drip irrigated to essentially eliminate any water stress encountered during the growing season. All trees were on a standard insect/disease control program for South Carolina I>eaches. This program consisted of cover sprays of encapsulated methyl parathion and captan with blossom and preharvest sprays of thiophanate· methyl (Gorsuch and Miller 1981). Trees were selected for uniformity in size and treatments were randomly applied at three density levels.

High and medium mite density treatment levels were created by placing TSSM infested lima bean plants in the center of the trees. These trees were sprayed six times during the season with carbaryl to reduce natural enemy populations and increase mite populations. Low mite populations were maintained by two applications of cyhexatin. Trees with low densities of mites were considered to be mite· free for comparative purposes.

Mite numbers were estimated by picking 20 leaves at random from each tree on a weekly basis before harvest and biweekly after harvest. Leaf samples were inserted into labeled paper bags, placed in a cooler, and taken to the laboratory for examination. A leaf bmshing machine was used to remove the mites from the leaves. All live mites were counted and the mean number of mites per leaf per tree was recorded. Mite-days were then calculated (Hoyt et 01. 1979).

Each tree was hand thinned to a spacing of ca. 15 em between fruit for maximum fruit size (Daniell 1983). Fruit was harvested when ripe and trees were stripped of all fruit during the final picking. All fruit from each tree was counted, measured and weighed. Peaches were grouped by size into four categories; less than 5.0 em, 5.0 to 5.7 em, 5.7 to 6.4 em, and greater than 6.4 CIlL Fruit yield was expressed 8S mean fruit weight per tree and the percent fruit in the four size categories.

Leaf loss was recorded by grading trees on a scale from zero to ten and percent leaf loss was determined. Trees were graded until all were naturally defoliated.

48 J. Agric. Entomol. VoL 2, No.1 (1985)

The following spring, five branches per tree were randomly selected. Shoot length of each branch was measured and all flowers counted. In this way, bloom density pCI' tree was derived. Fruit set and yield were not obtained due to two killing frosts that occurred in the spring of 1982.

A concurrent test was conducted to determine the effects of the additional chemical applications on mean fruit weighL Each chemical that was applied to manipulate mite populations in experimental treatments wns applied to individual branches of 12 additional mite-free 'Redhaven' trees. Three limbs from each tree were randomly selected. One limb on each trcc was a control limb. A second limb was sprayed six times during the season with carbaryl (l50g AI/IOO liters). The third limb received two applications of cyhexatin (45g AlIIOO liters). This test was conducted to differentiate the effects of chemical treatments and mite population density on mean fruit weight All data were analyzed statistically using analysis of variance and means were separated by Duncan's multiple range test (P .:$; 0.05).

RESULTS AND DISCUSSION

No difference was found between the mean fruit weight from individual limbs treated with no pesticides, si.'X applications of carbaryl or two applications of cyhexatin.

Table 1 shows the number of fruit per tree after hand thinning, mean fruit weight, mean mite-days and mites/leaf. There was no difference in number of fruit per tree between mite-infested trees and mite-free trees. Any difference in mean fruit weight and fruit size which may occur between treatments was not due to thinning or additional chemical application.

Table 1. Comparison of the mean number of fruit per tree present on 'Redhaven' trees and mean fruit weight at different TSSM density levels!

Mite Meant Meant Mean density mite peak Mean no. of fruit weight level (N) days mites/leaf fruit per tree (g)

low (32) 5 ± 15 a 1 ± 1.7 a 694 ± 42 a 97.7 ± 2.8 ab med (9) 467 ± 9 b 48 ± 3.3 b 654 ± 80 a 107.8±5.2 a high (7) 852 ± 32 c 105±3.7 c 658±91 a 92.8 ± 5.9 b • McnAll ± S,E, in ellth colullIn follo....ed by tho: some letter are not llignificantly difrerenl III the a'i:. le"eL Duncan's

multiple runge tesL t Prior 10 hlll'""ul.

A difference in mean fruit weight between high (92.8g) and medium (107.8g) density trees occurred, but no difference was detect.ed between mite-free and mite· infested trees. A more sensitive method of determining differences in yield between treatments is by fruit size.

Figure 1 shows the percent fruit in the various size categories at the different mite density levels. There was no difference in fruit size between high mile density trees and mite-free trees_ When comparing the medium density trees (467 mite­days) to the mite-free trees, however, a difference occurred in the number of fruit in the largest and smallest categories. The medium density trees produced a higher percentage of fruit over 6.4 em (11 %) and a lower percentage of fruit under 5.0 cm (38%). This indicates that mite populations with a peak of 48 TSSMlleaf present before haIVest may increase fruit size on healthy, irrigated (.fees.

49

MITE DENSITY Q

60 LOW

MED

HIGH

50 >­a: o ~40 t­et u ILl 30 N fI)

c·-20 ";!­

10

'<;5.7 ~6.4 >6.4

KOVACH and GORSUCH: Effect of Mites on Pecan Trees

FRUIT SIZE (em)

Fig. 1. Effect of TSSM populations on fruit size: • Means in each utegof)' rollowed by [hI'! SlIme leiter .~ not 5i~nili<:anll)' d,f{uenl al the $0';0 level. Duncpn', mu[ti!l]e

rnnge te!;l.

The relationship of population density to leaf loss and bloom density is presented in Table 2. High mite density trees (3203 mite-days) had larger bloom densities the following year than the trees that accumulated 1745 mite-days. High mite populat.ions also caused trees to lose their leaves sooner than mite-free trees. This earlier leaf drop did not seem to influence the bloom density of the tree the following season. Although it was not statistically significant (P :$. 0.05) medium mite densities seemed to produce an earlier leaf drop than mite-free trees.

50 J. Agric. Entomol. Vol. 2. No.1 (1985)

Table 2. Effect of TSSM populations on leaf loss and bloom density of 'Redhaven' peach trees.-

Mite density Total % leaf loss Bloom density level (N) mite·days 88 days post-harvest (flowers/em)

low (32) 82 ± 63 a 28 ± 1.3 a OA1±0.01 ab med (9) 1745 ± 119 b 35 ± 2.7 ab 0040 ± 0.02 b high (7) 3203 + 136 e 36 ± 2.7 b 0046 ± 0.02 a - Means ± S,E" in ellch ~Ollll1ln folloll"ed b)" the ~llme leiter lite not sjgnificllntb" different Itl the a~~ le'·el. Duncan's

Illultiple rAnge (elli.

Because there is a constant competition between vegetative tissue and fruit tissue for water and photosynthates produced by the tree, any factor that limits vegetative growth will result in more photosynthates becoming available to fruiting tissue (Coston 1983). An example of this phenomenon is placing peach trees under water st.ress during the early stages of fruit development when vegetative growth is rapid. By withholding water, vegetative growth is suppressed and the capacity to compete for photosynthesis is reduced. After adequate water is restored, fruit. growth is rapid and produces larger fruit than trees that were never under water stress (Chalmers et al. 1981).

The mites. by rupt.uring leaf cells. not only increased transpiration and reduced net photosynthesis. but, after an accumulation of 420 mite-days reduced non· structural carbohydrate levels (Ferree and Hall 1980), This reduction in leaf carbohydrate levels will increase the osmotic potential of the injured leaves suppressing vegetative growth and increasing water movement to fruiting tissue. The medium mite densities in this experiment may have reduced photosynthesis and leaf carbohydrate levels at a critical period of fruit development to shift the within tree competition for water and photosynthates in favor of the fruiting tissue resulting in larger fruit at harvest. If mites influence this within tree competition then flower bud initiation (bloom density) should also be affected.

Flower bud initiation in peach trees occurs in late June or early July when mite populations were highest during this experiment. At medium mite densities the fruit receives more photosynthates resulting in larger fruit. At high densities more photosynthates are apparently routed toward production of next year's nower buds as evident by the increase in bloom density. What effect high mite density has on fruit set is not known.

The results in this experiment may explain why some growers do not see any reduction in yield during TSSM outbreaks. In any orchard during an outbreak, mite densities will vary from tree to tree. This mite variation will cause individual tree yields to vary. Any yield loss that may occur from extremely high mite density trees could be compensated for by higher yields from medium mite density trees. This would result in no apparent yield loss from mite infestation.

The results of this study indicate that in some instances certain densities of mites (48 TSSfvl/leaO accumulated over a short period. (467 mite-days), may increase fruit size without adversely affecting bloom density or leaf loss. Mite density peaks of 105 mites/leaf did not reduce yield of irrigated peach trees. but leaf drop was earlier. The long term effect of TSSM populations on peach trees needs to be determined before an accurate damage threshold level of TSSM can be derived for pest. management practices.

51 KOVACH and GORSUCH: Effect of Mites on Pecan Trees

REFERENCES CITED

Bailey, P. 1979. Effect of late season population of twospolled mite on yield of peach trees. ,J. Econ Entomol. 72: 8-10.

Briggs, J. 8., and D. J. Avery. 1968. Effects of infestation with fruit tree red spidel' mite, PanOIl}'dIUS ulmi (Koch), on the gt'Owth and cropping of young trees. Ann. Appl. BioI. 61: 269-276.

Chalmers, D. J., P. D. -"1itchell. Ilnd L. van Heck. 1981. Control of peach tree growth and productivity by regulated wnler supply, tree density and summer pruning. ,J. Am. Soc. Hortie. Sci. 106: 307 -312.

Chapman, P. J., S. E. Lienk, and O. F. Curtis. 1952. Responses of apple trees to mile infestation: I. J. Econ Enlomol. 45: 815-82!.

Coston, D. C. I98:l. Peach tree physiology. In M. E. Ferree and P. K. Bertran leds.l. Peach Gro""'ers Handbook. University of Georgin Cooperative Extension Service.

Daniell, J. W. 1983. Thinning. III 1'0'1. E. Ferree and P. F. Bertran leds.l. Peach Growers Handbook. University of Georgia Cooperative Extension Service.

Ferree, D. C., and r. R. Hall. 1980. Effects of soil water stress on two-spotted spider mites on nel. photosynthesis and transpiration of apple leaves. Phot.osynt.h. HeB. 1(3): 189· 198.

Gorsuch, C. S., and R w. r..'liller. 1981. 1981 Commercial peach spray guide. Clemson University Cooperative Extension Service. Informat.ion card 72.

Hoyi. S. C., L K Tanigosh~ and R. W. Browne. 1979. Economic injury level studies in relation to mites on apples, pp. 3·12. In J. G. Hodriguel. led.l. Recent Ad\'ances in Acarology. Academic press Inc., New York. Vol. I. 631 pp.

Lienk, S. E., P. J. Chapman, and O. F. Curtis. 1956. Responses of apple trees to mite infestations: II. J. Econ. Entomol. 49: 350-353.

Light, W. I. SL G., lind F. A. B. Ludlam 1972. The effects of fruit tree red spider mite (Pnnonychus ulmi) on yield of apple trees in I<enl. Plant Pathol. 21: 175-181.

Wcst.igard, P. H., P. B. Lombnl'd, and J. H. Grim. 1966. Preliminary investigntions of the effect of feeding of various levels of two spotted spider mite on its Anjou pellr host. Proc. Am. Soc. Hortic. Sci. 89: 117-122.

Zwick.. R. W., G. J. Fields, and W. M. Mellenthin 1976. Effects of mite population density on 'Newlon' und 'Golden Delicious' apple lree performance. J. Am. Soc. Hortic. Sci. 101: 123-125.