Gustavo Rodrigues Alves

Lethal and Sublethal Effects of Insecticides Used on Citrus, on the Ectoparasitoid Tamarixia radiata

Huanglongbing (HLB) is a disease associated with the bacteria “Candidatus Liberibacter spp.” and has been devastating citrus orchards around the world. Its management involves control of the insect vector, the Asian citrus psyllid Diaphorina citri Kuwayama. However, the indiscriminate use of chemicals has caused pest outbreaks and eliminated the natural enemies of the vector, such as the parasitoid Tamarixia radiata (Waterston), the main agent for biological control of D. citri. This study assessed the lethal and sublethal effects of insecticides recommended for integrated production of citrus on the parasitoid T. radiata. When adult parasitoids were exposed to residues of 25 insecticides, 20% of them, i.e., gamma-cyhalothrin, etofenprox, azadirachtin, tebufenozide and pyriproxyfen, were considered as harmless (Class 1), 12% as slightly harmful (Class 2), 12% as moderately harmful (Class 3) and 56% as harmful (Class 4), according to the classification proposed by the IOBC/WPRS. Afterward, 14 insecticides (5 harmless and 9 harmful) were sprayed on the parasitoid pupae. Of the 14 insecticides tested, only the organophosphates dimethoate and chlorpyrifos affected the parasitoid emergence. The effects of insecticides on the parasitism capacity of adults exposed to residues of azadirachtin, etofenprox, gamma-cyhalothrin, pyriproxyfen and tebufenozide (harmless) were also evaluated. Tebufenozide and gamma-cyhalothrin affected the parasitism of the F0 generation, but did not affect the emergence of the F1 and F2 generations. Therefore, for an effective IPM program, selective insecticides or harmful pesticides to adult parasitoids could be used in the field, provided that the adults do not occur naturally and the chemical applications do not coincide with parasitoid releases.

Curry leaf smells better than citrus to females of Diaphorina citri (Hemiptera: Liviidae)

The Asian citrus psyllid (ACP) Diaphorina citri Kuwayama has a host range of about 20 species of the family Rutaceae, including Citrus spp. However, few studies have reported on its host preference. This study evaluated the host-choice behavior of ACP in curry leaf (Murraya koenigii L.), through free-choice test and bioassays with a type ‘Y’ olfactometer, and also characterized the volatiles involved in attracting the ACP. In the free-choice test, the number of adults per plant on curry leaf was higher than the number on citrus plants. When the ACP was tested in the olfactometer, the females showed preference for curry leaf over citrus plants. Sixteen volatile compounds were identified in citrus and curry leaves. Qualitative and quantitative differences in the compounds released by citrus and curry leaves were determined. The volatiles present in these hosts may play an important role in the attraction of D. citri. With this information, further studies should be done to develop new management strategies for the ACP.

Acute toxicity of fresh and aged residues of pesticides to the parasitoid Tamarixia radiata and to the HLB-Bacteria Vector Diaphorina citri

One method for controlling the Asian citrus psyllid (ACP) Diaphorina citri Kuwayama, the vector of the putative causal agent of Huanglongbing, uses the parasitoid Tamarixia radiata (Waterston). However, the general intensive use of insecticides has reduced the numbers of this parasitoid. This study evaluated the effect of the residual action of 24 insecticides on T. radiata and also determined the differential toxicity of insecticides to D. citri and T. radiata, using three bioassays. In the first, when adults of the parasitoid were exposed to residues of the 24 insecticides, ten were considered short-life (class 1), six slightly persistent (class 2), five moderately persistent (class 3), and three insecticides were considered persistent (class 4), under the IOBC/WPRS classification system. The second bioassay evaluated the sublethal concentrations of the persistent insecticides (formetanate, dimethoate, spinosad). Increasing the concentrations of the insecticides increased the number that were classified as persistent. In the third bioassay, evaluation of the differential toxicity of eight insecticides to the ACP and the parasitoid showed that chlorpyrifos and bifenthrin were more harmful to T. radiata. Therefore, these two insecticides are not recommended for application at the time of parasitoid release. Cypermethrin, imidacloprid, and dimethoate caused higher mortality of D. citri and are most often recommended in IPM programs. The choice of an insecticide for the control of citrus pests must be made with care, aiming to preserve the natural enemies in the ecosystem, and thereby contribute to the success of biological control.

Does the scion or rootstock of Citrus sp. affect the feeding and biology of Diaphorina citri Kuwayama (Hemiptera: Liviidae)?

The Asian citrus psyllid (ACP) Diaphorina citri is the main vector of the bacteria associated with Huanglongbing, and can exploit more than 50 species of the family Rutaceae as hosts. The number of possible hosts is even higher if plant varieties are included. This study evaluated the influence of different combinations of scion and rootstock citrus varieties on the development and feeding of ACP. Survival rates for the egg stage were highest on the Valencia and Sicilian varieties, both grafted on Sunki mandarin, with means of 87.99 and 87.98%, respectively; and lowest (67.63%) on Hamlinxa0×xa0Rangpur lime. The lowest levels of both nymphal and total viability (egg-adult) were obtained on Hamlin, regardless of the rootstock used. The total development time (egg-adult) ranged from 17.92 to 19.33xa0days for the Pêraxa0×xa0Sunki and Hamlinxa0×xa0Swingle combinations, respectively. Cluster analysis separated the hosts into two groups, the first consisting of the combinations of the Hamlin variety, and the second group formed by the other varieties. The highest food value (assessed by the area of honeydew produced) was observed for the orange scion varieties, and among these, the highest value was observed on Valencia (0.902xa0cm2); the smallest honeydew area was obtained on Ponkan (0.269xa0cm2). The rootstocks did not affect the feeding behavior of D. citri. The results of this study could aid in the development of management techniques, mainly in providing information for the installation of new citrus groves and assistance in crop-improvement research.

Can the Choice Behavior and Fitness of Tamarixia radiata (Hymenoptera: Eulophidae) be Affected by the Citrus (Sapindales: Rutaceae) Variety Used to Rear the Asian Citrus Psyllid (Hemiptera: Liviidae)?

Abstract Several studies have shown that the host plant influences the bioecological parameters of both pest insects and their natural enemies; however, few studies have evaluated the effect of varieties of the same plant species. This study evaluated the host-choice behavior (by a free-choice test) of Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) on nymphs of Diaphorina citri Kuwayama (Hemiptera: Liviidae) reared on 4 sweet orange (Sapindales: Rutaceae) varieties (Hamlin, Pêra, Natal, and Valencia), and its development (biology) in nymphs reared on these same varieties, on a mandarin (cv. Ponkan), and on orange jasmine. The host-choice behavior was not affected by the host on which D. citri was reared, showing that T. radiata has a degree of plasticity in interpreting the cues for host location. In the free-choice test, the hosts did not affect the percentage of parasitism. The emergence parameter, regardless of the host used, showed viability (egg to adult) above 80% for all varieties, with the highest level for Pêra sweet orange (90.06%). The parasitoids development period in nymphs reared on Ponkan (12.4 d) was statistically similar to the period in nymphs reared on orange jasmine (11.80 d) but statistically longer than in nymphs reared on other citrus varieties. No significant difference in the sex ratio was observed. Tamarixia radiata as a biological control agent of D. citri can be an additional component in integrated pest management in citrus groves. It can be used regardless of the citrus variety, because its behavior and biology are similar for the main commercial varieties of citrus grown in São Paulo.