The red spider mite, Tetranychus marianae McGregor (Acari: Tetranychidae), is one of the most serious pests on vegetable crops, particularly on eggplant (Solanum melongena L.; Solanaceae) and tomato (Solanum lycopersicum L.; Solanaceae), in the Mariana Islands (Reddy et al. 2011). This mite species is widespread in the Pacific Islands, including the Mariana Islands, where it was first reported (Reddy & Bautista 2012). In fact this species was described first from specimens collected in Tinian, Mariana Islands (McGregor 1950). Tetranychus marianae is also a pest of several annual crops and certain perennials (Reddy et al. 2013). This mite could also become important in pasturelands and rangeland (Reddy & Kikuchi 2011). Heavily infested tomato leaves become yellowish-green to yellowish brown (Wene 1956). Moreover, feeding due to T. marianae causes a silvering on the shoulders of the fruit, and this silvery area become russet brown in appearance. Denmark (1970) stated that the mites cause tomato leaves to become chlorotic and to curl. This species has been shown to migrate 10 m from one field to another (Wene 1956). While T marianae is known to be polyphagous, it prefers to feed and live on solanaceous plants (Bolland et al. 1998).

The tomato fruitworm, Helicoverpa armigera(Hübner) (Lepidoptera: Noctuidae), is a polyphagous pest attacking tomato, peas and all other crops that are grown in Guam and other neighboring islands (Reddy & Kikuchi 2011). Due to its high fecundity, polyphagous nature and high reproductive potential, control of this pest has become very difficult (Reddy & Manjunatha 2000). Females oviposit on the flowering and fruiting structures of these crops. Larvae feed voraciously, leading to significant crop losses (Reed & Pawar 1982). The capability of ovipositing females to find and exploit a wide range of hosts from a number of plant families is one of the main reasons contributing to the pest status of this species (Zalucki et al. 1986). Helicoverpa armigera larvae are enormously damaging because they prefer to feed and develop on the reproductive structures of crops which are rich in nitrogen (Fitt 1989). On tomato, the fruits are preferred for feeding, but flowers, stems and leaves can also be injured. Fruit damage can result in rot or attack of secondary pests. If larvae are big and the fruits are small, one larvae can attack more than one fruit per day (Reed & Pawar 1982).

Tomato growers in the Mariana Islands apply nearly 15 chemical sprays per cropping period, which often leads to the development of resistance of insect pests to the chemicals. The development of resistance in H. armigera, a wide range of pesticides has been documented (McCaffery et al. 1991; Jouβen et al. 2012). Similarly, the concern for the development of resistance in T marianae to insecticides and acaricides led to the search for alternative pesticides since at least 1964 (Wolfenbarger & Getzin 1964).

The use of action threshold in pest management programs results in reduced insecticide usage (Farrington 1977). However, no action threshold levels have been developed for T. marianae and H. armigera on tomato in the Mairana Islands. Growers in the Mariana Islands are unaware of the adverse effects of chemical sprays on natural enemies and of the benefits of using action threshold.

The objective of the present study was to develop action threshold levels for timing pesticide sprays for managing T. marianae and H. armigera on tomato.

MATERIALS AND METHODS

RESULTS

DISCUSSION

This study indicated that an action thresholdbased approach for determining when to use an insecticide application for pest control in tomato growing was operative and resulted in fewer insecticide applications. Growers should be recom- mended to follow the practice of action thresholdbased chemical sprays in managing major pests for tomatoes and avoid the traditional regular chemical sprays. From our results, the thresholdbased chemical sprays could reduce infestation of both T. marianae and H. armigera better than the regular chemical sprays.

TABLE 1.

THE DAY AFTER TRANSPLANTATION (DAT) AT WHICH A CHEMICAL TREATMENT WAS IMPOSED AFTER GETTING THE ESTIMATED THRESHOLD LEVEL OF MEAN NUMBER OF TETRANYCHUS MARIANAE IN COMPARISON TO REGULAR SPRAY SCHEDULE ON TOMATO.

TABLE 2.

PERCENTAGE OF MITE DAMAGED LEAVES BY TETRANYCHUS MARIANAE IN DIFFERENT TREATMENTS IMPOSED ON TOMATO.

The action threshold-based chemical spray has been used to control T. marianae with Sunspray 6E on other crops such as eggplant, with the optimum threshold of 4 mites per leaf during the dry season and 8 mites per leaf in the wet season (Reddy et al. 2013). For our present study, TABLE 3. MEAN NUMBER OF TETRANYCHUS MARIANAE RECORDED IN DIFFERENT TREATMENTS IMPOSED ON TOMATO. the same Sun-spray 6E was used and resulted in the mean number of T. marianae and infested leaves of tomato plants being significantly fewer in threshold-based chemical spray treatments than in regular spray treatments. It was obvious that spraying the plants when 8–12 mites were observed per leaf diminished the infestation significantly. However, we found no significant difference in reducing infestation incidence among T1, T2, and T3 in the dry season and among T1, T2, T3, and T4 in the wet season for both locations (Table 2, 3). Therefore, instead of spraying 7 times, the number of sprays could be reduced to 4 in the dry season and only 3 in the wet season (Table 1).

TABLE 3.

MEAN NUMBER OF TETRANYCHUS MARIANAE RECORDED IN DIFFERENT TREATMENTS IMPOSED ON TOMATO.

Fig. 1.

Marketable yield (tonnes/ha) of tomato after chemical application in different treatments on tomato grown in dry and wet season T1: TCS (8 mites/leaf); T2: TCS (10 mites/leaf); T3: TCS (12 mites/leaf); T4: TCS (14 mites/leaf); T5: TCS (16 mites/leaf); T6: RSS (15 DAT); T7: RSS (30 DAT); T8: RSS (45 DAT); T9: RSS (60 DAT); T10: Control (no spray). Abbreviations: DAT = days of after transplantation; TCS = Threshold-based chemical sprays; RSS = Regular spray schedule.

The season also affected the level of infestations for T. marianae. In the previous study conducted on eggplant, different threshold levels were indicated for T. marianae during the dry and wet seasons (Reddy et al. 2013), dry and warm climate contributes to higher potential for T marianae populations to increase (Denmark 1970). This could be the explanation in the present study that T4 (14 mites/leaf) did not differ significantly from T1, T2 and T3 in the wet season (Table 2,3) but did for the dry season. Thus, a threshold of 12 mites/leaf would be indicated for the dry season, and a threshold of 14 mites/leaf would be indicated for wet season.

For H. armigera, the spray thresholds were based on surveys of the eggs in the field, but egg counts were found previously to be an unreliable criterion, due to high mortality rates of egg populations, while larval counts were shown to be more reliable indicators of the pest (Kfir & Vanhamburg 1983). Because H. armigera is a polyphagous pest, which damages many plant species, there have been some studies on using action threshold-based sprays to control this pest. For cotton genetically modified to express the endotoxin of Bacillus thuringiensis, the current spray threshold was set at 5 larvae/24 plants and 12 eggs/24 plants, with third instars each counted as 2 larvae (Basson 1987; Nel et al. 2002). Russell et al. (1998) suggested that an egg action threshold of 1 egg per cotton plant but the action threshold for larvae differed according to difference in stages in the crop's phenology. For tomato, Cameron & Walker (1988) developed a scouting procedure for the implementation of the economic thresholds when making early evaluation with H. armigera in New Zealand. They found larval populations to be significant predictors of damage in tomatoes. Spraying insecticide based on the 1- min scouting threshold of 1 larva per tomato plant was suggested for the control of H. armigera especially in temperate climate (Cameron et al. 2001).

TABLE 4.

THE DAY AFTER TRANSPLANTATION AT WHICH A CHEMICAL TREATMENT WAS IMPOSED AFTER GETTING THE ESTIMATED THRESHOLD LEVEL OF MEAN NUMBER OF HELICOVERPA ARMIGERA IN COMPARISON TO THE REGULAR SPRAY SCHEDULE ON TOMATO.

TABLE 5.

PERCENTAGE OF FRUIT DAMAGE AND MARKETABLE YIELD OF TOMATOES IN DIFFERENT TREATMENTS WITH VARIOUS ACTION THRESHOLDS FOR HELICOVERPA ARMIGERA .

Damage threshold levels have also been reported for other species of fruitworms including H. zea (Boddie). In fresh tomatoes (Kuhar et al. 2006), the action threshold regimen with the highest yields, lowest fruit damage and fewest number of insecticide applications was when the first spray was applied when H. zea eggs were observed on ≥ 10% of plants sampled, and when subsequent sprays were made only when ≥ 3 of 100 unripe fruits were observed with damage. In the present study, no differences in the percentage of damaged fruit occurred when the first insecticide sprays were applied when 1 or 2 H. armigera eggs were detected on 10 leaf samples and when subsequent sprays were applied either every 7 days until the end of harvest or when 1–2 damaged fruit or H. armigera larvae were observed per 50 immature fruits (T1, T2, T5, T6) (Table 5). These results are similar to those of Zehnder et al. (1995) which indicated that the application of sprays based on an action threshold of 1 egg/10 plants reduced the number of sprays needed for effective insect control. However, in the present study, applying the initial spray when 2 H. armigera eggs were detected on 10 of the samples, and applying subsequent sprays only if 2 damaged fruit or larvae were detected per 50 immature fruits (T6) could lead to the highest marketable yield.

A large number of pesticides are sprayed on tomato because this crop is subject to attack by multiple pest species from the time plants first emerge in the seed bed until harvest (Godfrey 2011). Spider mites and fruitworms threaten young plant-bed tomatoes. Spider mites often become a problem after applying pesticides, because natural enemies of the mites may be reduced and because certain insecticides may stimulate mite reproduction. Godfrey (2011) showed that spider mites reproduced faster following exposure to carbaryl. Pyrethroids are another group of pesticides which have been used widely to control mites (Godfrey 2011). Similarly, dicofol has been used to control Panonychus ulmi (Koch) and Tetranychus urticae Koch throughout the world for nearly 30 years (Dennehy & Granett 1984a, b), and has been found to be very effective. Dicofol continues to be an important acaricide for many crops such as California cotton (Dennehy & Granett 1984a, b), New York apples (Dennehy et al. 1988), and Brazilian and Japanese citrus (Inoue 1979; Gravena 1988). Recently, there have been new acaricides registered and introduced to control of spider mites; for example, milbemectin, fenproximate, acequincyl, spiromesifen. However, resistance to these acaricides could develop especially when excessive repeated applications occur such as in the case of dicofol, which was found to be resisted by spider mites (Singh 2010). Also Tetranychus cinnabarinus (Boisduval) was proven to have resistance to dicofol (Dağli & Tunç 2001).

There are also a number of plant extracts formulated as acaricides that exert an effect on spider mites. These include garlic extract, clove oil, mint oil, rosemary oil, cinnamon oil and others (Godfrey 2011). Another option to control mites is the use of horticultural oils, either petroleum or vegetable based. The primary mode of action of horticultural oil is suffocation. The oil blocks the spiracles through which insects breathe. Moreover, oils have also been shown to have antisettling and anti-feeding effects for some insects such as aphids and leaf hoppers (Smith 2012). Sun spray oil is one horticultural oil, which is an effective acaricide with low toxicity to the environment (Miller 1997). Baxendale & Johnson (1988) reported that 3% oil was highly effective in controlling spider mites on greenhouse specimens of large-flowered tuberous dahlia (Dahlia spp.; Asteraceae) without phytotoxicity. However, the repeated applications of oil could also depress tomato yields due to its scorching effect, which leads to phytotoxicity (Muqit et al. 2006).

Helicoverpa armigera is also capable of becoming resistant to insecticides. Resistance to synthetic pyrethroids has been reported in Thailand and India, and Heliothis virescens has become resistant in the USA (Riley 1989). Helicoverpa armigera has a history of resistance to DDT and has also developed resistance to endosulfan (Forrester et al. 1993), carbamates and organophosphates (Gunning et al. 1992). A few alternative controls have been introduced to control the fruit worms including mating disruption with pheromone (Rothschild et al. 1982; Kehat & Dunkelblum 1993; Chen et al. 1995), neem extracts (Thakur et al. 1988; Rao et al. 1990), and microbial insecticides including nuclear virus (Tinsely 1979; Bell 1982; Mckinley 1982), also Bacillus thuringiensis and Bacillus thuringiensis (Bt) transgenic crops (Downes & Mahon 2012). The tetranotriterpenoid, azadirachtin, is the most promising insecticidal compound found in seeds and leaves of the neem tree (Azadirachta indica A. Juss; Meliaceae) (Butterworth & Morgan 1968; Spollen & Isman 1996). It has been reported that neem products are less toxic to natural enemies than synthetic pesticides and that they have several biological effects on insects including antifeedant, insect growth regulator and repellency effects (Zehnder & Warthen 1988; Saxena 1989). Wakil et al. (2012) reported that 2nd instars of H. armigera were more susceptible to the neem products than other life stages. The maximum mortality (50%) was observed in the treatment where the larvae were exposed 72 h to the neem leaf extracts and neem seed kernel oil. This same treatment resulted in 40% mortality of 4th instars at the same exposure interval. However, Reddy & Manjunatha (2000) proposed that no single method of control can be expected to provide an acceptable solution to all insect problems where a complex of pests is involved. They evaluated the use of parasitoids, NPV, chemical insecticides and pheromone in the control of H. armigera on cotton and suggested that the IPM strategy is the most profitable and sustainable in the management of this pest in order to minimize harm to the environment and delay the development of resistance.

Additional study is needed to evaluate if there is an interaction between the control of H. armigera and T. marianae. The Sun-spray 6E might have contributed to the control H. armigera also. The synergism or additive effects could occur because the dates of application overlapped.

In summary, the present study established thresholds to control 2 of the key tomato pests in a typical warm, tropical climate using environmentally friendly pesticides. For T. marianae initial sprays should be applied when 8 to 14 mites/leaf are observed during the wet season and when 8 to 12 mites/leaf are observed during the dry season. For H. armigera, the initial spray could be applied when 2 eggs are detected on 10 leaf samples and subsequent sprays would be applied only if 2 damaged tomatoes or 2 borer larvae are found per 50 immature tomatoes. The use of such action thresholds should enable growers to better time pesticide applications, and thus reduce the potential for resistance development and environmental harm.