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The Management of Spider Mites, with a Special Emphasis on the Cultural Practices of Growers
Masateru Inoue
Nara Prefectural Fruit Tree Research Center
1345, Yushio, Nishiyoshino-mura
Nara-Ken 637-01, Japan, 1995-06-01

Abstract

This paper discusses the efforts by farmers to identify and control mite problems in Nara Prefecture, Honshu, Japan. In the case of eggplant, farmers routinely apply miticides to their fields, usually at least three times during the growing season. In two adjacent fields, eggplant in one field may be covered in mites while there are none to be found in the neighboring field. The reasons for this difference in the degree of damage are discussed, particularly the way in which farmers perceive and respond to the presence of mites on their crops. Improved cultural practices such as proper disposal of crop residues are suggested.

Key words: Cultural practices, feeding marks, farm management, miticides, spider mites, spraying technology

Introduction

Spider mites are noxious pests which growers of fruits, vegetables, flowers etc. find very difficult to control. In Nara Prefecture in northern Honshu, Japan, extensive damage has been caused by spider mites, most of it by two species, the two-spotted spider mite ( T. urticae) and the Kanzawa spider mite ( T. kanzawai). Approximately 220 ha in Nara Prefectaure are planted in eggplant. The average growing area per farmer is around 0.1 ha, so in this prefecture there are around 2,200 fields of eggplant. In all these fields, for seven months from the beginning of planting in May to the end of the harvest in November, miticides are sprayed at least three times and sometimes more. This shows that growers feel that spider mites are highly noxious pests. However, in fact there are only a few example of very heavy field losses caused by spider mites. In many other fields, the damage is medium or very slight ( Fig. 1(1209)). It is interesting that in two adjacent fields, eggplant in one field may be covered in mites while there are none to be found in the other field. This is not necessarily because growers in the fields with no mites have paid a lot more attention to pest control.

Why does this difference in the degree of damage occur ( Fig. 1(1209))? This paper discusses the possible factors which influence an increase or decrease in damage by spider mites. It is based on a method of arranging the factors according to what I have called "the grower-field system" (GFS).

The Grower-Field System (GFS)

The grower-field system means the flow of the farm work of growers in their field, as shown in Fig. 2(1026). The grower is represented by the section above the horizontal line drawn across the center, and his field by the lower section. The right of the dotted vertical line represents the farmer on the neighboring farm (upper right) and his field (lower right). The plants planted in both fields are transmitting various signals. These might be, for example: a signal that water stress is withering the plant; or a signal that a shortage of nitrogen or magnesium is discoloring the plant. If the plants are attacked by diseases or insect pests, separate specific signals appear from the plants affected. This signal is Arrow 1 in Fig. 2(1026). By the stage of Arrow 2, the grower has seen the signal for himself. At the 3 rd step, he judges what this signal means. At Arrow 4, the countermeasures for the causes of the signals are determined. At the 5 th step, this decision is implemented. However, it is considered that these measures are not directed towards the individual plants affected, but towards the ecosystems of the field as a whole. If the measures are the right ones, at the 6 th step the plant starts transmitting a signal indicating that there is the longer any abnormality. If the field is small and there are many fields within a small area, as is often the case in Japan, the field is influenced by work carried out in the adjacent field.

Countermeasures against spider mites are executed according to the flow chart in Fig. 2(1026). If mites attack the plant, a signal referred to as a "feeding mark" _ usually a group of small, white spots _ appears. The grower finds it and judges it to ave been caused by the spider mites. He decides what pesticide to use and how to spray it, and then does the spraying. By this series of individual steps, the infestation by spider mites is stopped.

Constraints to Countermeasures against Spider Mites

If the grower can not find any feeding marks of the spider mites, or if he wrongly judges that the marks were caused by some disease, the damage continues to advance. Knowing what obstacles or constraints may get in the way of a correct decision is very important when a manual is being made for farmers or extension staff on countermeasures against spider mites.

Reliability of the feeding mark (signal of the plant attacked by the spider mites)

(Arrow 1 in Fig. 2(1026))

Spider mites are small and hide on the backs of the leaves, so they are not easily seen at the initial stage of infestation. Looking for the feeding marks that appear on the surface of a leaf as soon as the spider mite attacks the back of the leaf is less time-consuming than looking for the spider mites themselves. However, it is dangerous for farmers to rely only on this signal if we do not know how reliable the signal is. Fig. 3(953) gives the ratio between feeding marks and actual mites on a strawberry plant, and shows that the reliability of the signal varies depending on the season. If signals appear on all leaves where spider mites are living on the back of the leaf, the reliability is 100%. The greater the number of leaves on which no signals appear in spite of the existence of the spider mites, the smaller the reliability becomes. As shown in the Figure, during March and April the number of mites on strawberries cultivated in vinyl houses in Nara Prefecture were not accurately reflected in the number of feeding marks, which were too few.

How many spider mites can growers observe?

(Arrow 2 in Fig. 2(1026))

Table 1(1253) shows the results of a study on how many Kanzawa spider mites can be observed by growers. The number of growers capable of observing spider mites directly with the naked eye decreases with their increasing age, while the number of growers who can only see them with a magnifying glass increases. Growers who cannot find spider mites on the leaves even when they use a magnifying glass begin to appear from the age of 50. This result indicates that the passage in the manual stating that

"If 5 spider mites are found on 100 observed leaves, pesticides shall be sprayed"

is not a reliable guide to growers.

Is the feeding mark identified correctly?

(Arrow 3 in Fig. 2(1026))

In many cases, the feeding mark of the spider mites is a group of very small white spots. If the attack by the spider mites intensifies, the leaves turn yellow or their growth stops. The signals transmitted from the leaves of a persimmon tree attacked by the Kanzawa spider mites are rather similar to those transmitted by infection with powdery mildew. We were surprised to come across many cases in which growers made a wrong judgment when they observed these signals. Table 2(1161) is a list of examples in which growers made wrong judgments when observing plants attacked by spider mites. This type of incorrect judgment is particularly common when growers begin the cultivation of a different type of plant, or when they are using greenhouses for the first time.

Effect of spraying apparatus used

(Arrow 4 in Fig. 2(1026))

Even if the grower correctly reads the signals on the plant and sprays it with miticides, it is often impossible to stop the attack of the spider mites. One of the causes for this is the wrong choice of spraying apparatus. Fig. 4(1003) shows the results of a study of the number of leaves on which spider mites remained alive after spraying. The vertical axis shows the proportion of leaves on which spider mites remained alive, and the horizontal axis shows the amount of pesticide that has reached the leaves. This Figure indicates that when a gun nozzle was used, it was difficult to kill all the spider mites, no matter how much pesticide was used. The gun nozzle is not a suitable tool for killing spider mites.

Effect of spraying technology

(Arrow 5 in Fig. 2(1026))

Growers always carry out farm work only in their own fields. This means that the field of a grower using inappropriate spray technology is repeatedly sprayed in a less effective way. Information for farmers about pests and pesticides is mainly about the ecology of pests, effective pesticides, and the timing of spraying. Little information is available to help farmers improve their spraying techniques.

Fig. 5(1001) shows the results of a study on how much spraying techniques differ from one grower to another. Before spraying took place, pieces of a special paper which changes color from yellow to blue when pesticide touches it were attached to the leaves of a pear tree. Three farmers then sprayed pesticides using exactly the same type of sprayer. The Figure indicates the extent of the color changes in the paper between 0 and 8. Obviously, the greater the extent of the color change, the better the coverage of the tree with pesticide spray.

It shows that the pesticide sprayed by Grower 3 had a sufficient coverage of the whole tree, whereas Grower 2 (and to a lesser extent Grower 1) left much of the tree untouched. Thus, it is very important to recognize that spraying techniques differ markedly from one grower to another, and provide information for improving the spraying technology of growers.

Effect of the ecological system of the field

(Arrow 6 in Fig. 2(1026))

Our research for this portion of the Figure has not yet advanced far enough to give an objective picture of the problem.

Leaving plant residues in the field and migration of mites

(Arrow 7 in Fig. 2(1026))

Mites may be living in the plant residues after harvest. Many growers abandon these in or near the field without paying them any particular attention. However, migration of the spider mites soon takes place from residues that have been cut and have dried rapidly. Most of the spider mites migrate to new host plants within two or three days after cutting. Fig. 6(1054) shows the migration of mites from abandoned strawberry plant residues left in a grassy place near a field of snap (French) beans. The Figure shows the total number of mites which reached the weeds near where the plants were abandoned, and the number reaching the leaves of the snap bean plants in Ridges 1 _ 6 near the same place. It shows clearly that the day after the strawberry plants were abandoned; numerous mites started escaping from these residues into nearby weeds. Therefore, if it is not possible to find some way to prevent the escape of spider mites from plant residues, these should be abandoned further away from any crops or weeds that can serve as host plants. Since migrating spider mites have a strong tendency to climb up any obstacles they reach, I prevented their movement by building a low wall of plastic film, the top of which was folded over, as shown in Fig. 7(1226). Since the spider mites cannot climb over a wall of this kind, I recommend its use to prevent the migration of spider mites into greenhouses etc..

Ordinary Farm Work and Extent of Damage Caused by Spider Mites

It is very important to teach growers about efficient control methods in cases where spider mite populations are becoming established and are increasing. However, such control imposes a heavy burden in terms of the labor and pesticides required.

It is much better if damage can be avoided without increasing the burden on growers. In this regard, I should like to describe the results of research on the protected cultivation of asparagus in Japan.

In Japan, asparagus plants have withered away from the cold by December ( Fig. 8(1046)-1). The whole ridge is covered with black polyethylene mulch to prevent the growth of weeds except the central part through which the asparagus stems are growing ( Fig. 8(1046)-2). The growers cut the stalks À of the dead plants and carry them out of the greenhouse. They then remove the old polyethylene mulch ­ covering the ridges and carry that too out of the greenhouse. Their next job is to heat-treat the residual stalks and the soil surface with a burner to prevent disease (À-­-HT). However, in fields that have suffered heavy damage from spider mites at the initial stage of the harvest, the cut stems are heated with a burner immediately after they are taken out of the greenhouse, before the film is removed. (À-HT-­). A detailed study was made of the degree of carbonization of the stalks and the status of the spider mites in the fragments of stalk which had dropped onto the ridges. It found that if the heat-treatment is carried out before the plastic film is removed; growers unconsciously operate the burner in such a way as to avoid scorching the film. In this way, spider mites spending winter in the stalks were able to survive. Instructions for changing this procedure have been distributed to farmers, together with an illustration to show why it is such a disastrous practice. This prevented damage of this kind from occurring again the following year. It must be remembered that growers bear, not only an economic burden and a heavy labor demand, but also the mental burden of having to constantly adapt to new technology.

It is also worth noting that the best way for the growers of spinach plants to protect their crop from spider mites is to be rather idle while the spinach plants are very young. Diligent weeding in and around fields of very young spinach chases spider mites living in the weeds onto the spinach plants. Spinach plants old enough to have four well-grown leaves do not suffer damage from any spider mites that move onto them.

Conclusion

Around 20 years ago, in March 1974, a DC10 airplane that had taken off from Orley airport in Paris, France, crashed just before reaching a height of 4000m, killing all its passengers. The cause was that one of its many doors had not been properly shut. As the doors of the DC10 were known to have a design problem, there was a written warning on how to compensate for this _ but it was written in English. However, the airport worker who shut the door was French, and could not understand the English words. After this accident, all the doors of the DC10 were modified to prevent similar accidents. If it was the worker who shut the door who was responsible for this accident, the same accident would surely have been repeated again and again all over the world. Just as airport workers are not designers of airplanes, so growers are not researchers of pests. If the growers cannot follow the instructions of the researcher and this causes a disaster, the instructions themselves should be seen as having problems. We scientists should do research, not only on how damage caused by spider mites can be prevented, but also equally on whether the growers can execute our recommendations, given their limited supply of labor.

Index of Images

Figure 1 Amount of Damage Caused by Spider Mites

Figure 1 Amount of Damage Caused by Spider Mites

Figure 2 Flow Chart of Typical Farm Work in the Grower-Field System

Figure 2 Flow Chart of Typical Farm Work in the Grower-Field System

Figure 3 Ratio of Leaves on Which Feeding Marks Appear, Compared to the Number of Leaves on Which Spider Mites Are Living

Figure 3 Ratio of Leaves on Which Feeding Marks Appear, Compared to the Number of Leaves on Which Spider Mites Are Living

Figure 4 Pesticide Control of the Spider Mites on Pear Trees, According to Type of Spray Nozzle Used.

Figure 4 Pesticide Control of the Spider Mites on Pear Trees, According to Type of Spray Nozzle Used.

(SprayingwascarriedoutonJuly19,1994,inthefieldofpeartrees,100%ofwhichwereinfestedwithspidermites.Twotesterssprayedpesticidesusingdifferentnozzles,andinvestigatedthenumberofinfestedleavesthreedayslater).
  • ·:Circular10headatomizingport1:Gunnozzle
  • Figure 5 Appearance of Test Papers, Indicating the Frequency with Which They Came into Contact with Pesticide Sprayed by Different Growers Using the Same Spraying Apparatus. (Spray Applied on July 12, 1994).

    Figure 5 Appearance of Test Papers, Indicating the Frequency with Which They Came into Contact with Pesticide Sprayed by Different Growers Using the Same Spraying Apparatus. (Spray Applied on July 12, 1994).

    Figure 6 Prevention of Diffusion of Spider Mites with Simple Device around Plant Residues.

    Figure 6 Prevention of Diffusion of Spider Mites with Simple Device around Plant Residues.

    Withoutdevice, Residuessurroundedbydevice

    Table 1 Ability of Growers to Observe Spider Mites

    Table 1 Ability of Growers to Observe Spider Mites

    Figure 7 Structure with Vinyl Walls Folded at Various Angles, to Prevent Invasion of Spider Mites

    Figure 7 Structure with Vinyl Walls Folded at Various Angles, to Prevent Invasion of Spider Mites

    Figure 8 Status of Aspargus Plant in December (Winter).

    Figure 8 Status of Aspargus Plant in December (Winter).

    Table 2 Example of Incorrect Diagnosis of Plants Damaged by Spider Mites

    Table 2 Example of Incorrect Diagnosis of Plants Damaged by Spider Mites

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