Influence of Insect Growth Regulators on Stephanitis pyrioides (Hemiptera: Tingidae) Eggs and Nymphs

Abstract

The azalea lace bug, Stephanitis pyrioides (Scott) (Hemiptera: Tingidae), is an important insect pest of azaleas (Rhododendron L. spp.) in the USA. Stephanitis pyrioides feeds on azalea foliage and causes extensive chlorosis, which reduces the aesthetic value and marketability of these plants. Because the use of neonicotinoid insecticides has been dramatically reduced or discontinued, growers and landscape managers are seeking alternative tools or strategies to control this insect. Although insect growth regulators (IGRs) are known for their activity against immature insect stages, their activity against egg hatching has not been addressed thoroughly, specifically against S. pyrioides. Thus, a series of experiments was conducted to understand the ovicidal activity of IGRs using novaluron, azadirachtin, pyriproxyfen, and buprofezin against S. pyrioides. The number of newly emerged young instars was significantly lower when leaves implanted with eggs were sprayed on both sides with novaluron, azadirachtin, and buprofezin compared to nontreated and pyriproxyfen treatments. When IGRs plus adjuvant were applied to the adaxial surface of the leaves, the densities of the newly emerged nymphs were significantly lower under the novaluron treatment compared to the nontreated leaves. However, there was no significant difference in the number of nymphs that emerged in the absence of adjuvant. Furthermore, close monitoring revealed reduced levels of egg hatching in the presence of adjuvant with novaluron compared to its absence. The data show that the survival of S. pyrioides first instars was not affected by exposure to dried IGR residues.

Ingestion of Novaluron Elicits Transovarial Activity in Stephanitis pyrioides (Hemiptera: Tingidae)

“…The leaves treated with novaluron alone and novaluron with any adjuvant exerted similar effects on nymphal densities. In contrast, a previous study showed that adding an adjuvant improved the ovicidal activity of novaluron in S. pyrioides [18]. This suggests that adding an adjuvant to novaluron may present value in suppressing the S. pyrioides population in the long run.…”

“…This means that the deposited novaluron residues are not required to travel through several cell layers to be accessible to adults and nymphs of S. pyrioides. Although the length of the stylets may be shorter in the first and second instars of S. pyrioides than in adults, the effects of novaluron were evident even in young nymphs [18], which suggests that novaluron without any adjuvant can cross the leaf blade and intoxicate young nymphs through ingestion. A previous study also showed that young instars directly exposed to novaluron residues on the leaf surface did not exhibit mortality [18].…”

“…A previous study showed that among novaluron, buprofezin, pyriproxyfen and azadirachtin, novaluron was by far the most effective IGR, with transovarial effects on adult S. pyrioides [17]. In addition, another study showed that nymphs of S. pyrioides were susceptible to novaluron when the residues were deposited on the adaxial side of the leaves [18]. This suggests that novaluron can be effective in reducing field populations of S. pyrioides by affecting egg hatching through transovarial activity as well as disrupting nymphal development through abnormal ecdysis.…”

“…All the S. pyrioides adults used in the experiments were obtained from laboratory colonies maintained on live 'George Tabor' azalea plants in 3.7 L pots. The details of S. pyrioides rearing are described in [17,18]. At biweekly intervals, completely depleted plants were swapped with fresh non-infested plants to ensure uninterrupted food and water sources for S. pyrioides development.…”

“…When novaluron was topically applied on adults of S. pyrioides, transovarial activity was observed, with the viability of the eggs being considerably reduced [17]. Similarly, when novaluron was applied to eggs and nymphs of S. pyrioides, ovicidal and nymphicidal activities, respectively, were observed [18]. Further studies are warranted to develop novaluron as an alternative tool and to incorporate it into integrated pest management programs for S. pyrioides.…”