Sterilization of Cosmopolites sordidus (Germar, 1834) (Coleoptera: Curculionidae) Eggs with Physiologic Insecticides as Integrated Pest Management Strategy

Introduction

The corm weevil Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) is a major banana insect pest (Musa spp.). This pest’s larvae damages the corm and roots. Consequently, the emergence of new roots has delayed, and the nutrient uptake reduced. Weevil attack can prevent crop formation, cause significant yield reductions in ratooning cycles and contribute to a shortened plant life [1, 2, 3].

In addition, the yellowing of the leaves and the reduction of the size and weight of banana clusters has observed after the pest attack. Along with this, pathogenic microorganisms such as Fusarium oxysporum f. cubense responsible for the disease known as Panama disease, reach and install themselves through the galleries opened by C. sordidus [4, 5].

In traditional growing areas of central Uganda, western Tanzania and Costa Rica the weevil has been blamed as the primary factor contributing to the decline and disappearance of the highland cooking banana (Musa spp., genome group AAA-EA) [1, 2, 4].

Corm weevil damage and yields losses tend to increase with time, achieving losses up to 100% in cases of severe infestation [3]. It is estimated to occur an average 30% reduction in production due to its attack in Brazil [5].

Main contributing factors are the pest’s biology and behavior. The pest adults can live for more than one year and lay about 100 eggs in the banana tree rhizome tissue [1, 6]. In Brazil, the breaking out period for adults is between march and may and the adults can found throughout the entire year [7].

Currently in the world, many integrated pest management exist (IPM) to maintain the banana corm weevil populations at low levels, like a planting material free of egg, crop sanitation through destruction C. sordidus residues, trapping to monitoring and control, host plant resistance, biological control and chemical control [8, 9].

In spite this, an interesting IPM strategy is the pseudostem with lethal compounds to trap adult weevils it has aroused the interest of several researchers [1, 10]. In this context, Aby, et al. research on entomopathogenic fungus Metarhizium and Beauveria bassiana with pseudo stem traps to control adults [3].

However, the best results of entomopathogenic fungus are on the adult phase of insect, penetrating through its integument [11]. Therefore, it would be interesting to use products that inhibit the pest at the egg stage, once the damage caused by the larvae.

To test sterilization effect in C. sordidus, two commercial formulations of physiological insecticides were evaluated. Two concentrations of lufenuron (Match 50 EC, Syngenta Crop Protection, Sao Paulo, Brazil) and abamectin (Vertimec 18 EC, Syngenta Crop Protection, Sao Paulo, Brazil) were tested (Table 1). All commercial products were diluted in 1 liter of water.

In this sense, physiological insecticides or insect growth regulator (IGR) were also an option to control this pest because these compounds sterilizing activity on insect pest. According Schroeder, et al. Nakano, et al. Grosscurt and Avila and Nakano, physiological insecticides have greater stability and could have a longer sterilizing activity and sterilization is still the only effective way to eradicate pests on short notice [12, 13, 14, 15].

Based on banana corm weevil`s behavior and the sterilization characteristic of some physiological insecticides, lay forward the possibility of controlling this pest on banana orchards by using baits, taking into consideration that this pest does not scatter but is transported with the seedlings.

For this, the hypothesis based on C. sordidus female ingest bait containing physiological insecticide and hatching of the subsequently laid eggs will be prevent.

Material and Methods

The trial took place between February and May 2016 at the Department of Entomology and Acarology Laboratory of Sao Paulo State University, Luiz de Queiroz College of Agriculture, Piracicaba, São Paulo.

Results

Results of total eggs demonstrated that there was no significant difference between treatments (P=0.3122; F=1.306). In contrast, fertilized eggs (P=0.0001; F=57.723) and feasibility of eggs (P=0.0001; F=70.963) had statistically significant differences (Tables 2 and 3).

This result confirms the insecticides sterilizing effect efficacy, where avermectin 1.5 and 3.0 ml demonstrated 35.09 and 47.92% respectively. On the other hand, the efficacy rate of lufenuron 0.5 and 1.0 ml was 87.93 and 90.48% respectively (Table 3).

1Rate of commercial product per liter; 2Percentage of feasible eggs after C. sordius adults exposure to pseudo stem baits contaminated with physiological insecticides; 3Insecticide sterilant effect (%E) calculated according to the Henderson and Tilton efficacy formula (1955); 4Numbers followed by the same letter do not differ statistically in column at the 5% probability level according to the Tukey’s test.

Discussion

C. sordidus is the most important insect pest in banana orchard. To management of this pest, it is necessary understand your biology cycle and behavior in the field to chose the best IPM tool. Thus, several studies have shown pseudo banana bait capacity for capturing insect pest adults and use this trap with pesticides to control as a good strategy in banana IPM programs [3, 13, 17, 18, 19, 20, 21, 22, 23].

According to our study, statistical analysis of data shows that lufenuron in all concentrations was efficient to control banana corn weevil when compared to other treatments. Lufenuron was highly efficient for the control of banana beetles by sterilizing adults during the entire 70days period.

The efficiency of all lufenuron concentrations was between 87.93 and 90.48%, which was considered satisfactory, given that the samples collected from the field were post-contaminated, even though the effect was present and the eclosion was impracticable.

Silva obtained similar results with other benzoylphenyl ureas insecticide. In this case, when baits had diflubenzuron at 0.25% as the active ingredient and the number of laid eggs was higher than the control but had no eclosion, showing the sterilizing effect [19].

Physiological insecticides or insect growth regulators (IGR), group such as lufenuron, have shown to have transovarian activity for Grapholita molesta, Plutella xylostella, Ceratitis capitata and Delia antiqua [24, 25, 26, 27]. This insecticide group exerts its action by inhibiting the production of chitin, a major component of the insect exoskeleton.

In this sense, it is possible that effect over the spermatogenesis or oogenesis in insect occurred, although study shows that the effect of lufenuron on Diabrotica speciosa females wasn´t necessarily sterilizing [28].

According this authors, this means that the product did not cause sterilization when observing unborn embryos formation, as possibly the females were fed the product, which was transferred to the embryos through the ovary, precluding larvae eclosion.

Silva developed anatomic ovary studies of C. sordidus and did not notice any apparent abnormality on the structure and formation of ovules on diflubenzuron tests [19]. Although larvae viability was low even for the control, the results obtained in this bioassay with lufenuron allow for some confidence with the data shown by Silva where lufenuron would be affecting directly the embryo formation in the eggs inside the ovaries [19].

The drastic effect would be occurring because the beetle goes for the inner tissue of baits, causing contamination by ingestion and by contact, different from Avila and Nakano that poisoned only through ingested feed, having no effect on contact. Even more the usage concentration here was 0.50% while Avila and Nakano used 0.033% doses.

A number of laid eggs on the lufenuron treatment compared to the control would indicate poisoning that usually accelerates weakening of organism. Silva also relates that thanks to Beauveria bassiana, a natural parasitism occurs in the populations of banana corm weevil in the field [19]. In contrast to lufenuron, avermectin in all the concentrations used, was not efficient and did not have great sterilization effect. Avermectin in all concentrations was between 35.09 and 47.92%, which was not considered satisfactory to control this pest.

This insecticide had not much of an effect on the reproduction process but appeared to be promising at high concentrations. Silva studied the abamectin effect on the same pest and found that at doses of 2.8 ml with 1.8% or 0.05% of active ingredient, showed eclosion inhibition [19].

Unlike lufenuron’s mode of action, abamectin activate the GABA-gated chloride channel through a GABA-agonist type action in nervous system. In this case, activation of the channel suppresses neuronal activity, resulting in ataxia, paralysis and death of insects characterizing knock down insecticide and not “physiological” compounds [29, 30].

Other hypothesis is that abamectin have is quickly photo-degraded and the insecticidal effect has been lost faster [31, 32]. For this, to abamectin exert toxicity in C. sordidus eggs in field, it is necessary put the baits in place with sun brightness to avoid this possible effect.

Although in our study did not demonstrate abamectin good sterelizing effect on C. sordidus eggs, there is differences when comparing both concentrations. In this sense, it is possible increased the sterilization effect with higher doses. However, studies to evaluate efficacy and economic feasibility will be necessary.

In general, this work be many advantages although further work have to be conducted to test lufenuron pseudo stem baits C. sordidus eggs sterilization efficiency in the field, for example: lufenuron acted as specific hormones on arthropods, and this is more safe for natural enemies and mammals; lufenuron in low concentrations showed efficient in C. sordidus control and when used in baits could avoid application of pesticides in large scale and reduce environmental impact; lufenuron could kill larvae in your damage phase before, which is interesting in crop protection [33, 34].

Conclusion

The physiological insecticide lufenuron at 0.5 and 1.0ml L-1concentrations showed sterilant effect on C. sordidus eggs, and this control method can be considered a good strategy to management this pest on banana orchards.