Phytophagous insects use a variety of sensory capabilities to orient and recognize the appropriate host plants 1. Tephritid fruit flies (Diptera: Tephritidae) may use chemical stimuli in the form of nutrients to orient towards host plants 2. Food sources that are rich in nitrogen have a strong influence on the physiology and behaviour of tephritid flies 34. This behaviorally-based tactic targets female fruit flies primarily based on the female’s need of protein for ovarian development and egg production 56.

Use of proteinaceous bait sprays (protein mixed with a toxicant) are one effective method for suppressing populations of many tephritid species 789. Bait sprays mainly work on female fruit flies, which are strongly attracted to protein source from which ammonia emanate, causing flies to ingest a lethal dose of insecticide together with the protein 1011. Response of fruit flies to traps baited with synthetic lures versus those with liquid protein bait tends to be variable and host/population level may be the cause in such variation 56789101112.

Baited insecticides are an attractive alternative to conventional pesticides that used in fruit fly control, in part because the environmental impact is reduced compared with broadcast foliar sprays. GF-120 ^GF has been the most frequently tested bait against fruit flies 91314. It contains a much lower concentration of active ingredient than in broadcast spray formulations, such as in the unbaited SpinTor formulation of spinosad (0.2% compared with 22.8% in SpinTor) and it has a lower effect on non-target insects 71315. GF-120 contains 1% ammonium acetate (wt:vol) as an attractant 16. Spinosad is an insecticide derived from fermentation products of the bacterium, Saccharopolyspora spinosa Mertz and Yao that has a high safety profile 17.

Bait sprays using GF-120 became the primary tool for covering a wide area and for suppressing the tephritid fruit fly populations 181920. Although protein baits do not attract flies from long distances, GF-120 apparently is attractive to some species and has been used to suppress larval infestations in many fruit orchards 2122. In contrast, some researchers reported that GF-120 is not highly attractive to some fruit flies (i.e.Rhagoletis flies) 141823. Accordingly, Pelz-Stelinski et al. 19 emphasized the need to improve the efficiency of GF-120 bait to be more attractive.

However, given the zero tolerance for fruit flies at fruit harvest, a higher level of control is required. Therefore, increasing attractiveness of GF-120 is required to increase its commercial use for attracting fruit flies. This increase would be expected to promote greater likelihood that flies ingest spinosad, thus improving the efficacy of this bait spray. On another hand, ammonia is associated with protein-rich foods and has long been known to attract fruit flies 24252627. Yee and Landolt 28 found that increasing the concentration of ammonia in lures significantly increased their attraction to apple maggot fly, Rhagoletispomonella (Walsh). Indeed, adding more ammonium acetate to GF-120 enhanced its attractiveness to some eastern fruit fly, Rhagoletiscingulata (Loew) 19. On the other hand, pH-level of the baits plays a fundamental role in attracting fruit flies, since the effectiveness of bait is diminished as the pH-level decreased 2930313233. This information therefore could be used in chemical analysis for identification of new attractants from preferred bait formulations.

In Egypt, the peach fruit fly, Bactrocerazonata (Saunders), is a serious pest attacking a wide range of fruits that differ in their ripening time stage during the year. It is a polyphagus insect attacking more than 50 species of fruit and vegetable crops as well as wild host plants 34. It causes 25-50% damage to fruits particularly in the summer season 35. This fly could be effectively controlled using GF-120 enhanced with ammonium compounds, but the attractiveness of these baits needs to be determined.

The purposes of this study were to 1) enhance the ability of GF-120 to attract B. zonata by adding di- ammonium phosphate, ammonium carbonate and ammonium acetate, and to 2) study the relationship between pH-level and captures of B. zonata flies on traps.

It is likely that the ammonia emission rate is a significant determinant for the efficacy of candidate products, which is supported by the significantly higher catches of flies 39. Results here suggest that a change to GF-120 compassion is warranted. According to Yee 36, addition of ammonium carbonate and ammonium acetate enhanced the ability of GF-120 to attract R. pomonella under field conditions. Similarly, in this study, the ability of GF-120 bait to attract B. zonata flies was enhanced by adding di-ammonium phosphate, ammonium carbonate or ammonium acetate to the bait. The addition of these ammonium compounds (especially with concentrations of 3 and 4%) to the bait significantly attracted more B. zonata flies than bait alone. Pelz et al. 18 and Pelz-Stelinski et al. 19 found that fruit flies spent more time around GF-120 bait with additional ammonium acetate than GF-120 without it. They suggest that higher levels of ammonium acetate in GF-120 can increase arrestment of foraging flies. In another study, adding ammonium acetate to protein baits potentially increased the bait’s efficacy in attracting, monitoring and control of Ceratitis capitata (Wiedemann) 5. The current results are further confirmed by El-Metwally 40 who reported that addition of di-ammonium phosphate, ammonium carbonate and ammonium acetate to GF-120 bait improved its ability to attract C. capitata flies.

Among the ammonium compounds tested, di-ammonium phosphate improved the ability of GF-120 to attractive B. zonata flies the most, followed by ammonium carbonate and ammonium acetate. The same results were obtained for C. capitata40. Furthermore, Hemeida et al. 41 reported that di-ammonium phosphate was more effective in enhancing the ability of protein-biased baits (Buminal, Agrinal and Amadine) to attract B. zonata flies than ammonium acetate. In contrast, attractiveness of R. cingulata flies to GF-120 bait increased as concentrations of ammonium acetate increased compared to di-ammonium phosphate 19. But ammonium carbonate was more effective in improving the GF-120 bait to attract R. pomonella than ammonium acetate 36. This variation may be specific to fruit fly species or compound concentrations. Yee and Landolt 28 found that captures of R. pomonella flies in traps increased as concentrations of ammonia in lures increased from 0 to 29.3%. Limited improvement in attractiveness to R. pomonella also was reported for ammonium bicarbonate added to the protein hydrolysate bait, NuLure 42.

Results showed that attractiveness of highly concentrated treatments (especially the highest concentrations of di-ammonium phosphate) decreased after two days of hanging traps. This was explained by Yee 36 who documented that the lack of differences between GF-120 alone and GF-120 enhanced by ammonium carbonate or ammonium acetate was likely due to ammonia release rates from enhanced drops decreased quickly after sprays, so after a few days or even less time the enhanced GF-120 was the same as GF-120 alone in attractiveness. Ammonium acetate dissipates rapidly because of its high volatility 9. This volatilization is likely the cause of the loss of GF-120 attractiveness to the melon fly, BactroceracurcurbitaeCoquillett, observed within the first day after application. Further, the present results are consistent with the trends observed by Epsky et al. 4344. They mentioned that volatile attractant chemicals from Nulure/borax solution may be released at levels too high for effective fly capture when freshly prepared but are too low to have an effect on fly capture by 4-5 days after preparation. Vargas et al. 13 noted that aged baits (4 days-old) were as not attractive to C. capitata as were fresh baits.

The ability of GF-120 to attract B. zonata was found to be basically dependent on the ammonium compound, the concentrations, and pH with positive relationships. The degree of pH was positively affected by the concentration of added ammonium compound to GF-120. For example, each increase in the concentration of di-ammonium phosphate by one percent increased pH by 0.88, 0.71 and 0.86 degrees in fresh bait and at the end of treatment under high and low populations, respectively. These results are consistent with those of Bateman and Morton 45, Heath et al. 46, El-Gendy 32 and El-Metwally 40; they mentioned that the attracted fruit flies to ammonium compounds, food baits and GF-120 (that enhanced by ammonium compounds) were found to be strongly dependent on concentration of ammonium compound and therefore on pH level. Further, there were positive relationships between pH level of some protein-based baits and the attracted flies of B. zonata and C. capitata31. El-Gendy 32 added that borax makes the solution of ammonium compounds more alkaline and therefore increases release of ammonia from the bait solution (which increased the attracted B. zonata flies). Also, addition of 1-10% borax to 10% Nulure solution (as food attractant) increased pH level, resulting in increased captures of Anastrephasuspensa (Loew) and C. capitata in traps under field conditions 293031323334353637383940414243. Mazor et al. 47 mentioned that the elevation of the pH of the liquid commercial baits, Buminal and Naziman, increased the efficacy of the latter for C. capitata but the increased stimulation could not be strictly correlated with the increased rate of ammonia release. The highest captures of C. capitata in McPhail traps occurred with Milhocina and borax adjusted to a pH of 8.5 33. In the present study, the highest attracted B. zonata flies were recorded when pH more than 7.00. On the other hand, B. zonata females were more responsive to the increase of pH and concentrations of ammonium compounds than males. Similar results were obtained by El-Metwally 40, who mentioned that females of C. capitata were more responded to the increase of pH in GF-120 preparations.

At the end of experiments, pH level decreased under high and low population levels; which resulted in decreased fly captures. These results are in agreement with El-Gendy 31, who mentioned that the properties of protein bait solutions in traps were changed along the elapsed time. Heath et al. 46 added that resultant pH affected by factors such as age of bait solution increased attraction of flies. According to the present study it may be affected by ecological factor such as air temperature degrees, relative humidity and/or population level of the attracted flies. Also, many authors showed through regression analysis that attraction of some ammonium compounds and food attractants decreased by time after treatment 24252648. Epsky et al. 44 and El-Gendy 32 mentioned that there were no significant differences in pH levels of food baits and ammonium compounds over the time period of the test. Climatic and habitat differences may explain the inconsistency between the present results and others.

According to the relationships between concentrations of ammonium compounds and the resulted pH in the prepared solutions, there were positive relationships between them. The degrees of pH were more affected by the concentrations of ammonium compounds in fresh baits compared to those in baits at the end of experiment under field conditions. This may be attributed to the effects of ecological factors on the baits in the field. At the end of experiment under field conditions, pH varied from high to low pest population levels; whereas, the general pH-levels were higher in the high than low population experiment. This may be attributed to the high numbers of dead flies in traps in the high population experiment, which may have changed the chemical composition of the bait and therefore modified its pH level and therefore its attraction. This theory is agreement with that of Rousse et al. 30, who documented that the chemical composition of the bait may change because of dead flies, increasing its attraction to fruit flies, while living flies might emit attractive or repulsive volatiles.

The obtained results showed that GF-120 alone or enhanced by ammonium compounds attracted females more than males. These results are in agreement with those obtained by Yee 36 who reported that females of R. pomonella were more responsive to the GF-120 lures on traps than males. He added that the lures affected the sexes similarly in terms of relative responses. Also, many studies reported that ammonium compounds and protein-biased baits attracted more female than male flies 24252629313240414349. Rousse et al. 30 showed that numerically more females than males of B. cucurbitae were caught in traps baited with food attractants especially when they were protein starved. Because fruit flies require a source of protein to complete egg maturation 50 this requirement is probably the main cause for the strong attraction of females towards decomposing proteinaceous substances 51.

The present results showed that the sex ratios of B. zonata (as number of attracted females per one male) obtained by GF-120 alone or enhanced by ammonium compounds were higher under low population level in comparison with high population level. This may be explained as follows: data in the high population level experiment were obtained in guava orchard at the end of the fruit ripening period (9-20 October 2017), where as in the low population level experiment they were obtained in navel orange orchard before the fruit ripening period (20-31 October 2017). Thus, many infested guava fruits were fallen under trees (as field observation) and most flies (females and males) were newly emerged; therefore both sexes of flies were attracted to food sources for nutrition. In navel orange orchard, females occurred as visitors searching for hosts to deposit their eggs. The present results are supported by El-Metwally 40, who mentioned that the sex ratios of attracted C. capitata to GF-120 enhanced with ammonium compounds tended to females in guava orchards more than those in navel orange orchards. Also, Pinero et al. 5 and Hemeida et al. 41 reported that adding di-ammonium phosphate, ammonium carbonate or ammonium acetate to a variety of protein baits and materials increased the numbers of attracted females of C. capitata and B. zonata in comparison with attracted males.

The ability of GF-120 to attract B. zonata flies can be enhanced by additing di-ammonium phosphate or ammonium carbonate at concentrations of 3 or 4%. In comparison with males, females of B. zonata were more responsive to the increase of pH. Attractiveness of GF-120 preparations is depending on pH; so, it can be used in chemical analysis for identification of new attractants from preferred bait formulations. Information regarding the effect of pH on the efficiency of different attractants in attracting fruit flies under various ecological situations should be studied.

The author wishes to thank Dr. Mostafa M. El-Metwally and Dr. Reda A. El-Sharkawy, Plant Protection Research Institute, Agricultural Research Center; for assisting in the field bioassay and determining pH in the present study.