The Asian Citrus Psyllid in California

In May of last year, a small insect about the size of an aphid – but looking more like a miniature cicada – was discovered in California near San Diego. Known as the Asian citrus psyllid, this small creature has caused a large amount of concern.

The reason is that the psyllid is known to carry a disease that is devastating to citrus trees. In Florida, the disease (known as citrus greening) is killing so many trees that some people are predicting the end of the state’s citrus industry within 15 years.

Fortunately for Californians, the disease hasn’t been detected here yet even though the disease’s vector – the psyllid - has. Even so, we’re not taking any chances. In fact, earlier this week, a room full of entomologists and other interested parties met for a full day near Sea World to discuss the issue.

For a day-length symposium, it was surprising how well attended every talk was – even the last ones. It was also surprising how engaged everybody seemed to be throughout. A big feature of the morning session was the presence of scientists from Florida discussing what had been happening in their state – including what successes they had had; and, more importantly, what had not worked well.

Florida’s number one lesson for Californians is that waiting to control the insect (and hoping that the disease doesn’t show up) is a very bad idea. The reason for this is that the disease can remain dormant in trees for over a year. Florida learned this lesson the hard way. The psyllid showed up in 1998 and a bit later the disease was detected. Efforts were made to control the insect but the focus was on those areas were the disease was apparent. As a result an ever-increasing number of trees, infected with the dormant disease, went undetected. Almost over-night, it seemed, when these trees started showing symptoms, it was discovered that much of the state was infected. Not enough attention had been given to the insects.

The bad news for California is that this lesson is more easily understood than acted upon. The psyllids are too mobile and are quite good at remaining undetected. The California Department of Food and Agriculture (CDFA) has acted promptly in putting together a team to deal with this problem, and significant efforts are being made to discover the insects and kill them, but there are two major reasons that their efforts will probably not be successful at completely containing the insect.

The biggest problem is that the psyllid has not only been feeding on citrus trees in San Diego County but it has also been feeding south of the border. In fact there have been many times more captures of the insect in Mexico than there have been in California. We can continue to kill every insect we find but unless something is done to control the pests outside our jurisdiction, our efforts will be no more effective than a fly swatter is for killing mosquitoes in a swamp.

The other major problem is that there are just too many citrus trees in California, and many of them will probably not be sampled or effectively treated no matter how much money we throw at the problem. If we only had to worry about citrus orchards, we might have a chance. It is known that the psyllids prefer feeding on new leaves from trees on the borders of orchards. It is conceivably possible to monitor these places and either treat them with effective insecticides, or else remove the trees so they can’t be reservoirs of the disease. Unfortunately, there are just too many trees in urban areas and even in rural areas outside of orchards. It isn’t possible to find all of these trees even if we had the resources.

In the last several months, despite our significant effort, the psyllid has moved from San Diego County to Imperial County, and it is now in Riverside County too. It seems to be only a matter of time before it jumps over the Transverse Ranges (or skirts them from the east) and enters the Central Valley. We seem to be pretty good at monitoring the critters, but this isn’t enough. We’re just watching them expand their range.

And then there remains the big unknown threat of a yellowing disease on tomatoes and potatoes. A graduate student has recently shown that the bacterium causing this disease is closely related to citrus greening. The bad news is that it can also infect citrus trees. It seems that it isn’t happening very often because the insect that feeds on tomatoes and potatoes doesn’t like to feed on citrus too (thus transferring the disease). The big unknown is whether or not our newly-arrived psyllid – the Asian citrus psyllid - might do a much better job at carrying it around. We might end up with an entirely different citrus greening (or yellowing) disease on our hands.

So what are citrus growers to do? Watch and despair – and pray that the disease doesn’t show up? Well, things probably aren’t that bad yet – although a bit of prayer might not hurt. We do have tools.

For starters, we have good systemic products that kill psyllids and that have been very effective in Florida on young trees – both as a preventative treatment and to already infested plants. It is applied to the roots and is absorbed by the plant - moving into the newer plant tissue. This is where the psyllids are feeding or laying eggs. It would probably work on older trees too but the rate would have to be quite a bit higher than is currently allowed. Putting this large amount on the trees, besides being illegal, might only make for super-resistant bugs. There are a few people looking into higher rates in order to save the more mature trees and this may be an option later on under certain conditions. We’ll have to wait and see.

Another important precaution is to watch your trees - especially the new growth - for signs of the insects. They’re small so you have to pay attention but they tend to like the outside trees of an orchard. If you can catch them early and kill them with a good insecticide, you may be able to keep things under control. The best time for spraying for maximum effect is just prior to the adult flush – to prevent egg-laying and subsequent feeding by the nymphs. In California, this timing isn’t worked out yet as well as it is in Florida, so we’ll have to pay extra careful attention until it is.

And remember, Southern California is now a war zone, and like any effective military effort, we need to anticipate where the enemy is headed. Quite simply, it is headed to any and all unprotected citrus plants. It’s true that citrus growers are the first that need to take precautions, but this is not enough. For every commercial citrus grower there are many more home owners that are not paying attention to the looming crisis. Growers need to make as large of a “no-fly” zone as possible around their trees. This means talking to neighbors and thinking like the enemy. It’s the newly-potted nursery plant sitting on an apartment balcony that’s going to be forgotten. We need snipers everywhere (polite ones, for sure). CDFA, for all their help, will never make this work alone. It’s time to bring out the troops. If we fail, we may lose the citrus industry in the Golden State.