Washington governor Jay Inslee announced almost two weeks ago that a plague of non-native insects posed an “imminent danger” to his state.
The US Department of Agriculture (USDA) says the Asian gypsy moth (AGM) - one of two types of the invasive moth - "could cause serious, widespread damage to our country’s landscape and natural resources".
In an emergency proclamation on 6 May, Mr Inslee warned that two types of the plant pests, both the Asian and the Asian-European hybrid gypsy moths, would cause serious damage to the state’s agricultural, economic, and social well-being.
Mr Inslee’s proclamation added that “the imminent threat of infestation has created a state of emergency”.
Large infestations of both forms of gypsy moth can completely defoliate trees and weaken plants to the point that they are vulnerable to disease. The USDA adds that “repeated defoliation can lead to the death of large sections of forests, orchards and landscaping”.
Both the Asian gypsy moth and the European hybrid are thought to have taken hold in parts of Snohomish County, where the state department for agriculture began spraying a biological insecticide this weekend to stop their spread.
On Twitter on Sunday, it documented the plane carrying the insecticide bacterium Bacillus thuringiensis to kill the gypsy moth in its caterpillar stage.
“We really need to try and get this treatment in today. The weather forecast this week is not good and this may be our only window,” said the department. “We must do 3 total treatments while the caterpillars are small for this to be effective.”
Most gypsy moths hatch in spring with new larvae being about 2mm long, uniformly dark in colour and very hairy.
When fully grown, a female gypsy moth has white wings featuring a dark wavy patterns, and and a wingspan of between 40mm and 70 mm. Male moths are grey or brown, and half the size. According to the USDA a single female can produce hundreds of eggs.
Although the two types of gypsy moths are similar, the USDA warns that Asian gypsy moths are more dangerous because they feed on a much broader range of plants and trees – as many as 500 different species. These include alder, larch, apple, poplar, oak, willow, linden and elm trees.
At the same time, the Asian moth can up fly up to 25 miles, meaning the potential extent of its destructive feeding is huge.
The Asian-European hybrid moth, which can be found in the northeastern corner of the US, mainland Europe and southeast England, is less of a threat because it does not fly as far and is not known to feed on as many species.
The USDA says that cargo ships coming from Asian countries where the gypsy moth are known to exist are the most likely cause of the Washington infestation.
The department's own gypsy moth notice on shipping inspections for Asian gypsy moths, issued in 2019, states that “populations are prevalent in some seaport areas in far east Russia, Japan, Korea, and northern China.”
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