- Author: Kathy Keatley Garvey
“Weather patterns associated with climate change may adversely affect the performance of some of the most important insecticides, systemic insecticides, against coleopteran and lepidopteran pests,” Nansen says.
“These insecticides depend on efficient water and nutrient uptake through the root system and on vascular flow – both mechanisms may be partially compromised under drought conditions,” Nansen points out. “This leads to lower uptake and non-uniform distribution of systemic insecticide in plant tissues – and therefore, higher risk of pests NOT acquiring a lethal dosage.”
In an opinion-based article published in the current edition of the Journal of Economic Entomology, Nansen and three lab researchers take it one step further, and argue that non-uniform distribution of systemic insecticide in plant tissues may also select for behavioral resistance in target pest populations.
The research, “Does Drought Increase the Risk of Insects Developing Behavioral Resistance to Systemic Insecticides?” is the work of Nansen, doctoral students Trevor Fowles and Emily Bick and researcher Haleh Khodaverdi. The researchers cite some 60 references.
“Increases in severity and frequency of drought periods, average global temperatures, and more erratic fluctuations in rainfall patterns due to climate change are predicted to have a dramatic impact on agricultural production systems,” they wrote in the abstract. “Insect pest populations in agricultural and horticultural systems are also expected to be impacted, both in terms of their spatial and temporal distributions and in their status as pest species.”
The UC Davis researchers discussed how indirect effects of drought may adversely affect the performance of systemic insecticides and also lead to increased risk of insect pests developing behavioral insecticide resistance. “We hypothesize that more pronounced drought will decrease uptake and increase the magnitude of nonuniform translocation of systemic insecticides within treated crop plants, and that may have two concurrent consequences: 1) reduced pesticide performance, and 2) increased likelihood of insect pests evolving behavioral insecticide resistance,” they wrote in the abstract.
“Under this scenario, pests that can sense and avoid acquisition of lethal dosages of systemic insecticides within crop plants will have a selective advantage. This may lead to selection for insect behavioral avoidance, so that insects predominantly feed and oviposit on portions of crop plants with low concentration of systemic insecticide.”
Nansen noted that limited research has been published on the effect of environmental variables, including drought, on pesticide performance. It's important to study the many ways environmental factors can affect, directly and indirectly, both the performance of insecticides and the risk of target insect pests developing resistance, Nansen says.
Potato beetles and diamondback moths are two of the pests mentioned in the research article. Potato beetle larvae attack potato crop foliage, while the larvae of the diamondback moths are major pests of cabbage.
- Author: Kathy Keatley Garvey
Yes, it does, says UC Davis agricultural entomologist Christian Nansen of the Department of Entomology and Nematology, who set out to investigate whether there's a relationship between the “physiological” and “behavioral” resistance in insects and found “some quite interesting patterns.”
In a first-of-its-kind research, published March 4 in the Public Library of Science (PLOS), Nansen and his colleagues discovered that “certainly there was an effect of level of physiological resistance or susceptibility of moth strains” – this they demonstrated by comparing two moth strains with high and low levels of insecticide resistance. But they also found intriguing differences in life stages. “We found that ovipositing females and developing larvae may not show the same levels of behavioral responses to insecticides.”
“This is all very interesting and clearly links theoretical evolutionary biology with applied pest management,” he said, concluding that “behavioral avoidance ought to be considered in evaluating the performance of an insecticide.”
The research, “Behavioral Avoidance—Will Physiological Insecticide Resistance Level of Insect Strains Affect their Oviposition and Movement Responses,” by Christian Nansen and fellow scientists Maria Nansen of UC Davis and Olivier Baissac, Kevin Powis and Greg Baker, all of Australia, targeted the Brassica pest, Plutella xylostella, commonly known as “the cabbage moth.” It is responsible for an estimated $4 to $5 billion loss annually in the United States, Nansen said. Cole crops are the moth's host plant. It lays its eggs only on the family Brassicaceae. Its larvae or caterpillars feed on the leaves, floral stalks and flower buds.
The main objective of the Christian Nansen-led study was to quantify two possible types of behavioral avoidance:
- under choice conditions with leaves having different levels of pesticide spray coverage (including an untreated control leaf), females oviposit predominantly on leaf surfaces without insecticides, and
- larvae avoiding insecticide-treated leaf surfaces.
“As a model system, we studied movement and oviposition responses by two strains of DBM denoted ‘single resistance' and ‘double resistance' based on their levels of physiological resistance to two insecticides: gamma-cyhalothrin and spinetoram,” they wrote.
The researchers compared behavioral responses by these two strains as part of characterizing the relative effect of levels of physiological resistance on the likelihood of insects showing signs of behavioral avoidance.
“Although we are unaware of any theoretical framework providing clear predictions of expected behavioral responses by phenotypes with different of physiological resistance," Nansen said, "we predicted that: (1) DBM individuals with confirmed physiological resistance to a given combination of dosage and insecticide show similar movement and oviposition responses to host plant surfaces with/without insecticides, and (2) DBM individuals should avoid insecticide treated surfaces and show significant changes in movement and oviposition behavior, if they are exposed to a combination of dosage and insecticide to which they are susceptible.”
Nansen said the study “highlights the possibility of associations between physiological resistance and avoidance responses by ovipositing females. In addition, larvae from the single resistance strain moved significant faster than those from the double resistance strain, when the entire arena was treated with either gamma-cyhalothrin or spinetoram.”
"Our study highlights the importance of conducting behavioral studies as part of characterizing effects of selective pressures by insecticides and as part of performance evaluations of insecticides.”
The diamondback moth, thought to be of European origin, is found throughout the Americas and in Europe, Asia, Africa, Australia, New Zealand and the Hawaiian Islands. It was first observed in North America in 1854 in Illinois and by 1883 had spread to Florida and the Rocky Mountains, data shows.
The diamondback moth was the first insect found to have become resistant to biological control by the Bt toxin (from Bacillus thuringiensis) in the field. In the 1980s, the moth developed resistance to pyrethroids, and today, virtually all insecticides are ineffective, entomologists say.
Cold winters help to kill off overwintering pests. In California, natural enemies can often effectively control the diamondback moth, according to the UC Integrated Pest Management (UC IPM) Program website. “In southern California, the ichneumonid wasp, Diadegma insularis, has been identified as the most common parasite. Trichogramma pretiosum may also attack diamondback eggs," IPM says. "Various predators such as ground beetles, true bugs, syrphid fly larvae, and spiders can be important factors in controlling populations.”